References:- Organic Chemistry, 6th ed., by: Robert Morrison and Robert Boydm, New York, University, Prentice-Hall of India, private limited, New Delhi – 2002.- Organic Chemistry, Paula Bruice, 4th ed., Prentice Hall; 2003.

Part two

HydrocarbonsCertain organic compounds contain only elements of hydrogen and carbon therefore are known as hydrocarbons. On the basis of structure, hydrocarbons are divided into two main classes: aliphatic and aromatic. Aliphatic hydrocarbons are divided into families: alkanes, alkenes, alkynes and cycloalkanes (alicyclics).

First you will learn how to name alkanes because they form the basis for the names of almost all organic compounds. Alkanes are composed of only carbon atoms and hydrogen atoms and contain only single bonds. Compounds that contain only carbon and hydrogen are called hydrocarbons, so an alkane is a hydrocarbon that has only single bonds. Alkanes in which the carbons form a continuous chain with no branches are called straight-chain alkanes. The names of several straight-chain alkanes are given in Table 2.1. It is important that you learn the names of at least the first 10.The family of alkanes shown in the table is an example of a homologous series. A homologous series (homos is Greek for “the same as”) is a family of compounds in which each member differs from the next by one methylene (CH2) group. The members of a homologous series are called homologs. Propane (CH3CH2CH3) and butane (CH3CH2 CH2CH3) are homologs.If you look at the relative numbers of carbon and hydrogen atoms in the alkanes listed in Table 2.1, you will see that the general molecular formula for an alkane is CnH2n+2 where n is any integer. So, if an alkane has one carbon atom, it must have four hydrogen atoms; if it has two carbon atoms, it must have six hydrogens.

1

In Kekulé structures, the bonding electrons are drawn as lines and the lone-pair electrons are usually left out entirely. Condensed Structures Frequently, structures are simplified by omitting some (or all) of the covalent bonds and listing atoms bonded to a particular carbon (or nitrogen or oxygen) next to it with a subscript to indicate the number of such atoms.

Compounds such as butane and isobutane that have the same molecular formula but differ in the order in which the atoms are connected are called constitutional isomers—their molecules have different constitutions.

2

There are three constitutional isomers alkanes with molecular formula C5H12

There are five constitutional isomers alkanes with molecular formula C6H14

3

There are nine constitutional isomers alkanes with molecular formula C7H16

This method of nomenclature is called systematic nomenclature. It is also called IUPAC nomenclature because it was designed by a commission of the International Union of Pure and Applied Chemistry at a meeting in Geneva, Switzerland, in 1892. Names such as isobutane and neopentane—nonsystematic names—are called common names.

Nomenclature of Alkyl Substituents

Removing a hydrogen from an alkane results in an alkyl substituent (or an alkyl group). Alkyl substituents are named by replacing the “ane” ending of the alkane with “yl.” The letter “R” is used to indicate any alkyl group.

4

If a hydrogen of an alkane is replaced by an (-OH), the compound becomes an alcohol; if it is replaced by an (-NH2 ) the compound becomes an amine; and if it is replaced by a halogen, the compound becomes an alkyl halide.

Two alkyl groups—a propyl group and an isopropyl group—contain three carbon atoms. A propyl group is obtained when a hydrogen is removed from a primary carbon of propane. A primary carbon is a carbon that is bonded to only one other carbon. An isopropyl group is obtained when a hydrogen is removed from the secondary carbon of propane. A secondary carbon is a carbon that is bonded to two other carbons.

A tertiary carbon is a carbon that is bonded to three other carbons.

5

Nomenclature of Alkanes

The systematic name of an alkane is obtained using the following rules:1. Determine the number of carbons in the longest continuous carbon chain. Thischain is called the parent hydrocarbon.

2. The name of any alkyl substituent that hangs off the parent hydrocarbon is cited before the name of the parent hydrocarbon, together with a number to designate the carbon to which the alkyl substituent is attached. The chain is numbered in the direction that gives the substituent as low a number as possible. The substituent’s name and the name of the parent hydrocarbon are joined in one word, and there is a hyphen (-) between the number and the substituent’s name.

3. If more than one substituent is attached to the parent hydrocarbon, the chain is numbered in the direction that will result in the lowest possible number in the name of the compound.

6

4. When both directions lead to the same lowest number for one of the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents.

5. If the same substituent numbers are obtained in both directions, the first group cited receives the lower number.

6. If a compound has two or more chains of the same length, the parent hydrocarbon is the chain with the greatest number of substituents.

7

7. Names such as “isopropyl,” “sec-butyl,” and “tert-butyl” are acceptable substituent names in the IUPAC system of nomenclature

8

The simplest of alkane family is methane, CH4. We shall ordinarily write methane with a dash to represent each pair of electrons shared by carbon and hydrogen (I). To focus our attention on individual electrons, we may sometimes indicate a pair of electrons by a pair of dots (II). Finally, when we wish to represent the actual shape of the molecule, we shall use a simple three-dimensional formula like III or IV.

Physical propertiesMethane is non-ionic compound, whether solid, liquid, or gas, is the molecule. Because the methane molecule is highly symmetrical, the polarities of the individual carbon-hydrogen bonds cancel out; as a result, the molecule itself is non-polar. Attraction between such non-polar molecules is limited to van der Waals forces; for such small molecules, these attractive forces must be tiny compared with the enormous forces between, say, sodium and chloride ions. It is not surprising, then, that these attractive forces are easily overcome by thermal energy, so that melting and boiling occur at very low temperatures: m.p. is (-183oC), b.p. is (- 101.5 oC). Methane is colorless and, when liquefied, is less dense than water (specific graphity 0.4). In agreement with the rule of thumb that "like dissolves like," it is only slightly soluble in water, but very soluble in organic liquids such as gasoline, ether, and alcohol.

SourcesMethane is an end product of the anaerobic (without air) decay of plants, that is, of the breakdown of certain very complicated molecules. As such, it is the major constituent (up to 97) of natural gas. It is the dangerous gas in the coal mine, and can be seen as marsh gas bubbling to the surface of swamps. If methane is wanted in very pure form, it can be separated from the other constituents of natural gas (mostly other alkanes) by fractional distillation. Most of it, of course, is consumed as fuel without purification.

ReactionsIn its chemical properties as in its physical properties, methane sets the pattern for the alkane family. Typically, it reacts only with highly reactive substances or under very vigorous conditions.

1- Oxidation. Heat of combustionCombustion to carbon dioxide and water is characteristic of organic compounds; under special conditions it is used to determine their content of carbon and hydrogen. Combustion of methane is the principal reaction taking place during the burning of natural gas. The important product is not carbon dioxide or water but heat. Burning of hydrocarbons takes place only at high temperatures, as provided, for example, by a flame or a spark. Once started, however, the reaction gives offحرر heat which is often sufficient to maintain the high temperature and to permit burning to continue. The quantity of heat evolved when one mole of a hydrocarbon is burned to carbon dioxide and water is called (heat of combustion) for methane, its value is 213 kcal.Through controlled partial oxidation of methane and the high-temperature catalytic reaction with water, methane is an increasingly important source of products other than heat: of hydrogen, used in the manufacture of ammonia; of mixtures of carbon monoxide and hydrogen, used in the manufacture of methanol and other alcohols; and of acetylene, itself the starting point of large-scale production of many organic compounds.

9

2- Chlorination: a substitution reactionChlorination is a typical example of a broad class of organic reactions known as substitution. Under the influence of ultraviolet light or at a temperature of 250-400oC, a mixture of the two gases, methane and chlorine, reacts vigorously to yield hydrogen chloride and a compound of formula CH3Cl. We say that methane has undergone chlorination, and we call the product, CH3C1, chloromethane or methyl chloride (CH3 = methyl). A chlorine atom has been substituted for a hydrogen atom of methane, and the hydrogen atom thus replaced is found combined with a second atom of chlorine.

The methyl chloride can itself undergo further substitution to form more hydrogen chloride and CH2C12, dichloromethane or methylene chloride (CH2 = methylene).

In a similar way, chlorination may continue to yield CHCl3, trichloromethane or chloroform, and CC14, tetrachloromethane or carbon tetrachloride.

Methane reacts with bromine, again at high temperatures or under the influence of ultraviolet light, to yield the corresponding bromomethanes: methyl bromide, methylene bromide, bromoform, and carbon tetrabromide.

Bromination takes place somewhat less readily than chlorination. Methane does not react with iodine at all. With fluorine it reacts so vigorously that, even in the dark and at room temperature, the

10

reaction must be carefully controlled: the reactants, diluted with an inert gas, are mixed at low pressure. We can, therefore, arrange the halogens in order of reactivity.

Mechanism of chlorination. Free radicalsThe chlorination of methane is an example of a chain reaction, a reaction that involves a series of steps, each of which generates a reactive substance that brings about the next step.

The first step is the breaking of a chlorine molecule into two chlorine atoms. Like the breaking of any bond, this requires energy, the bond dissociation energy, (is 58 kcal/mole). The energy is supplied as either heat or light.

The chlorine molecule undergoes homolysis that is; cleavage of the chlorine-chlorine bond takes place in a symmetrical way, so that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired as are all the other electrons of the chlorine atom; that is, it does not have a partner of opposite spin. An atom or group of atoms possessing an odd (unpaired) electron is called a free radical. First in the chain of reactions is a chain-initiating step, in which energy is absorbed and a reactive particle generated; in the present reaction it is the cleavage of chlorine into atoms (step 1). There are one or more chain-propagating steps, each of which consumes a reactive particle and generates another; here they are the reaction of chlorine atoms with methane (step 2), and of methyl radicals with chlorine (step 3). Finally, there are chain-terminating steps, in which reactive particles are consumed but not generated; in the chlorination of methane these would involve the union of two of the reactive particles, or the capture of one of them by the walls of the reaction vessel.

Transition stateClearly, the concept of act is to be our key to the understanding of chemical reactivity. To make it useful, we need a further concept: transition state, A chemical reaction is presumably المحتمل a منcontinuous process involving a gradual transition from reactants to products. It has been found extremely helpful, however, to consider the arrangement of atoms at an intermediate stage of reaction as thoughكانما it were an actual molecule. This intermediate structure is called the transition state; its energy content corresponds to the top of the energy hill.

11

The reaction sequence is now:

Just as is the difference in energy content between reactants and products, so act is the difference in energy content between reactants and transition state.

Heat of reactionBy using the values of bond dissociation energies, we can calculate the energy changes that take place in a great number of reactions. In the conversion of methane into methyl chloride, two bonds are broken, CH3-H and Cl-Cl, consuming 104 + 58, or a total of 162 kcal/mole. At the same time two new bonds are formed, CH3- Cl and H-Cl, liberating 84 + 103, or a total of 187 kcal/mole. The result is the liberation of 25 kcal of heat for every mole of methane that is converted into methyl chloride; this is, then, an exothermic reaction. When heat is liberated, the heat content (enthalpy), H, of the molecules themselves must decrease; the change in heat content, , is therefore given a negative sign. (In the case of an endothermic reaction, where heat is absorbed, the increase in heat content of the molecules is indicated by a positive ).

