ORGANIC CHEMISTRY

Hydrocarbons: Alkenes, Cycloalkenes, Dienes and Alkynes

Dr. Geetu Gambhir E-340, Greater Kailash II

New Delhi -110048

(31.07.2006)

CONTENTS Alkenes

StructureNomenclature of alkenesMethods of preparation

Dehydration of AlcoholsPropertiesRegioselectivityPolymerization of AlkenesIndustrial Application of ethylene and Propene

Cycloalkenes Nomenclature

ConformationsReactions

DienesTypes of DienesNomenclature

AllenesMethods of PreparationReactions

1, 3 – ButadieneMethods of preparationReactions

AlkynesStructureNomenclatureMethods of preparationPropertiesAcidity of terminal hydrogenReactions

1

Alkenes Alkenes are the unsaturated hydrocarbons having one or more carbon – carbon double bond. These are also termed as olefins since ethene (or ethylene – common name), the simplest alkene forms an oily liquid when treated with chlorine. General formula: CnH2n

Structure The double bond geometry of alkenes is typical of that found in ethene. Each of the double bonded carbon atom is Sp2 hybridized, molecular geometry of which requires it to have trigonal planar geometry i.e. all the atoms surrounding each carbon atom lie in the same plane with bond angles approximating 120°. Of the three hybrid orbitals formed by Sp2 hybridization two overlap with s orbital of each hydrogen forming sigma bond. The third hybrid orbital overlapps with the Sp2 hybrid orbital of adjacent carbon atom forming another sigma bond. Pure p orbital on each of the two adjacent carbon atoms undergoes sideways overlap to form a pi bond. As a result of the double bond the carbon bond length is shorter for alkenes in comparison to the alkanes (also because in Sp2 hybrid orbitals there is large amount of s character (33%)and therefore density with in Sp2 orbitals is concentrated closer to the nucleus). e.g. C2 H2

Nomenclature of alkenes Nomenclature of alkenes can be derived by simply modifying the alkane nomeclature.

1. An unbranched alkene is named by replacing the “ane” suffix in the corresponding alkane with “ene”.

2. Carbon atoms are numbered from one end of the chain to other so that the double bond receives the lowest number.

3. For the branched alkenes, the principal chain is defined as the carbon chain containing the greatest number of double bonds even if it is not the longest (if more than one chain with equal numbers of double bonds then the longer one is the principal chain).

4. The principal chain is numbered from the end that results in the lowest numbers for the carbon of the double bond.

5. Alkene containing the alkyl substituent the position of the double bond and not the position of branch determines the numbering of the chain.

6. Position of the double bond is cited in the name after the name of the alkyl group.

7. If the compound contains more than one double bond, the “ane” ending of the corresponding alkane is replaced by “adiene” or “atriene” and so on for two three or more double bonds.

2

Example:

Isomerism Isomeric alkenes that differ in position of the double bond are the constitutional isomers. Alkenes with identical connectivities that differ in the spatial arrangement of their atoms are called stereoisomers. For any alkene with two different groups around the double bonded carbon atom, interchanging the two groups at either carbon of the double bond gives different molecules hence stereoisomers (The two can not be interchanged by simple rotation as any such rotation would break the pi bond). For example:

Methods of preparation 1. Partial reduction of alkynes: to form alkenes can be brought about by number of reducing

agents like Na/liq NH3, hydrogen in presence of palladium poisned with BaSO4 or CaCO3 along with quinoline (Lindlar catalyst) , Hydrogen in presence of nickel boride.

3

2. Dehydrohalogenation of alkyl halides: by action of strong base like ethanolic potassium hydroxide. It proceeds by elimination of hydrogen halide leading to formation of alkene.

4

Mechanism • The reaction is referred as 1,2 elimation or β-elimation • It is a base catalyzed reaction • Hydrogen is abstracted by a base as a proton leaving behind it’s electron pair and the

leaving group (halogen ) leaves the substrate molecule (alkylhalide) as halide ion taking its electron pair along with it. An extra electron pair on the carbon atom is responsible for the pi bond between the carbon atoms.

• It is a single step bimolecular elimination reaction. • Rate determining step involves cleavage of C - H and C - L bonds. • The energy required to break these bonds come from the energy released due to

formation of B-H bond and pi bond • The cleavage of C-X bond in rate determine step implies the reactivity of alkyl halides to

follow the order R-1>R-Br>RCl which matched the bond dissociation energy of C-X bond.