From the table of bond dissociation energies, we can calculate for each of the four halogens the for each of the three steps of halogenation. Since Eact (The minimum amount of energy that must be provided by a collision for reaction to occur is called the energy of activation) has been measured for only a few of these reactions.

12

Alkanes

The next in size after methane is ethane, C2H6.

All bonds C-H and C-C are bonds. Electron diffraction, X-ray and spectroscopic studies have verified this structure in all respects, giving above the following measurements for the molecule: bond

13

angles, 109.5o; C-H length, 1.10 Å; C-C length, 1.53 Å. Similar studies have shown that, with only slight variations, these values are quite characteristic of carbon-hydrogen and carbon-carbon bonds and of carbon bond angles in alkanes.

Free rotation about the carbon-carbon single bond.Change from one to another involves rotation about the carbon-carbon bond, we describe this freedom to change by saying that there is free rotation about the carbon-carbon single bond. Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformations. I is called the eclipsed conformation; II is called the staggered conformation.

The energy required to rotate the ethane molecule about the carbon-carbon bond is called torsional energy. Torsional strain is the resistance to bond twisting

The highly useful representations of the kind are called after M. S. Newman, of The Ohio State University, who first proposed their use.

14

The 3-kcal barrier is not a very high one; even at room temperature the fraction of collisions with sufficient energy is large enough that a rapid interconversion between staggered arrangements occurs.

Propane and the butanesThe next member of the alkane family is propane, C3H8. Again following the rule of one bond per hydrogen and four bonds per carbon, we arrive at structure I. Here, rotation can occur about two carbon-carbon bonds, and again is essentially free. Although the methyl group is considerably larger than hydrogen, the rotational barrier (3.3 kcal/mole) is only a little higher than for ethane.

15

When we consider butane, C4H10, we find that there are two possible structures, II and III. II has a four-carbon chain and III has a three-carbon chain with a one-carbon branch.

Conformations of n-butane. Van der Waals repulsion,Let us look more closely at the n-butane molecule and the conformations in which it exists. Focusing our attention on the middle C-C bond, we see a molecule similar to ethane, but with a methyl group that replaces one hydrogen on each carbon. Due to the presence of the methyl groups, two new points are encountered here: first, there are several different staggered conformations; and second, a factor besides torsional strain (is the resistance to bond twisting) comes into play to affect conformational stabilities.

There is the anti conformation, I, in which the methyl groups are as far apart as they can be (dihedral angle 180o ). There are two gauche conformations, II and III, in which the methyl groups are only 60 o

apart.

16

When the methyl groups are crowded together, under these conditions, van der Waals forces are repulsive and raise the energy of the conformation. We say that there is van der Waals repulsion (or steric repulsion) between the methyl groups, and that the molecule is less stable because of van der Waals strain (or steric strain occurs when nonbonded atoms are forced closer to each other).

Physical properties Alkane molecules are either non-polar or very weakly polar. The forces holding non-polar molecules together (van der Waals forces) are weak and of very short range. The first four in alkanes (Table below) are gases, but, as a result of the rise in boiling point and melting point with increasing chain length, the next 13 alkanes are liquids, and those containing 18 carbons or more are solids. Melting points depend on molecular weight and boiling points depend on molecular weight and the structure of molecule. We see that in every case a branched-chain isomer has a lower boiling point than a straight-chain isomer, and further, that the more numerous the branches, the lower the boiling point. That branching should lower the boiling point is reasonable: with branching the shape of the molecule tends to approach that of a sphere; and as this happens the surface area decreases, with the result that the intermolecular forces become weaker and are overcome at a lower temperature. In agreement with the rule of thumb, "like dissolves like," the alkanes are soluble in non-polar solvents such as benzene, ether, and chloroform, and are insoluble in water and other highly polar solvents.

17

Industrial sourceThe principal source of alkanes is petroleum, together with the accompanying natural gas.

18

19

Preparation

Each of the smaller alkanes, from methane through n-pentane and isopentane, can be obtained in pure form by fractional distillation of petroleum and natural gas.

1. Hydrogenation of alkenes.By far the most important of these methods is the hydrogenation of alkenes. When shaken under a slight pressure of hydrogen gas in the presence of a small amount of catalyst, alkenes are converted smoothly and quantitatively into alkanes of the same carbon skeleton.

2- Hydrolysis of the Grignard reagent (organometallic compound),When a solution of an alkyl halide (different alkylhalide) in dry ethyl ether, is allowed to stand over turnings of metallic magnesium, a vigorous reaction takes place: the solution turns cloudy, begins to boil, and the magnesium metal gradually disappears. The resulting solution is known as a Grignard reagent, after Victor Grignard (of the University of Lyons) who received the Nobel prize in 1912 for its discovery. The Grignard reagent has the general formula RMgX, and the general name alkylmagnesium halide.

The Grignard reagent is highly reactive. It reacts with water to yield alkane:

3- Coupling of alkyl halides with organometallic compounds,

To make an alkane of higher carbon number than the starting material requires formation of carbon-carbon bonds, most directly by the coupling together of two alkyl groups. Coupling takes place in the reaction between a lithium dialkylcopper, R2CuLi, and an alkyl halide, R'X. (R' stands for an alkyl group that may be the same as, or different from, R.)

20

An alkyllithium, RLi, is prepared from an alkyl halide, RX, in much the same way as a Grignard reagent. To it is added cuprous halide, CuX, and then, finally, the second alkyl halide, R'X.

For good yields, R'X should be a primary halide; the alkyl group R in the organometallic may be primary, secondary, or tertiary. For example:

Reactions of alkanes

Much of the chemistry of alkanes involves free-radical chain reactions, which take place under vigorous conditions and usually yield mixtures of products.

1-Halogenation,Under the influence of ultraviolet light, or at 250-400oC, chlorine or bromine converts alkanes into chloroalkanes (alkyl chlorides) or bromoalkanes (alkyl bromides); an equivalent amount of hydrogen chloride or hydrogen bromide is formed at the same time.

21

Bromination gives the corresponding bromides but in different proportions:

2-Combustion,The reaction of alkanes with oxygen to form carbon dioxide, water, and most important of all heat

3-Pyrolysis: cracking,Decomposition of a compound by the action of heat alone is known as pyrolysis.

22

Alkenes

Hydrocarbons that contain a carbon–carbon double bond are called alkenes. On the basis of this observation, alkenes were originally called olefins (oil forming). Alkenes play many important roles in biology. Ethene, for example, is a plant gaseous hormone-a compound that controls the plant’s growth and other changes in its tissues. Ethene affects seed germination (إنبات ), flower maturation (نضوج ), and fruit ripening(نضوج ),. Insects communicate by releasing pheromones—chemical substances that other insects of the same species detect with their antennae. Many of the flavors and fragrances produced by certain plants also belong to the alkene family.

The general molecular formula for an acyclic alkene is also CnH2n because, as a result of the carbon–carbon double bond, an alkene has two fewer hydrogens than an alkane with the same number of carbon atoms. Thus, the general molecular formula for a cyclic alkene must be CnH2n-2 .

Because alkanes contain the maximum number of carbon–hydrogen bonds possible— that is, they are saturated with hydrogen—they are called saturated hydrocarbons. In contrast, alkenes are called unsaturated hydrocarbons, because they have fewer than the maximum number of hydrogens. The total number of bonds and rings in an alkene is called its degree of unsaturation.

23

Nomenclature of Alkenes

The systematic (IUPAC) name of an alkene is obtained by replacing the “ane” ending of the corresponding alkane with “ene.” For example, a two-carbon alkene is called ethene and a three-carbon alkene is called propene. Ethene also is frequently called by its common name: ethylene.

1. The longest continuous chain containing the functional group (in this case, the carbon–carbon double bond) is numbered in a direction that gives the functional group suffix the lowest possible number.

2. The name of a substituent is cited before the name of the longest continuous chain containing the functional group, together with a number to designate the carbon to which the substituent is attached.

3. If a chain has more than one substituent, the substituents are cited in alphabetical order,

4. If the same number for the alkene functional group suffix is obtained in both directions, the correct name is the name that contains the lowest substituent number.

24

5. In cyclic alkenes, a number is not needed to denote the position of the functional group, because the ring is always numbered so that the double bond is between carbons 1 and 2.

6. If both directions lead to the same number for the alkene functional group suffix and the same low number(s) for one or more of the substituents,

25

EthyleneThe simplest number of alkene family is ethylene, C2H4. If we arrange the two carbons and four hydrogens of ethylene to perm it maximum overlap of orbitals, we obtain the structure shown

Carbon atom lays at the center of a triangle, at whose corners are located the two hydrogen atoms and the other carbon atom. Every bend angle is 120o. The molecule is not yet complete, however. In forming the sp2 orbitals, each carbon atom has used only two of its three p orbitals. The remaining p orbital it is formed bond by the overlap of p orbitals.

***There is hindered rotation about the carbon-carbon double bond.

26

The butyleneGoing on to the butylenes, C4H8, we find that there are a number of possible arrangements. First of all, we may have a straight-chain skeleton as in n-butane, or a branched-chain structure as in isobutane.

Experiment has shown that not three but four alkenes of the formula C 4H8 exist; they have the physical properties shown in Table 5.1.

**Configuration: the arrangement of atoms that characterizes a particular stereoisomer. Conformation is different spatial arrangements of a molecule that are generated by rotation about single bonds. Stereoisomers are the isomers that have the same constitution but differ in the arrangement of their atoms in space.

Geometric isomerismSince the isomeric 2-butenes differ from one another only in the way the atoms are oriented in space (but are like one another with respect to which atoms are attached to which other atoms), they belong to the general class we have called stereoisomers. The particular kinds of stereoisomers that owe their existence to hindered rotation about double bonds are called geometric isomers.

27

There is hindered rotation about any carbon-carbon double bond, but it gives rise to geometric isomerism only if there is*a certain relationship among the groups attached to the doubly-bonded carbons.

The prefixes cis and trans work very well for disubstituted ethylenes and some trisubstituted ethylenes. But how are we to specify configurations like these?

We arrange its two atoms or groups and then take the group of higher priority on the one carbon and the group of higher priority on the other carbon, and tell whether they are on the same side of the molecule or on opposite sides. So that it will be clear that we are using this method of specification, we use the letter Z to mean on the same side, and the letter E to mean on opposite sides. (From the German: zusammen, together, and entgegen, opposite.)

Physical propertiesAs a class, the alkenes possess physical properties that are essentially the same as those of the alkanes. They are insoluble in water, but quite soluble in nonpolar solvents like benzene, ether, chloroform, or ligroin. They are less dense than water.

28

As we can see from Table 5.2, the boiling point rises with increasing carbon number; as with the alkanes, the boiling point rise is 20-30oC for each added carbon, except for the very small homologs. As before, branching lowers the boiling point. Like alkanes, alkenes are at most only weakly polara small difference in polarity is reflected in the higher boiling point of the cis-isomer. This same relationship exists for many pairs of geometric isomers. Because of its higher polarity the cis-isomer is generally the higher boiling of a pair; because of its lower symmetry it fits into a crystalline lattice more poorly, and thus generally has the lower melting point. The differences in polarity, and hence the differences in melting point and boiling point, are greater for alkenes that contain elements whose electronegativities differ widely from that of carbon.

Industrial sourceAlkenes are obtained in industrial quantities chiefly by the cracking of petroleum. The smaller alkenes can be obtained in pure form by fractional distillation and are thus available for conversion into a large number of important aliphatic compounds.

Preparation

Alkenes containing up to five carbon atoms can be obtained in pure form from the petroleum industry. Pure samples of more complicated alkenes must be prepared by:

1- Dehydrohalogenation of alkyl halides,Alkyl halides are converted into alkenes by dehydrohalogenation: elimination of the elements of hydrogen halide. Dehydrohalogenation involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one bearing the halogen.

29

The alkene is prepared by simply heating together the alkyl halide and a solution of potassium hydroxide in alcohol. For example:

Mechanism of dehydrohalogenation, The function of hydroxide ion is to pull a hydrogen ion away from carbon; simultaneously a halide ion separates and the double bond forms.

30

2-Dehydration of alcohols,An alcohol is converted into an alkene by dehydration: elimination of a molecule of water. Dehydration requires the presence of an acid (sulfuric or phosphoric acid ) and the application of heat. The various classes of alcohols differ widely in ease of dehydration, the order of reactivity being:

Ease of dehydration of alcohols: 3o > 2 o > 1 o

Mechanism of dehydration of alcohols,The alcohol unites (step 1) with a hydrogen ion to form the protonated alcohol, which dissociates (step 2) into water and a carbonium ion; the carbonium ion then loses (step 3) a hydrogen ion to form the alkene.

The first step of the mechanism is more properly represented as

31

We see the carbonium ion within the mechanism above, the carbonium ion (carbocation), is a group of atoms that contains a carbon atom bearing only six electrons. Carbonium ions are classified as primary, secondary or tertiary after the carbon bearing the positive charge.

According to the laws of electrostatics, the stability of a charged system is increased by dispersal of the charge.

32

Rearrangement of carbonium ionsThe double bond appears in unexpected places; sometimes the carbon skeleton is even changed. For example:

A migration of hydrogen with a pair of electrons is known as a hydride shift; a similar migration of an alkyl group is known as an alkyl shift. The 1,2-shifts: rearrangements in which the migrating group moves from one atom to the very next atom.

33

Reactions of the C-C double bond in alkenes

Addition Reactions

1. Addition of hydrogen. Catalytic hydrogenation

2. Electrophilic addition of halogens.Alkenes are readily converted by chlorine or bromine into saturated compounds that contain two atoms of halogen attached to adjacent carbons in an inert solvent like carbon tetrachloride (CCl4); iodine generally fails to react.

Examples:

34

The mechanism,The bond joining the two halogen atoms is relatively weak and, therefore, easily broken. When the -electrons of the alkene approach a molecule of Br2 or Cl2, one of the halogen atoms accepts the electrons and releases the shared electrons to the other halogen atom. Therefore, in an electrophilic addition reaction, Br2 behaves as if it were Br+and Br-, and Cl2 behaves as if it were Cl+ and Cl-.

The cyclic bromonium ion is more stable than the carbocation would have been, since all the atoms (except hydrogen) in the bromonium ion have complete octets, whereas the positively charged carbon of the carbocation does not have a complete octet.

3. Electrophilic addition of hydrogen halides. Markovnikov's rule,An alkene is converted by hydrogen chloride, hydrogen bromide, or hydrogen iodide into the corresponding alkyl halide.

Markovnikov's rule: In the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom that already holds the greater number of hydrogens. Using Markovnikov's rule, we can correctly predict the principal product of many reactions. For example:

35

In 2-pentene each of the doubly-bonded carbons holds one hydrogen and roughly equal quantities of the two isomers actually being obtained. The mechanism of electrophilic addition:

36

Addition of hydrogen bromide. Peroxide effect,Addition of hydrogen chloride and hydrogen iodide to alkenes follows Markovnikov's rule., but in the presence of alkyl peroxide [ROOR, as (CH3)3C-O-O-C(CH3)3], the addition of HX will be anti Markovnikov’s addition.

37

Explanation of anti Markovnikov’s addition

The mechanism of anti Markovnikov’s addition

38

4. Electrophilic addition of sulfuric acid.Alkenes react with cold concentrated sulfuric acid to form compounds of the general formula ROSO3H, known as alkyl hydrogen sulfates.

The mechanism of addition of sulfuric acid,

If the sulfuric acid solution of the alkyl hydrogen sulfate is diluted with water and heated, there is obtained an alcohol bearing the same alkyl group:

5. Electrophilic addition Addition of water. Hydration.

Water adds to the more reactive alkenes in the presence of acids (H2SO4, HCl) to yield alcohols The catalyst in the hydration of an alkene is an acid, so the reaction is said to be an acid-catalyzed reaction. Since this addition, too, follows Markovnikov's rule. (The addition of water to a molecule is called hydration)

39

Example:

The mechanism of addition of water in the presence of acid,

6. Halohydrin formation.Addition of chlorine or bromine in the presence of water can yield compounds containing halogen and hydroxyl groups on adjacent carbon atoms. These compounds are commonly referred to as halohydrins.

40

7. Dimerization. Addition of alkenes.Under proper conditions, isobutylene is converted by sulfuric or phosphoric acid into a mixture of two alkenes of molecular formula C8H16 . Hydrogenation of either of these alkenes produces the same alkane,

The mechanism of reaction,

41

Step (1) addition of a hydrogen ion to isobutylene to form the carbonium ion; the tertiary cation would, of course, be the preferred ion. Step (2), then, addition of the tert-butyl cation to isobutylene; again, the orientation of addition is such as to yield the more stable tertiary cation. The carbonium ion undergoes a reaction familiar to us: loss of a hydrogen ion (step 3). Since the hydrogen ion can be lost from a carbon on either side of the positively charged carbon, two products should be possible.

8. Alkylation. Addition of alkanes.Isobutylene and isobutane are allowed to react in the presence of an acidic catalyst (industrial method for produce isooctane). This reaction is, in effect, addition of an alkane to an alkene.

The mechanism of reaction,

9- Hydroxylation. Glycol formation.Certain oxidizing agents convert alkenes into compounds known as glycols. Glycols-are simply dihydroxy alcohols.

Of the numerous oxidizing agents that cause hydroxylation, two of the most commonly used are (a) cold alkaline KMnO4 , and (b) peroxyformic acid, HCO2OH.

42

10- Halogenation. Allylic substitution.Alkanes undergo substitution by halogen at high temperatures or under the influence of ultraviolet light, and generally in the gas phase: conditions that favor formation of free radicals. We know that alkenes undergo addition of halogen at low temperatures and in the absence of light, and generally in the liquid phase:

3-chloro-l-propene, known as allyl chloride (CH2=CH-CH2 = allyl).

43

The stability of allyl radical comes from a resonance hybrid of the two structures, I and II.

If, however, the groups attached to the two carbons of the allylic radical are not the same, we expect two substitution products are formed:

Because the groups of two carbons of the allylic radical are not the same in both resonance contributors, so, we got two substitution products are formed:

Its important to show that, N-Bromosuccinimide (NBS) is frequently used to brominate allylic positions because it allows a radical substitution reaction to be carried out without subjecting the reactant to a relatively high concentration of Br2 that could add to its double bond.

11- Ozonolysis.The classical reagent for cleaving the carbon-carbon double bond is ozone. Ozonolysis (cleavage "by ozone) is carried out in two stages: first, addition of ozone to the double bond to form an ozonide ; and second, hydrolysis of the ozonide to yield the cleavage products.

44

12- Addition of AlcoholsAlcohols react with alkenes in the same way that water does. Like the addition of water, the addition of an alcohol requires an acid catalyst. The product of the reaction is ether.

The mechanism for the acid-catalyzed addition of an alcohol is essentially the same as the mechanism for the acid-catalyzed addition of water.

13- Addition of Borane: hydroboration–oxidation reaction,1 mole of BH3 (borane) reacts with 3 moles of alkene to form 3 moles of alcohol.

45

The reaction of borane with alkene is addition reaction:

The mechanism of the oxidation reaction shows that a hydroperoxide ion (a Lewis base) reacts with R3B (a Lewis acid). Then, a 1,2-alkyl shift displaces a hydroxide ion. These two steps are repeated two more times. Then, hydroxide ion (a Lewis base) reacts with (RO)3B (a Lewis acid), and an alkoxide ion is eliminated. Protonation of the alkoxide ion forms the alcohol. These three steps are repeated two more times.

Because carbocation intermediates are not formed in the hydroboration reaction, carbocationrearrangements do not occur. Examples:

Inductive effectInductive effect : withdrawing (I-)or donating (I+) group effect is transmitted through bondField effect: withdrawing (I-)or donating (I+) group effect is transmitted directly through space or solvent molecules.the C-C bond in chloroethane is polarized by the presence of the electronegative chlorine atom. This polarization is actually the sum of two effects. In the first of these, the C-1 atom, having been deprived of some of its electron density by the greater electronegativity of (-Cl) group is partially compensated

46

by drawing the C-C electrons closer to itself, resulting in a polarization of this bond and a slightly positive charge on the C-2 atom. This polarization of one bond caused by the polarization of an adjacent bond is called the inductive effect. The effect is greatest for adjacent bonds but may also be felt farther away; thus the polarization of the C-C bond causes a (slight) polarization of the three methyl C-H bonds. The other effect operates not through bonds, but directly through space or solvent molecules, and is called the field effect.

HyperconjugationAll of the delocalization discussed so far involves electrons. Another type, called hyperconjugation, involves electrons. When a carbon attached forms there is no bond at all between the carbon and hydrogen. The effect of 140 on the actual molecule is that the electrons in the C-H bond are closer to the carbon than they would be if 140 did not contribute at all.

Hyperconjugation in the above case may be regarded as an overlap of the orbital of the C-H bond and the orbital of the C-C bond, analogous to the – orbital overlap previously considered.

Alkynes

Alkynes are hydrocarbons that contain a carbon–carbon triple bond. Because of its triple bond, an alkyne has four fewer hydrogens than the corresponding alkane. Therefore, the general molecular formula for an acyclic (noncyclic) alkyne is CnH2n-2 and that for a cyclic alkyne is CnH2n-4. There are only a few naturally occurring alkynes. Examples include capillin, which has fungicidal activity, and ichthyothereol, a convulsion( تشنج) used by the Amazon Indians for poisoned arrowheads. A class of naturally occurring compounds called enediynes has been found to have powerful antibiotic and anticancer properties.