• A good leaving group is a weakly basic anion or molecule. • The reaction is not accompanied by rearrangement, which favours the given second order

kinetics. • The reaction proceeding by E1 mechanism or first order kinetics follows the mechanism

where substrate undergoes slow heterolysis to form halide ion and a carbocation. In the second step carbocation rapidly loses a proton to a base and forms the alkene.

The carbocation can very well combine with a nucleophile or undergo rearrangement. Example

• Ease of dehydrohalogenation of alkyl halides is 3° > 2° >1°.

• If the alkyl halide has two or more β carbon atoms then two or more alkenes are possible as products. It is the stability of the alkenes that decides the chief product. According to Saytzeff Rule dehydrohalogation of alkyl halides leads to the formation of that alkene which has maximum number of alkyl groups attached to >C = C< (more substituted alkene is more stable)

5

• If size of the base is increased it is easier to abstract proton from a less substituted β carbon atom of alkyl halide. Therefore less stable alkene is the chief product. This is known as Hoffmann rule

• Saytzeff product is the major product for alkyl iodides, alkyl bromides and alkyl chlorides.

• Hoffman product is the major product for alkyl flourides.

Reason : Increases in carbon - halogen bond dissocation energy makes difficult for leaving group to leave as Xθ. As a result reaction follows the alternate mechanism called elimination from conjugate base. Here the base first removes proton from the β carbon atom forming carbanion as the intermediate. The carbanion then attacks the α-carbon atom causing the removal of F.

Dehydration of Alcohols Alcohols when treated in presence of H2SO4, P2O5, BF3 or by passing the vapour of alcohol over a catalyst commonly alumina (Al2O3) at high temperature undergo a loss of water molecule with the formation of alkenes.

• Mechanism:

Ist step

-OH group of alcohol is protonated in a fast reversible reaction (Acid transforms the poor leaving group (-OH) (strongly basic) into a good leaving group O+H2 (weakly basic water molecule).

IInd step Water molecule is lost with the formation of carbonium ion in the rate determining step.

6

IIIrd Step Carbonium ion loses proton from its adjacent carbon atom to form stable alkene. The anion of the acid or another alcohol molecule functions as a base and facilitates the loss of proton.

• The ease of dehydration of alcohols 3° > 2° > 1°.

Primary alcohols require strong conditions of conc. H2SO4 and high temperature (180° – 200°) while secondary alcohols dehydrate under milder condition with 85% H3PO4 at 160°C or 60% H2SO4 at 100°C. Tertiary alconds can be easily dehydrated by 20% H2SO4 at 85°-90°C

Reason: • Alchol reactivity parallels carbocation stability 3° > 2° > 1°. • More stable intermediate (carbocation) implies a lower activation energy (Ea).

This can be understood from the Hammonds postulate that for the endothermic conversions (as dissociation of alkyloxinium ion involves bond breaking without any bond making to compensate for energy required) as shown in the figure, the transition state resembles the high energy intermediate or product and tracks the energy of this intermediate if it changes. The change in transition state energy and activation energy is observed as the stability of intermediate (carbocation changes)

Since the activation energy path to the formation of more stable carbocation (3° > 2° > 1°) is the lowest, hence the reactivity of the alcohol also follows the similar order. • When more than one alkene can be formed the preferred product is more stable one.

7

• When carbonium in is the intermediate and structure permits rearrangement in which an atom or a cyclic ring expands so that more stable carbonium ion is formed, the rearrangement tends to take place example.

• Alkyl sulphonates undergo base promoted elimination closely analogues to dehydrohalogenation.

• The leaving group (sulphonyl group) is a weak base.

4. Dehalogenation of vicinal dihalides (where two halogen atoms are attached to the adjacent carbon atom)

• The two halogen atoms are lost in two steps and the two align themselves at 180 and in

the same plane before they are lost . NaI and Acetone

5. Cleavage of ethers: Treatment with strong bases such as sodamide or alkyl lithium or alkyl sodium, alkenes are generated

Mechanism: Reaction takes place through cyclic intermediate

8

• Reaction is aided by e- withdrawing groups at β position

6. Pyrolysis of esters Thermal cleavage of an ester usually acetate, involves the formation of cyclic transition state leading to the elimination of an acid leaving behind alkene as a product.

• This is cis elimination where both the leaving groups proton and carboxylate ion are in the cis position.

7. Hoffmann Degradation method: by heating quartenary ammonium hydroxide under

reduced pressure and at a temperature between 100ºC and 200ºC

• OHθ ion removes proton from β -carbon atom which gives more stable carbanion.