A few drugs contain alkyne functional groups, but they are not naturally occurring compounds. They exist only because chemists have been able to synthesize them.

47

In the structure of ethyne each carbon is sp hybridized, so each has two sp orbitals and two p orbitals. One sp orbital overlaps the s orbital of a hydrogen, and the other overlaps a sp orbital of the other carbon. Because the sp orbitals are oriented as far from each other as possible to minimize electron repulsion, ethyne is a linear molecule with bond angles of 180°.

Nomenclature of AlkynesThe systematic name of an alkyne is obtained by replacing the “ane” ending of the alkane name with “yne.” If the triple bond is at the end of the chain, the alkyne is classified as a terminal alkyne. Alkynes with triple bonds located elsewhere along the chain are called internal alkynes. For example, 1-butyne is a terminal alkyne, whereas 2-pentyne is an internal alkyne.

48

If the same number for the alkyne functional group suffix is obtained counting from either direction along the carbon chain, the correct systematic name is the one that contains the lowest substituent number. If the compound contains more than one substituent, the substituents are listed in alphabetical order.

The triple-bond-containing propargyl group is used in common nomenclature. It is analogous to the double-bond-containing allyl group.

Physical properties of alkynesBeing compounds of low polarity, the alkynes have physical properties that are essentially the same as those of the alkanes and alkenes. They are insoluble in water but quite soluble in the usual organic solvents of low polarity: ligroin( a mixture of hydrocarbons (C7-C11) including: aliphatic and aromatic compounds, this mixture has bp=60-90oC) , ether, benzene, carbon tetrachloride. They are less dense than water. Their boiling points (Table 8.1) show the usual increase with increasing carbon number, and the usual effects of chain-branching; they are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons.

49

Industrial source of acetyleneThe alkyne of chief industrial importance is the simplest member of the family, acetylene. It can be prepared by the action of water on calcium carbide, CaC2 , which itself is prepared by the reaction between calcium oxide and coke at the very high temperatures of the electric furnace. The calcium oxide and coke are in turn obtained from limestone and coal, respectively. Acetylene is thus obtained by a few steps from three abundant, cheap raw materials: water, coal, limestone.

Coal is black or brown rocks which is flammable and combustion. Coke is a carbonaceous material usable as fuel burned, and is manufactured by destructive distillation of coal

An alternative synthesis, based on petroleum, is displacing the carbide process. This involves the controlled, high-temperature partial oxidation of methane.

50

Preparation of alkynesA carbon-carbon triple bond is formed in the same way as a double bond: elimination of atoms or groups from two adjacent carbons.

Dehydrohalogenation of alkyl dihalides,Alkyldihalides readily obtained from the corresponding alkenes by addition of halogen. Dehydrohalogenation can generally be carried out in two stages as shown.

Carried through only the first stage, it is a valuable method for preparing unsaturated halides. The halides thus obtained, with halogen attached directly to doubly-bonded carbon, are called vinyl halides, and are very unreactive. Under mild conditions, therefore, dehydrohalogenation stops at the vinyl halide stage; more vigorous conditions use of a stronger base is required for alkyne formation.

Reactions of alkynes

Like alkenes, alkynes undergo electrophilic addition, Besides addition, alkynes undergo certain reactions that are due to the acidity of a hydrogen atom held by triply-bonded carbon.

1- Acidity of alkynes. Very weak acids,we took acidity to be a measure of the tendency of a compound to lose a hydrogen ion. hydrogen attached to triply-bonded carbon, as in acetylene or any alkyne with the triple bond at the end of the chain (RCC-H), shows appreciable acidity. For example, sodium reacts with acetylene to liberate hydrogen

Sodium metal reacts with ammonia to form sodamide, NaNH2, which is the salt of the weak acid, NH3 .

51

Addition of acetylene to sodamide dissolved in ether yields ammonia and sodium acetylide.

Addition of water to sodium acetylide forms sodium hydroxide and regenerates acetylene. The weaker acid, H-CC-H, is displaced from its salt by the stronger acid, H-OH. Thus we see that acetylene is a stronger acid than ammonia, but a weaker acid than water.

sp orbital has less p character and more s character, An electron in a p orbital is at some distance from the nucleus and is held relatively loosely; an electron in an s orbital, on the other hand, is close to the nucleus and is held more tightly. The acetylide ion is the weaker base since its pair of electrons is held more tightly, in an sp orbital.

2- Formation of heavy metal acetylides,The acidic acetylenes react with certain heavy metal ions, chiefly Ag* and Cu+, to form insoluble acetylides. Formation of a precipitate upon addition of an alkyne to a solution of AgNO 3 in alcohol, for example, is an indication of hydrogen attached to triply-bonded carbon.

3- Reaction of sodium acetylides with alkyl halides(primary). Sodium acetylides are used in the synthesis of higher alkynes. For example:

52

4. Addition of hydrogen halides.Addition of hydrogen, halogens, and hydrogen halides to alkynes is very much like addition' to alkenes, except that here two molecules of reagent can be consumed for each triple bond.

5. Addition of water. Hydration.The molecule of water can be added to the triple bond of alkyne in the presence of acid to give the intermediate I (enol form) that can be change to the compound II ( keto form) by the process of tautomerism.

53

Rearrangements of this enol-keto kind take place particularly easily because of the polarity of the O-H bond. A hydrogen ion separates readily from oxygen to form a hybrid anion; but when a hydrogen ion (most likely a different one) returns, it may attach itself either to oxygen or to carbon of the anion. When it returns to oxygen, it may readily come off again; but when it attaches itself to carbon, it tends to stay there. We recognize this reaction as another example of the conversion of a stronger acid into a weaker acid. Compounds whose structures differ markedly in arrangement ofatoms, but which exist in equilibrium, are called tautomers. The most common kind of tautomerism involves structures that differ in the point of attachment of hydrogen. In these cases, as in keto-enol tautomerism,

54

6. Addition of hydrogen. Reduction.Hydrogen adds to an alkyne in the presence of a metal catalyst such as palladium, platinum, or nickel in the same manner that it adds to an alkene. It is difficult to stop the reaction at the alkene stage because hydrogen readily adds to alkenes in the presence of these efficient metal catalysts. The product of the hydrogenation reaction, therefore, is an alkane.

The reaction can be stopped at the alkene stage if a “poisoned” (partially deactivated) metal catalyst is used. The most commonly used partially deactivated metal catalyst is Lindlar catalyst, which is prepared by precipitating palladium on calcium carbonate and (then treats it with lead acetate and quinoline to deactivate Pd catalyst), here the product is cis isomer .

Alkynes can be converted into trans alkenes using sodium (or lithium) in liquid ammonia. The reaction stops at the alkene stage because sodium (or lithium) reacts more rapidly with triple bonds than with double bonds.

Dienes

Dienes are simply alkenes that contain two carbon-carbon double bonds. They therefore have essentially the same properties as the alkenes we have already studied. Hydrocarbons with two double bonds are called dienes, and those with three double bonds are called trienes. Tetraenes have four double bonds, and polyenes have many double bonds. Although we will be concerned mainly with the reactions of dienes, the same considerations apply to hydrocarbons that contain more than two double bonds.

55

Dienes are divided into two important classes according to the arrangement of the double bonds, Double bonds that alternate with single bonds are said to be conjugated; double bonds that are separated by more than one single bond are said to be isolated.

The third class of dienes, of increasing interest to organic chemists, contain cumulated double bonds; these compounds are known as allenes:

Relative Stabilities of DienesThe relative stabilities of substituted alkenes can be determined by their relative values -o of for catalytic hydrogenation. Remember that the least stable alkene has the greatest value -o; the least stable alkene gives off the most heat when it is hydrogenated, because it has more energy to begin with.

From the relative -o values for the three pentadienes, we can conclude that conjugated dienes are more stable than isolated dienes, which are more stable than cumulated dienes.

56

(a) Double bonds are formed by p orbital–p orbital overlap. The two p orbitals on the central carbon are perpendicular, causing allene to be a nonplanar molecule. (b) 2,3-Pentadiene has a nonsuperimposable mirror image. It is, therefore, a chiral molecule, even though it does not have an asymmetric carbon.

Nomenclature of Alkenes with More than One Functional GroupTo arrive at the systematic name of a diene, we first identify the longest continuous chain that contains both double bonds by its alkane name and then replace the “ne” ending with “diene.” The chain is numbered in the direction that gives the double bonds the lowest possible numbers. The numbers indicating the locations of the double bonds are cited either before the name of the parent compound or before the suffix. Substituents are cited in alphabetical order. Propadiene, the smallest member of the class of compounds known as allenes, is frequently called allene.

If the functional groups are a double bond and a triple bond, the chain is numbered in the direction that yields the lowest number in the name of the compound.

57

If the same low number is obtained in both directions, the chain is numbered in the direction that gives the double bond the lower number.

If the second functional group suffix has a higher priority than the alkene, the chain is numbered in the direction that assigns the lowest possible number to the functional group with the higher priority.

Preparation and properties of dienes,Dienes are usually prepared by adaptations of the methods used to make simple alkenes. For example, the most important diene, 1,3-butadiene has been made in USA country by a cracking process, and in Germany by dehydration of an alcohol containing two O-H groups:

58

Conjugated dienes differ from simple alkenes in three ways: (a) they are more stable, (b) they undergo 1,4-addition, and (c) toward free radical addition, they are more reactive.

Resonance in conjugated dienes,Let us focus our attention on the four key carbon atoms of any conjugated diene system. We ordinarily write the C1=C2 and C3=C4 bonds as double, and the C2-C3 bond as single:

This would correspond to an orbital picture of the molecule, in which -bonds are formed by overlap of the p orbitals of C1 and C2 , and overlap of the p orbitals of C3 and C4 .

There could be a certain amount of overlap between the p orbitals of C2 and C3. The resulting delocalization of the electrons makes the molecule more stable: each pair of electrons attracts and is attracted by not just two carbon nuclei, but four.

We say that a conjugated diene is a resonance hybrid.

Consistent with partial double-bond character, the C2 C3 bond in 1,3- butadiene is 1.48 Å long, as compared with 1.53 Å for a pure single bond.

59

Resonance in alkenes. Hyperconjugation,Alkenes are stabilized not only by conjugation but also by the presence of alkyl groups : the greater the number of alkyl groups attached to the doubly-bonded carbon atoms, the more stable the alkene. Stabilization by alkyl groups has been attributed to the same fundamental factor as stabilization by a second double bond: delocalization of electrons, this time through overlap between a p orbital and a orbital of the alkyl group. Delocalization of this kind, involving a bond orbitals, its called as hyperconjugation.

Hyperconjugation occurs only if the bond orbital and the empty p orbital have the proper orientation. The proper orientation is easily achieved because there is free rotation about a carbon–carbon bond

Electrophilic Addition Reactions of Isolated DienesThe reactions of isolated dienes are just like those of alkenes. If an excess of the electrophilic reagent is present, two independent addition reactions will occur, each following the rule that applies to all electrophilic addition reactions: The electrophile adds to the sp2 carbon that is bonded to the greater number of hydrogens.