8. Wittig Reaction :Aldehydes and ketones are converted to alkenes by using special class of compounds called phosphorus ylides or wittig regent. The alkyl halide is treated with triphenyl phospine to produce phosphonium halide. A strong base like C6H5Li or n-C4H9Li converts it to phosphorane, which is stabilized by resonance.

Phosphorane reacts with aldehyde or ketone to produce alkene via a cyclic intermediate.

9. By cracking of petroleum number of alkenes like ethylene propene or butene can be

prepared. Properties Physical properties of Alkenes

• Like alkanes, alkenes are inflammable, non-polar compounds, less dense than and insoluble in water but soluble in nonpolar solvents like benzene petroleum ether etc.

9

• Lower molecular weight alkenes (C2-C4) are gases

• Sp2 hybridized carbon atom is more electronegative than Sp3 carbon atom so Sp2-Sp3 carbon - carbon bond has a small dipole directed towards Sp2 carbon atom.

Dipole moment of the compound is vector sum all the bond dipoles. This is why is cis 2-butene has a net dipole moment, while the dipole moment of trans 2-butane is zero.

• Boiling points increases (i) With increasing carbon content (ii) With decrease of branching

• cis alkenes boil at some what higher temperature than trans alkene due to its higher dipole moment.

• cis alkene have poor symmetry and do not fit in to crystalline lattice as compared to trans isomer therefore cis alkene has low melting point.

• Thermal stability of alkenes can be compared by determination of their heats of combustion. Lower is the – ∆H value of combustion, higher is the stability. In general order of stability is.

• More substituted alkenes are more stable.

Chemical properties • The most characteristic type of alkene reaction is electrophillic addition at carbon-carbon

double bond. The pi bond of the alkene and X-Y bond of the reagent are broken and new C-X and C-Y bonds are formed

• Majority of these reactions are exothermic due to the fact that the C-C pi bond is weak

relative to the sigma bonds formed to the atoms or groups of the reagent. Consequently bond energies of product molecules are greater than the bond energies of the reactants.

1) Hydrogenation ; Hydrogen adds to the double bond under pressure and in presence of

catalyst to form alkenes.

Heterogeneous catalyst: Catalyst such as finally divided Platinum or palladium black or nickel are useful as hydrogenation catalyst. They are used in conjunction with solid support materials such as alumnia (Al2O3), BaSO4 or activated charcoal.

• These are heterogeneous catalyst as they are insoluble in reaction solution • These noble metal catalysts can be filtered and reused.

10

Mechanism 1. Hydrogen and alkene are adsorbed on the surface of the catalyst 2. This chemisorption converts hydrogen molecule into hydrogen atoms and breaks weak pi

bond of the alkene 3. Activation energy of the hydrogen is lowered due to this action of the catalyst 4. Stereo selective syn addition (i.e. from same side of the double bonds) of the hydrogen

atoms takes place.

Homogenous catalysis: Homogenous catalyst is soluble in a reaction solution. Example:- Wilkinson catalyst [RhCl(Ph3P)3]

Mechanism 1. H2 adds to Rhodium complex and one of the Ph3P is lost 2. Oxidation state of Rhodium changes from +1 to +3 3. Coordination number of complex increases from 4 to 5 4. Alkene attacks Rhodium forming a pi complex 5. One of the hydrogen is then transferred to one of carbons of the double bond and other

carbon forms the sigma bond with Rhodium 6. Second hydrogen atom is transferred to the other carbon and the alkane is lost with the

regeneration of catalyst.

2. Addition of hydrogen halide: Hydrogen halides undergo addition on the double bond of alkenes to give alkyl halides

• Use of HF for making alkyl fluorides is avoided as HF is extremely hazardous. • Reaction is facilitated in moderately polar solvent acetic acid which will dissolve both

polar hydrogen halide and the non polar alkene

11

Regioselectivity Addition of H-X to alkenes is highly regioselctive because addition of the reagent across the double bond gives more than one product and only one of these constitutionally isomeric product is formed preferentially. Selectivity of this type is called regioselectivity.

• When number (not necessarily the nature) of alkyl substituents on the double bonded carbon atoms are same almost no regioselectivity is observed even if the alkyl group are of different size.