60

But if there is only enough electrophilic reagent to add to one of the double bonds and addition of HCl to the double bond on the right forms a tertiary carbocation.

Electrophilic Addition Reactions of Conjugated Dienes, If a conjugated diene, such as 1,3-butadiene, reacts with a limited amount of electrophilic reagent so that addition can occur at only one of the double bonds, two addition products are formed. One is a 1,2-addition product, which is a result of addition at the 1- and 2-positions. The other is a 1,4-addition product, the result of addition at the 1- and 4-positions.

To understand why both 1,2-addition and 1,4-addition products are obtained from the reaction of a conjugated diene with a limited amount of electrophilic reagent, we must look at the mechanism of the reaction. In the first step of the addition of HBr to 1,3-butadiene, the electrophilic proton adds to C-1, forming an allylic cation.

61

Addition of HBr to 1,3-butadiene yields both the 1,2- and the 1,4-products; the proportions in which they are obtained are markedly affected by the temperature.

62

Free-radical addition to conjugated dienesLike other alkenes, conjugated dienes undergo addition not only by electrophilic reagents but also by free radicals. In free-radical addition, conjugated dienes show two special features: they undergo 1,4-addition as well as 1,2-addition, Let us take, as an example, addition of HBr to 1,3-butadiene in the presence of a peroxide (di-tert-butyl peroxide (t-Bu-O-O-t-Bu)). The peroxide decomposes (step 1) to yield a free radical, which abstracts hydrogen atom from HBr (step 2) to generate a .Br radical. The .Br radical thus formed and adds to the butadiene (step 3). Addition to one of the ends of the conjugated system is the preferred reaction, since this yields a resonance-stabilized allyl free radical. The allyl free radical then abstracts hydrogen atom from a molecule of HBr (step 4) to complete the addition, and in doing so forms a new .Br radical which can carry on the chain. In step (4) hydrogen atom can become attached to either C-2 or C-4 to yield either the 1,2- or 1,4-product.

Cycloalkanes, cyclic aliphatic (alicyclic)compounds

Cycloalkanes are alkanes with their carbon atoms arranged in a ring. Because of the ring, a cycloalkane has two fewer hydrogens than an acyclic (noncyclic) alkane with the same number of carbons. This means that the general molecular formula for a cycloalkane is CnH2n.

Nomenclature of CycloalkanesCycloalkanes are alkanes with their carbon atoms arranged in a ring. Because of the ring, a cycloalkane has two fewer hydrogens than an acyclic (noncyclic) alkane with the same number of carbons. This means that the general molecular formula CnH2n for a cycloalkane is Cycloalkanes are named by adding the prefix “cyclo” to the alkane name that signifies the number of carbon atoms in the ring.

63

Cycloalkanes are almost always written as skeletal structures. Skeletal structures show the carbon–carbon bonds as lines, but do not show the carbons or the hydrogens bonded to carbons.

Acyclic molecules can also be represented by skeletal structures. In a skeletal structure of an acyclic molecule, the carbon chains are represented by zigzag lines.

The rules for naming cycloalkanes resemble the rules for naming acyclic alkanes:

1. In the case of a cycloalkane with an attached alkyl substituent, the ring is the parent hydrocarbon unless the substituent has more carbon atoms than the ring. In that case, the substituent is the parent hydrocarbon and the ring is named as a substituent. There is no need to number the position of a single substituent on a ring.

2. If the ring has two different substituents, they are cited in alphabetical order and the number 1 position is given to the substituent cited first.

3. If there are more than two substituents on the ring, they are cited in alphabetical order. The substituent given the number 1 position is the one that results in a second substituent getting as low a number as possible. If two substituents have the same low number, the ring is numbered—either clockwise or counterclockwise—in the direction that gives the third substituent the lowest possible number.

64

Polycyclic compounds contain two or more rings that share two or more carbon atoms. We can illustrate the naming system with norbornane, whose systematic name is bicyclo[2.2.1]heptane: (a) heptane, since it contains a total of seven carbon atoms; (b) bicydo, since it contains two rings, that is, breaking two carbon-carbon bonds converts it into an open-chain compound; (c) [2.2.1], since the number of carbons between bridgeheads (shared carbons) is two (C-2 and C-3), two (C-5 and C-6), and one (C-7).

65

Industrial sourcePetroleum from certain areas, is rich in cycloalkanes, known to the petroleum industry as naphthenes. Among these are cyclohexane, methylcyclohexane, methylcyclopentane, and 1,2-dimethylcyclopentane. Addition of hydrogen to aromatic compounds yields cyclic aliphatic compounds, specifically cyclohexane derivatives. An important example of this is the hydrogenation of benzene to yield pure cyclohexane.

As we might expect, hydrogenation of substituted benzenes yields substituted cyclohexanes. For example:

66

Preparation Preparation of alicyclic hydrocarbons from other aliphatic compounds. The method applied to a dihalide can bring about coupling between two alkyl groups that are part of the same molecule:

Also there are other methods to prepare cyclic aliphatic compounds like:

- Diazomethane reacts with alkene in the presence of light:

- Diels-Alder reaction can be used to prepare cyclic aliphatic compounds:

Reactions of cycloalkanes

With certain very important and interesting exceptions, alicyclic hydrocarbons undergo the same reactions as their open-chain analogs.Cycloalkanes undergo chiefly free-radical substitution. For example:

67

Cycloalkenes undergo chiefly addition reactions, both electrophilic and free radical; like other alkenes, they can also undergo cleavage and allylic substitution. For example:

These addition reactions destroy the cycldpropane and cyclobutane ring systems, and yield open-chain products. For example:

68

Cyclobutane does not undergo most of the ring-opening reactions of cyclepropane; it is hydrogenated, but only under mote vigorous conditions than those required for cyclopropane,

Cycloalkanes: Ring Strain

In 1885, the German chemist Adolf von Baeyer proposed that the instability of three- and four-membered rings was due to angle strain. We know that, ideally, an sp3 hybridized carbon has bond angles of 109.5°. Baeyer suggested that the stability of a cycloalkane could be predicted by determining how close the bond angle of a planar cycloalkane is to the ideal tetrahedral bond angle of 109.5°. The angles in an equilateral triangle are 60°. The bond angles in cyclopropane, therefore, are compressed from the ideal bond angle of 109.5° to 60°, a 49.5° deviation. This deviation of the bond angle from the ideal bond angle causes strain called angle strain. The angle strain in a three-membered ring can be appreciated by looking at the orbitals that overlap to form the bonds in cyclopropane (Figure 2.6). Normal bonds are formed by the overlap of two sp3 orbitals that point directly at each other. In cyclopropane, overlapping orbitals cannot point directly at each other. Therefore, the orbital overlap is less effective than in a normal C-C bond. The less effective orbital overlap is what causes angle strain, which in turn causes the C-C bond to be weaker than a normal C-C bond. Because the C-C bonding orbitals in cyclopropane can’t point directly at each other, they have shapes that resemble bananas and, consequently, are often called banana bonds. In addition to possessing angle strain, three-membered rings have torsional strain because all the adjacent bonds are eclipsed.The bond angles in planar cyclobutane would have to be compressed from 109.5° to 90°, the bond angle associated with a planar four-membered ring. Planar cyclobutane would then be expected to have less angle strain than cyclopropane because the bond angles in cyclobutane are only 19.5° away from the ideal bond angle.

69

Baeyer predicted that cyclopentane would be the most stable of the cycloalkanes because its bond angles (108°) are closest to the ideal tetrahedral bond angle. He predicted that cyclohexane, with bond angles of 120°, would be less stable and that as the number of sides in the cycloalkanes increases, their stability would decrease.

Contrary to what Baeyer predicted, cyclohexane is more stable than cyclopentane. Furthermore, cyclic compounds do not become less and less stable as the number of sides increases. The mistake Baeyer made was to assume that all cyclic molecules are planar. Because three points define a plane, the carbons of cyclopropane must lie in a plane. The other cycloalkanes, however, are not planar. Cyclic compounds twist and bend in order to attain a structure that minimizes the three different kinds of strain that can destabilize a cyclic compound:

1. Angle strain is the strain induced in a molecule when the bond angles are different from the ideal tetrahedral bond angle of 109.5°.2. Torsional strain is caused by repulsion between the bonding electrons of one substituent and the bonding electrons of a nearby substituent.3. Steric strain is caused by atoms or groups of atoms approaching each other too closely.

Although planar cyclobutane would have less angle strain than cyclopropane, it could have more torsional strain because it has eight pairs of eclipsed hydrogens, compared with the six pairs of cyclopropane. So cyclobutane is not a planar molecule—it is a bent molecule. One of its methylene groups is bent at an angle of about 25° from the plane defined by the other three carbon atoms. This increases the angle strain, but the increase is more than compensated for by the decreased torsional strain as a result of the adjacent hydrogens not being as eclipsed, as they would be in a planar ring.

Conformations of CyclohexaneThe cyclic compounds most commonly found in nature contain six-membered rings because such rings can exist in a conformation that is almost completely free of strain. This conformation is called the chair conformation (Figure 2.7). In the chair conformer of cyclohexane, all the bond angles are 111°, which is very close to the ideal tetrahedral bond angle of 109.5°, and all the adjacent bonds are staggered.

70

The chair conformer is such an important conformer that you should learn how to draw it:1. Draw two parallel lines of the same length, slanted upward. Both lines shouldstart at the same height.

2. Connect the tops of the lines with a V; the left-hand side of the V should be slightly longer than the right-hand side. Connect the bottoms of the lines with an inverted V; the lines of the V and the inverted V should be parallel. This completes the framework of the six-membered ring.

3. Each carbon has an axial bond and an equatorial bond. The axial bonds are vertical and alternate above and below the ring. The axial bond on one of the uppermost carbons is up, the next is down, the next is up, and so on.

4. The equatorial bonds (balls) point outward from the ring. Because the bond angles are greater than 90°, the equatorial bonds are on a slant. If the axial bond points up, the equatorial bond on the same carbon is on a downward slant. If the axial bond points down, the equatorial bond on the same carbon is on an upward slant.

Notice that each equatorial bond is parallel to two ring bonds (two carbons over) and parallel to the opposite equatorial bond.

71

Remember that cyclohexane is viewed on edge. The lower bonds of the ring are in front and the upper bonds of the ring are in back.

Cyclohexane rapidly interconverts between two stable chair conformations because of the ease of rotation about its carbon–carbon bonds. This interconversion is known as ring flip.

Cyclohexane can also exist in a boat conformation. Like the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable as the chair conformer because some of the bonds in the boat conformer are eclipsed, giving it torsional strain. The boat conformer is further destabilized by the close proximity of the flagpole hydrogens (the hydrogens at the “bow” and “stern” of the boat), which causes steric strain.

When the carbon is pulled down to the point where it is in the same plane as the sides of the boat, the very unstable half-chair conformer is obtained. Pulling the carbon down farther produces the chair conformer. The graph in Figure 2.10 shows the energy of a cyclohexane molecule as it interconverts from one chair conformer to the other; the energy barrier for interconversion is 12.1 kcal mol (50.6 kJ mol). From this value, it can be calculated that cyclohexane undergoes 105 ring flips per second at room temperature. In other words, the two chair conformers are in rapid equilibrium.

72

Conformations of Monosubstituted CyclohexanesThe two chair conformers of a monosubstituted cyclohexane such as methylcyclohexane are not equivalent. The methyl substituent is in an equatorial position in one conformer and in an axial position in the other, because substituents that are equatorial in one chair conformer are axial in the other.

This can be best understood by examining Figure 2.12, which shows that when the methyl group is in an equatorial position, it is anti to the C-3 and C-5 carbons. Therefore, the substituent extends into space, away from the rest of the molecule.

In contrast, when the methyl group is in an axial position, it is gauche to the C-3 and C-5 carbons (Figure 2.13). As a result, there are unfavorable steric interactions between the axial methyl group and both the axial substituent on C-3 and the axial substituent on C-5 (in this case, hydrogens).

73

Because the interacting substituents are on 1,3-positions relative to each other, these unfavorable steric interactions are called 1,3-diaxial interactions.

Conformations of Disubstituted CyclohexanesLet’s start by looking at 1,4-dimethylcyclohexane. First of all, note that there are two different dimethylcyclohexanes. One has both methyl substituents on the same side of the cyclohexane ring; it is called the cis isomer. The other has the two methyl substituents on opposite sides of the ring; it is called the trans isomer. cis-1,4-Dimethylcyclohexane and trans-1,4-dimethylcyclohexane are called geometric isomers.

First we will determine which of the two chair conformers of cis-1,4-dimethylcyclohexane is more stable. One chair conformer has one methyl group in an equatorial position and one methyl group in an axial position. The other chair conformer also has one methyl group in an equatorial position and one methyl group in an axial position. Therefore, both chair conformers are equally stable.

74

In contrast, the two chair conformers of trans-1,4-dimethylcyclohexane have different stabilities because one has both methyl substituents in equatorial positions and the other has both methyl groups in axial positions.

The chair conformer with both substituents in axial positions has four 1,3-diaxial interactions, which is less stable than the chair conformer with both methyl groups in equatorial positions. We can, therefore, predict that trans-1,4-dimethylcyclohexane will exist almost entirely in the more stable diequatorial conformation.

75

Alkyl Halides Alkyl halides are organic compounds with a general formula , R-X

Nomenclature of Alkyl HalidesAlkyl halides are compounds in which a hydrogen of an alkane has been replaced by a halogen.

The common names of alkyl halides (haloalkanes) consist of the name of the alkyl group, followedby the name of the halogen—with the “ine” ending of the halogen name replaced by “ide” (i.e., fluoride, chloride, bromide, iodide).

Physical propertiesBecause of greater molecular weight, haloalkanes have considerably higher boiling points than alkanes of the same number of carbons. For a given alkyl group, the boiling point increases with increasing atomic weight of the halogen, so that a fluoride is the lowest boiling, an iodide the highest boiling. In spite of their polarity, alkyl halides are insoluble in water, probably because of their inability to form hydrogen bonds. They are soluble in the typical organic solvents. lodo, bromo, and polychloro compounds are more dense than water. (table 14.1)

76

77

Industrial sourceCertain important halides are prepared by methods similar to those used in the laboratory; thus, for vinyl chloride:

Many fluorine compounds are not prepared by direct fluorination, but rather by replacement of chlorine, using inorganic fluorides:

Preparation of alkyl halides

1. from alcohols.Alcohols react readily with hydrogen halides or phosphorous halide to yield alkyl halides.

Examples:

78

2. Halogenation of certain hydrocarbons.

3. Addition of hydrogen halides to alkenes.

4. Addition of halogens to alkenes and alkynes

79

Substitution Reactions of Alkyl Halides

Organic compounds that have an electronegative atom or group bonded to an sp3 hybridized carbon undergo substitution reactions and/or elimination reactions. In a substitution reaction, the electronegative atom or group is replaced by another atom or group. In an elimination reaction, the electronegative atom or group is eliminated, along with hydrogen from an adjacent carbon. The atom or group that is substituted or eliminated in these reactions is called a leaving group

How Alkyl Halides ReactA halogen is more electronegative than carbon. Consequently, the two atoms do not share their bonding electrons equally. Because the more electronegative halogen has a larger share of the electrons, it has a partial negative charge and the carbon to which it is bonded has a partial positive charge.

It is the polar carbon–halogen bond that causes alkyl halides to undergo substitution and elimination reactions. There are two important mechanisms for the substitution reaction:

1. A nucleophile is attracted to the partially positively charged carbon (an electrophile). As the nucleophile approaches the carbon and forms a new bond, the carbon–halogen bond breaks heterolytically (the halogen takes both of the bonding electrons).

2. The carbon–halogen bond breaks heterolytically without any assistance from the nucleophile, forming a carbocation. The carbocation—an electrophile—then reacts with the nucleophile to form the substitution product.

80

Regardless of the mechanism by which a substitution reaction occurs, it is called a nucleophilic substitution reaction because a nucleophile substitutes for the halogen. We will see that the mechanism that predominates depends on the following factors:

• the structure of the alkyl halide• the reactivity of the nucleophile• the concentration of the nucleophile• the solvent in which the reaction is carried out

The Mechanism of SN2 Reaction

SN2 means (S: substitution, N: nucleophilic, 2: bimolecular). The rate of a nucleophilic substitution reaction such as the reaction of methyl bromide with hydroxide ion depends on the concentrations of both reagents.

Because the rate of this reaction depends on the concentration of two reactants, the reaction is a second-order reaction.Because a productive collision is a collision in which the nucleophile hits the carbon on the side opposite the side bonded to the leaving group, the carbon is said to undergo back-side attack.

81

82

Factors Affecting SN2 Reactions

1- The Leaving Group

From the relative reaction rates, we can see that the iodide ion is the best leaving group and the fluoride ion is the worst. This brings us to an important rule in organic chemistry—one that you will see frequently: The weaker the basicity of a group, the better is its leaving ability.

2- The Nucleophile

When we talk about atoms or molecules that have lone-pair electrons, sometimes we call them bases and sometimes we call them nucleophiles. Basicity is a measure of how well a compound (a base) shares its lone pair with a proton. The stronger the base, the better it shares its electrons.Nucleophilicity is a measure of how readily a compound (a nucleophile) is able to attack an electron-deficient atom. Nucleophilicity is measured by a rate constant (k). When comparing molecules with the same attacking atom, there is a direct relationship between basicity and nucleophilicity: Stronger bases are better nucleophiles

83

3- The Effect of the Solvent on Nucleophilicity

A protic solvent (water, methanol), a protic solvent makes strong bases less nucleophilic.

Aprotic polar solvents, such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO). anion can be a powerful nucleophile in an aprotic polar solvent.

84

4-Nucleophilicity Is Affected by Steric Effects

A bulky nucleophile cannot approach the back side of a carbon as easily as a less sterically hindered nucleophile can. Thus, the bulky tert-butoxide ion, with its three methyl groups, is a poorer nucleophile than ethoxide ion even though tert-butoxide ion is a stronger base.

Many different kinds of nucleophiles can react with alkyl halides. Therefore, a wide variety of organic compounds can be synthesized by means of SN2 reactions.

If the difference between the basicities of the nucleophile and the leaving group is not very large, the reaction will be reversible.

85

The Mechanism of an SN1 Reaction

SN1 means (S: substitution, N: nucleophilic,1: unimolecular). Knowing that the rate of this nucleophilic substitution reaction depends only on the concentration of the alkyl halide, we can write the following rate law for the reaction:

Because the rate of the reaction depends on the concentration of only one reactant, the reaction is a first-order reaction. The reaction between tert-butylbromide and water is an SN1 reaction, where “S” stands for substitution, “N” stands for nucleophilic, and “1” stands for unimolecular. Unimolecular means that only one molecule is involved in the rate-determining step. The mechanism of an SN1reaction is based on the following experimental evidence:

1. The rate law shows that the rate of the reaction depends only on the concentration of the alkyl halide.

2. When the methyl groups of tert-butyl bromide are successively replaced by hydrogens, the rate of the SN1 reaction decreases progressively (Table 10.4).

Unlike an SN2 reaction, where the leaving group departs and the nucleophile approaches at the same time, the leaving group in an SN1 reaction departs before the nucleophile approaches. In the first step of an SN1 reaction of an alkyl halide, the carbon–halogen bond breaks heterolytically, with the halogen retaining the previously shared pair of electrons, and a carbocation intermediate is formed. In the second step, the nucleophile reacts rapidly with the carbocation to form a protonated alcohol.

86

Because the rate of an SN1 reaction depends only on the concentration of the alkyl halide, the first step must be the slow and rate-determining step. The nucleophile, therefore, is not involved in the rate-determining step.

Factors Affecting SN1 Reactions

1- The Leaving GroupBecause the rate-determining step of an reaction is the dissociation of the alkyl halide to form a carbocation, two factors affect the rate of an reaction: the ease with which the leaving group dissociates from the carbon and the stability of the carbocation that is formed.

2-The NucleophileThe nucleophile reacts with the carbocation that is formed in the rate-determining step of an reaction. Because the nucleophile comes into play after the rate-determining step, the reactivity of the nucleophile has no effect on the rate of an reaction.

3- Carbocation RearrangementsA carbocation intermediate is formed in an SN1 reaction will rearrange if it becomes more stable in the process.

87

Elimination Reactions of Alkyl Halides

In addition to undergoing the nucleophilic substitution reactions, alkyl halides undergo eliminationreactions. In an elimination reaction, groups are eliminated from a reactant. For example, when analkyl halide undergoes an elimination reaction, the halogen (X) is removed from one carbon and a proton is removed from an adjacent carbon. A double bond is formed between the two carbons from which the atoms are eliminated. Therefore, the product of an elimination reaction is an alkene.

88

The E2 Reaction

Just as there are two important nucleophilic substitution reactions SN1 and SN2, there are two important elimination reactions: E1 and E2. The reaction of tert-butyl bromide with hydroxide ion is an example of an E2 reaction; “E” stands for elimination and “2” stands for bimolecular. The product of an elimination reaction is an alkene.

The rate of an E2 reaction depends on the concentrations of both tert-butyl bromide and hydroxide ion. It is, therefore, a second-order reaction

We see that an E2 reaction is a concerted, one-step reaction: The proton and the bromide ion are removed in the same step, so no intermediate is formed. Removal of a proton and a halide ion is called dehydrohalogenation. It is also called a 1,2-elimination reaction because the atoms being removed are on adjacent carbons. The carbon to which the halogen is attached is called the -carbon. A carbon adjacent to the -carbon is called a -carbon. Because the elimination reaction is initiated by removing a proton from a -carbon, an E2 reaction is sometimes called a -elimination reaction.

In contrast, 2-bromobutane has two structurally different -carbons from which a proton can be removed. So when 2-bromobutane reacts with a base, two elimination products are formed: 2-butene

89

and 1-butene. This E2 reaction is regioselective because more of one constitutional isomer is formed than the other.

Alexander M. Zaitsev, a nineteenth-century Russian chemist, devised a shortcut to predict the more substituted alkene product. He pointed out that the more substituted alkene product is obtained when a proton is removed from the -carbon that is bonded to the fewest hydrogens. This is called Zaitsev’s rule.

If the base in an E2 reaction is sterically bulky and the approach to the alkyl halide is sterically hindered, the base will preferentially remove the most accessible hydrogen.

90

If the alkyl halide is not sterically hindered and the base is only moderately hindered, the major product will still be the more stable product.

Although the major product of an E2 dehydrohalogenation of alkyl chlorides, alkyl bromides, and alkyl iodides is normally the more substituted alkene, the major product of the E2 dehydrohalogenation of alkyl fluorides is the less substituted alkene.

Stereochemistry of the E2 Reaction

An E2 reaction involves the removal of two groups from adjacent carbons. There are two ways in which C-H and C-X bonds can be in the same plane: They can be parallel to one another either on the same side of the molecule (syn-periplanar) or on opposite sides of the molecule (anti-periplanar).

If an elimination reaction removes two substituents from the same side of the C-C bond, the reaction is called syn elimination. If the substituents are removed from opposite sides of the C-C bond, the reaction is called anti elimination. Both types of elimination can occur, but syn elimination is a much slower reaction, so anti elimination is highly favored in an E2 reaction. One reason anti elimination is favored is that syn elimination requires the molecule to be in an eclipsed conformation, whereas anti elimination requires it to be in a more stable, staggered conformation.

91

For example, the major product formed from the E2 elimination of 2-bromopentane is 2-pentene.

The 2-pentene obtained as the major product from the elimination reaction of 2-bromopentane can exist as a pair of stereoisomers, and more (E)-2-pentene is formed than (Z)-2-pentene.

The E1 Reaction

The second kind of elimination reaction that alkyl halides can undergo is an E1 elimination. The reaction of tert-butyl bromide with water to form 2-methylpropene is an example of an E1 reaction; “E” stands for elimination and “1” stands for unimolecular.

An E1 reaction is a first-order elimination reaction because the rate of the reaction depends only on the concentration of the alkyl halide.

92

We know, then, that only the alkyl halide is involved in the rate-determining step of the reaction. Therefore, there must be at least two steps in the reaction.

When two elimination products can be formed in an E1 reaction, the major product is generally the more substituted alkene.

93

In the following reaction, the initially formed secondary carbocation undergoes a 1,2-hydride shift to form a more stable secondary allylic cation:

Stereochemistry of the E1 Reaction

We have seen that an E1 elimination reaction takes place in two steps. The leaving group leaves in the first step, and a proton is lost from an adjacent carbon in the second step, following Zaitsev’s rule in order to form the more stable alkene. The carbocation formed in the first step is planar, so the electrons from a departing proton can move toward the positively charged carbon from either side. Therefore, both syn and anti elimination can occur.

94

Competition Between E2 and E1 Reactions

Primary alkyl halides undergo only E2 elimination reactions. They cannot undergo E1 reactions because of the difficulty encountered in forming primary carbocations. Secondary and tertiary alkyl halides undergo both E2 and E1 reactions (Table 11.3).

E2 reaction is favored by a high concentration of a strong base and an aprotic polar solvent (e.g., DMSO or DMF), whereas an E1 reaction is favored by a weak base and a protic polar solvent(e.g.,H2O or ROH).

E2 Elimination from Cyclic Compounds

Elimination from cyclic compounds follows the same stereochemical rules as elimination from open-chain compounds.

95

Examples:

E1 Elimination from Cyclic Compounds

When a substituted cyclohexane undergoes an E1 reaction, the two groups that are eliminated do not have to both be in axial positions, because the elimination reaction is not concerted. In the following reaction, a carbocation is formed in the first step. It then loses a proton from the adjacent carbon that is bonded to the fewest hydrogens in other words, Zaitsev’s rule is followed.

96

Alcohols

Alcohols are organic compounds with general formula, R-OH.

Nomenclature of Alcohols

97

Alcohols are compounds in which a hydrogen of an alkane has been replaced by an OH group. Alcohols are classified as primary, secondary, or tertiary, depending on whether the OH group is bonded to a primary, secondary, or tertiary carbon—the same way alkyl halides are classified.

The common name of an alcohol consists of the name of the alkyl group to which the OH group is attached, followed by the word “alcohol.”

The functional group is the center of reactivity in a molecule. In an alcohol, the OH is the functional group. The IUPAC system uses a suffix to denote certain functional groups. The systematic name of an alcohol, for example, is obtained by replacing the “e” at the end of the name of the parent hydrocarbon with the suffix “ol.”

The following rules are used to name a compound that has a functional group suffix:

1. The parent hydrocarbon is the longest continuous chain containing the functional group.

2. The parent hydrocarbon is numbered in the direction that gives the functional group suffix the lowest possible number.

3. If there is a functional group suffix and a substituent, the functional group suffix gets the lowest possible number.

98

4. If the same number for the functional group suffix is obtained in both directions, the chain is numbered in the direction that gives a substituent the lowest possible number. Notice that a number is not needed to designate the position of a functional group suffix in a cyclic compound, because it is assumed to be at the 1-position.

5. If there is more than one substituent, the substituents are cited in alphabetical order.

Physical propertiesAlcohols, in contrast, contain the very polar -OH group. In particular, this group contains hydrogen attached to the very electronegative element, oxygen, and therefore permits hydrogen bonding. The physical properties (Table 15.1) show the effects of this hydrogen bonding.

99

100

Alcohols show increase in boiling point with increasing carbon number, and decrease in boiling point with branching that because the hydrogen bonds that hold the molecules together. The solubility behavior of alcohols also reflects their ability to form hydrogen bonds. In sharp contrast to hydrocarbons, the lower alcohols are miscible with water and solubility changes with increase the number of carbons.

Industrial source

(a) Hydration of alkenes. Alkenes containing four or five carbon atoms can be separated from the mixture obtained from the cracking of petroleum. Alkenes are readily converted into alcohols either by direct addition of water, or by addition of sulfuric acid followed by hydrolysis.

(b) Fermentation of carbohydrates. Fermentation of sugars by yeast, the oldest synthetic chemical process used by man, is still of enormous importance for the preparation of ethyl alcohol and certain other alcohols.

Preparation of alcoholsMost of the simple alcohols and a few of the complicated ones are available from the industrial sources. Other alcohols must be prepared by one of the methods outlined below.

1. Oxymercuration-demercuration.

The first stage, oxymercuration, involves addition to the carbon-carbon double bond of OH and HgOAc. Then, in demercuration, the HgOAc (CH3COOHg) is replaced by H via sodium borohydride, NaBH4. The reaction sequence amounts to hydration of the alkene, but is much more widely applicable than direct hydration.

101

2- Hydroboration-oxidation

With the reagent diborane, (BH3)2 , alkenes undergo hydroboration to yield alkylboranes, R3B, which on oxidation give alcohols. For example:

Hydroboration involves addition to the double bond of BH3 (or, in following stages, BH2R and BHR2), with hydrogen becoming attached to one doubly-bonded carbon, and boron to the other. The alkylborane can then undergo oxidation.

Examples:

102

3- Grignard synthesis of alcohols,The Grignard reagent, we recall, has the formula RMgX, and is prepared by the reaction of metallic magnesium with the appropriate organic halide. This halide can be alkyl (1, 2, 3), allylic, aralkyl (e.g., benzyl), or aryl (phenyl or substituted phenyl). The halogen may be -Cl, -Br or -I. (Arylmagnesium chlorides must be made in the cyclic ether tetrahydrofuran instead of ethyl ether.)

One of the most important uses of the Grignard reagent is its reaction with aldehydes and ketones to yield alcohols. Aldehydes and ketones have the general formulas:

The carbon-magnesium bond of the Grignard reagent is a highly polar bond, carbon being negative relative to electropositive magnesium. It is not surprising, then, that in the addition to carbonyl compounds, the organic group becomes attached to carbon and magnesium to oxygen.

103

Examples:

A related synthesis utilizes ethylene oxide to make primary alcohols containing two more carbons than the Grignard reagent.

104

Reactions of alcoholsWe can see that alcohols undergo many kinds of reactions, to yield many kinds of products.

1- Dehydration to yield alkenes

The mechanism of dehydration involves (1) formation of the protonated alcohol, ROH2+, (2) its slow

dissociation into a carbonium ion, and (3) fast expulsion of a hydrogen ion from the carbonium ion to form an alkene.

**The stability and hence rate of formation of the simple alkyl cations follows the sequence 3o > 2o > 1o

and also a carbonium ion can rearrange, and that this rearrangement seems to occur whenever a 1,2-shift of hydrogen or alkyl group can form a more stable carbonium ion.

2- Reaction with hydrogen halides

Alcohols react readily with hydrogen halides to yield alkyl halides. The least reactive of the hydrogen halides, HC1, requires the presence of zinc chloride for reaction with primary and secondary alcohols; on the other hand, the very reactive tert-butyl alcohol is converted to the chloride by simply being shaken with concentrated hydrochloric acid at room temperature. For example:

105

Alcohols react with hydrogen halides to follow the mechanism of nucleophilic substitution reaction (SN1 or SN2):

Mechanism SN1:

Mechanism SN2:

3- Alcohols as acids

The acidity of alcohols is shown by their reaction with active metals to form hydrogen gas, and by their ability to displace the weakly acidic hydrocarbons from their salts (e.g., Grignard reagents):

With the possible exception of methanol, they are weaker acids than water, but stronger acids than acetylene or ammonia:

106

As before, these relative acidities are determined by displacement. We may expand our series of acidities and basicities, then, to the following:

4- Oxidation of alcohols

Primary alcohols can be oxidized to carboxylic acids, RCOOH, usually by heating with aqueous KMnO4.

Primary alcohols can be oxidized to aldehydes, RCHO, by the use of K2Cr2O7. Aldehydes are themselves readily oxidized to acids, the aldehyde must be removed from the reaction mixture by special techniques before it is oxidized further.

Secondary alcohols are oxidized to ketones, R2CO, by chromic acid in a form selected for the job at hand: aqueous K2Cr2O7, CrO3 in glacial acetic acid, CrO3 in pyridine, etc. Hot permanganate also oxidizes secondary alcohols.

*With no hydrogen attached to the carbinol carbon, tertiary alcohols are not oxidized at all under alkaline conditions.

107

The oxidation mechanism of alcohol and the effect of hydrogen attached to the carbinol carbon,

chromic acid oxidation involves initial formation of an alkyl chromate:

This alkyl chromate then undergoes an elimination reaction to form the carbon–oxygen double bond.

When a primary alcohol is oxidized to a carboxylic acid, the alcohol is initially oxidized to an aldehyde, which is in equilibrium with its hydrate. It is the hydrate that is subsequently oxidized to a carboxylic acid.

The oxidation reaction can be stopped at the aldehyde if the reaction is carried out with pyridinium chlorochromate (PCC), because PCC is used in an anhydrous solvent. If water is not present, the hydrate cannot be formed.

- Primary alcohol + PCC aldehyde- Sec. alcohol + PCC ketone

108

Ethers

Ethers are organic compounds with general formula R-O-R or Ar-O-Ar. If the two groups are identical, the ether is said to be symmetrical (e.g., ethyl ether, phenyl ether), if different, unsymmetrical (e.g., methyl tert-butyl ether, anisole). There are also cyclic ethers:

109

Nomenclature of Ethers

Ethers are compounds in which an oxygen is bonded to two alkyl substituents. If the alkyl substituents are identical, the ether is a symmetrical ether. If the substituents are different, the ether is an unsymmetrical ether.

The common name of an ether consists of the names of the two alkyl substituents (in alphabetical order), followed by the word “ether.” The smallest ethers are almost always named by their common names.

The IUPAC system names an ether as an alkane with an RO substituent. The substituents are named by replacing the “yl” ending in the name of the alkyl substituent with “oxy.”

110

Physical properties of ethersSince the C-O-C bond angle is not 180, the dipole moments of the two C-O bonds do not cancel each other; consequently, ethers possess a small net dipole moment (e.g., 1.18 D for ethyl ether).

This weak polarity does not appreciably affect the boiling points of ethers, which are about the same as those of alkanes having comparable molecular weights, and much lower than those of isomeric alcohols. On the other hand, ethers show solubility in water comparable to that of the alcohols, both ethyl ether and w-butyl alcohol, for example, being soluble to the extent of about 8 g per 100 g of water. We attributed the water solubility of the lower alcohols to hydrogen bonding between water molecules and alcohol molecules; presumably the water solubility of ether arises in the same way.

Industrial sources of ethers. Dehydration of alcohols

A number of symmetrical ethers containing the lower alkyl groups are prepared on a large scale, chiefly for use as solvents. The most important of these is diethyl ether. These ethers are prepared by reactions of the corresponding alcohols with sulfuric acid. Since a molecule of water is lost for every pair of alcohol molecules, the reaction is a kind of dehydration.

Diethyl ether is prepared by heating a mixture of ethyl alcohol and concentrated sulfuric acid to 140oC, alcohol being continuously added to keep it in excess.

111

Preparation of ethers

The following methods are generally used for the laboratory preparation of ethers.

1. Williamson synthesis.

In the laboratory, the Williamson synthesis of ethers is important because of its versatility: it can be used to make unsymmetrical ethers as well as symmetrical ethers, and aryl alkyl ethers as well as dialkyl ethers.

For the preparation of methyl aryl ethers, methyl sulfate, (CH3)2SO4 , is frequently used instead of the more expensive methyl halides. (R-X usually uses as CH3-X or primary)

Examples:

In the preparation of ethyl tert-butyl ether, for example, the following combinations are conceivable:

112

The explanation:

2- Preparation of ethers. Alkoxymercuration-demercuration

Alkenes react with mercuric trifluoroacetate in the presence of an alcohol to give alkoxymercurial compounds which on reduction by sodium borohydride (NaBH4) yield ethers.

Examples:

Reactions of ethers. Reactions of ethers. Cleavage by acids

Ethers are comparatively unreactive compounds. The ether linkage is quite stable toward bases, oxidizing agents, and reducing agents, in so far as the ether linkage itself is concerned; ethers undergo just one kind of reaction, cleavage by acids:

113

Cleavage takes place only under quite vigorous conditions: concentrated acids (usually HI or HBr) and high temperatures. For example:

Cleavage involves nucleophilic attack by halide ion on the protonated ether, with displacement of the weakly basic alcohol molecule:

If SN1 reaction:

If SN2 reaction:

114

EpoxidesEpoxides are compounds containing the three-membered ring:

Ethers in which the oxygen atom is incorporated into a three-membered ring are called epoxides or oxiranes. The common name of an epoxide uses the common name of the alkene, followed by “oxide,” assuming that the oxygen atom is where the bond of an alkene would be. The simplest epoxide is ethylene oxide.

There are two systematic ways to name epoxides. One method calls the three membered oxygen-containing ring “oxirane,” with oxygen occupying the 1-position of the ring. Thus, 2-ethyloxirane has an ethyl substituent at the 2-position of the oxirane ring. Alternatively, an epoxide can be named as an alkane, with an “epoxy” prefix that identifies the carbons to which the oxygen is attached.

They are ethers, but the three-membered ring gives them unusual properties. By far the most important epoxide is the simplest one, ethylene oxide. It is prepared on an industrial scale by catalytic oxidation of ethylene by air.

Other epoxides are prepared by the following methods.

1- From halohydrins.

The conversion of halohydrins into epoxides by the action of base is simply because a cyclic compound is obtained via both alcohol and halide happen to be part of the same molecule. In the presence of

115

hydroxide ion a small proportion of the alcohol exists as alkoxide; this alkoxide displaces halide ion from another portion of the same molecule to yield the cyclic ether.

2. Peroxidation of carbon-carbon double bonds.

The carbon-carbon double bond may be oxidized directly to the epoxide group by peroxybenzoic acid :

116

Epoxidation of alkenes with peroxy acids is a syn addition to the double bond. Substituents that are cis to each other in the alkene remain cis in the epoxide; substituents that are trans in the alkene remain trans in the epoxide.

Reactions of epoxidesEpoxides owe their importance to their high reactivity, which is due to the ease of opening of the highly strained three-membered ring.

1- Acid-catalyzed cleavage of epoxides.

Like other ethers, an epoxide is converted by acid (any acid) into the protonated epoxide, which can then undergo attack by any. of a number of-nucleophilic reagents. An important feature of the reactions of epoxides is the formation of compounds that contain two functional groups. Thus, reaction with water yields a glycol ; reaction with an alcohol yields a compound that is both ether and alcohol.

117

If different substituents are attached to the two carbons of the protonated epoxide (and the nucleophile is something other than (H2O), the product obtained from nucleophilic attack on the 2-position of the oxirane ring will be different from that obtained from nucleophilic attack on the 3-position. The major product is the one resulting from nucleophilic attack on the more substituted carbon.

The explanation:

The best way to describe the reaction is to say that it occurs by a pathway that is Partially SN1 and partially SN2. It is not a pure SN1 reaction because a carbocation intermediate is not fully formed; it is not a pure SN2 reaction because the leaving group begins to depart before the compound is attacked by the nucleophile.

118

2- Basic-catalyzed cleavage of epoxides. When a nucleophile attacks an unprotonated epoxide, the reaction is a pure SN2 reaction. That is, the C-O bond does not begin to break until the carbon is attacked by the nucleophile. In this case, the nucleophile is more likely to attack the less substituted carbon because the less substituted carbon is more accessible to attack. (It is less sterically hindered.) Thus, the site of nucleophilic attack on an unsymmetrical epoxide under neutral or basic conditions (when the epoxide is not protonated) is different from the site of nucleophilic attack under acidic conditions (when the epoxide is protonated).

After the nucleophile has attacked the epoxide, the alkoxide ion can pick up a proton from the solvent or from an acid added after the reaction is over.

3- Reaction of ethylene oxide with Grignard reagents

Reaction of Grignard reagents with ethylene oxide is an important method of preparing primary alcohols.

119

Example:

MgBr

+ CH3 CH CH2

O

1)Et O2

2) H3O+CH2 CH CH3

OH

Thiols and Thioethers (Reference: R. J. Cremlyn “An Introduction to Organosulfur Chemistry” John Wiley and Sons: Chichester (1996)).

Thiols (R-SH)are sulfur analogs of alcohols. Thiols are named by adding the suffix “thiol” to the name of the parent hydrocarbon. If there is a second functional group in the molecule, the SH group can beindicated by its substituent name, “mercapto.”

The S-H bond is less polar than the O-H bond, and hydrogen bonding in thiols is much weaker than that of alcohols. Thus, methanethiol (CH3SH) is a gas at room temperature (bp 6°C), and methanol (CH3OH) is a liquid (bp 65°C).

The preparation of thiols involves nucleophilic substitution of the SN2 type on alkyl halides and uses the reagent thiourea as the source of sulfur.

And also, the direct reaction of a halogenoalkane with sodium hydrosulfide is generally inefficient owing to the competing formation of thioethers(sulfide):

CH3CH2Br + NaSH → CH3CH2SH + NaBr

CH3CH2Br + CH3CH2SH → (CH3CH2)2S + HBr

120

Reactions of thiols

1- The S-H bond is less polar than the O-H bond, and hydrogen bonding in thiols is much weaker than that of alcohols, Thiols are weak acids, but are far more acidic than alcohols. We have seen that most alcohols have Ka values in the range 10-16 to 10-19 (pKa = 16 to 19). The corresponding values for thiols are about Ka = 10-10 (pKa = 10).

2- S-alkylationThiols, or more particularly their conjugate bases, are readily alkylated to give thioethers:

RSH + R'Br + base → RSR' + HBr

3- Thiols, especially in the presence of base, are readily oxidized by reagents such as iodine to give an organic disulfide (R-S-S-R).

2 R-SH + I2 → R-S-S-R + 2 HI

Dimethyl disulfide can be produced by the oxidation of methanethiol, e.g. with iodine:

2 CH3SH + I2 → CH3SSCH3 + 2 HI

Thioethers (dialkylsulphide)

Thioethers (R-S-R) (analogs of ethers)are sometimes called sulfides, especially in the older literature and this term remains in use for the names of specific thioethers. The two organic substituents are indicated by the prefixes. (CH3)2S is called dimethylsulfide. Some thioethers are named by modifying the common name for the corresponding ether. For example, C6H5SCH3 is methyl phenyl sulfide. Thioethers are characterized by their strong odors, which are similar to thiol odor.

PreparationThioethers are typically prepared by the alkylation of thiols in presence of base:R-Br + HS-R' → R-S-R' + HBr

121

Reactions

1- OxidationEthers are non-oxidizeable, thioethers can be easily oxidized to the sulfoxides (R-S(=O)-R), which can themselves be further oxidized to sulfones (R-S(=O)2-R). Hydrogen peroxide is a typical oxidant. For example, dimethyl sulfide can be oxidized as follows:

S(CH3)2 + H2O2 → O=S(CH3)2 + H2O

O=S(CH3)2 + H2O2 → O2S(CH3)2 + H2O

2- HydrogenolysisThioethers undergo hydrogenolysis in the presence of certain metals Raney nickel (nickel-aluminium alloy,):

R-S-R' + 2 H2 → RH + R'H + H2SExample:(C2H5)2S + H2 C2H6 + H2S

122