(Nearly equal amounts )

• Addition of halogen acid to the carbon – carbon double bond of an unsymmetrical alkenes, follows the Markovnikov’s rule which states, “the halogen of hydrogen halide attaches itself to the carbon of the alkene bearing the lesser number of hydrogens and greater number of carbons or negative part of the unsymmetrical reagent goes to that carbon atom which bears less number of hydrogen atoms.

Reason As seen from the mechanism, pi bonds of the alkene double bond serves as a base and are attracted to the proton of a bronstead acid, generating a carbocation intermediate, which then combines with anionic conjugate base. Reaction progress can be studied by the given energy diagram for addition of hydrogen chloride to propane. The carbocation intermediate formed in the first step of addition reaction directly influences the activation energy for this step. A more stable carbocation intermediate is formed faster than the less stable alternative because the activation energy of the path to the former is lower of the two possibilities. The more stable secondary carbocation is formed preferentially which rapidly bonds to conjugate base of bronstead acid (Cl⎯ ) to form the final product.

• But in those cases where the rearrangement occurs the overall addition of HX to alkenes does not follow the markovnikov’s rule

12

• Rearrangement is favoured by increased stability of rearranged ion

• The carbocation that is formed in this first steps of addition is Sp2 hydridized and trigonal planar and is achiral. Therefore attack of the reagent on the flat carbocation leads to the formation of two enantiomers in equal amounts. Hence product of the reaction is a racemic mixture.

Addition of hydrogen bromide in presence of peroxides Addition of HBr in presence of peroxide, leading to reversal of orientation of addition i.e. anti markovnikov addition is generally known as peroxide effect or Kharasch effect. • The difference in addition is due to the fact that in markovnikov addition the reaction is

initiated by H+ ions in the polar medium and proceeds via the formation of the most stable carbonium ion while in the presence of peroxides (which are radical initiators) the reaction proceeds by the formation of Br, and stability of the radical is the driving force for the formation of the product.

Mechanism

Chain initiation

Since O – O bond is weak so homolytic cleavage of O – O is very favoured.

Chain Propagation

Only HBr (out of four halogen acids HF, HCl, HBr, HF) give the anitmarkovnikov’s addition product in presence of peroxides because of the mechanism which involves two steps;

• It is only for the HBr that both the steps are exothermic.

• For HF: Due to high strengh of H-F, step II is endothermic.

13

• HCl: Step II is endothermic (not to extent of H-F)

• HI: Step I is endothermic as energy released by C-I bond forming is not sufficient to compensate for the energy lost in breaking C=C.

3. Hydration of alkene: Addition of water takes place on the double bond of an alkene in presence of moderatly concentrated strong acids like H2SO4, HClO4 and HNO3.

• Hydration is acid catalysed reaction.

• Catalysing acid is soluble in the reaction solution therefore, it is a homogenous catalyst.

• The reaction is regioselective and addition on the double bond follows the Markovnikov’s rule in those cases where rearrangement is not involved.

• Some alkenes give rearranged hydration product which gives the evidence for the presence of carbocation intermediate in the mechanism.

• Acids that have weakly nucleophillic anions like (HSO4-) are chosen as catalyst so that

these anions offer little competition to the actual nucleophile H2O.

Mechanism Ist Step: It is a slow rate-limiting step where the double bond is protonated to give carbocation.

IInd Step: The carbocation is attacked by lewis base, water

IIIrd Step: Proton is lost to the solvent to give alcohol regenerating the catalyzing acid H3O⊕.

Formation of the stable carbocation via low activation energy pathway (as explained earlier in addition of halogen acids) is the governing factor for the product formed where the rearrangement is possible. The occurrence of carbocation rearrangement limits the utility of alkene hydration as laboratory method of preparing alcohols.

14

4. Addition of Halogens: Alkenes react rapidly with halogens at room temperature and in absence of light to form vicinal dihalides.

• Br2 and Cl2 are used most frequently for addition on double bonds. Flourine is so reactive

that it not only adds to the double bond but also rapidly replaces all the hydrogens with fluorine’s. Iodine adds to alkene at low temperature but most di-iodies are unstable and decompose to the corresponding alkene and I2 at room temperature.

• Inert solvents like CCl4 and CH2Cl2 are used because these solvents dissolve both halogens and alkene.

• The addition of Br2 to alkene is so fast that when bromine is added to alkene the reddish brown colours of bromine discharges instantly as long as alkene is present in excess. This serves as classical test for detection of unsaturation in a compound.

Example: Bromination.

Mechanism: 1. Involves a reactive intermediate called bromonium ion, formed as shown in the single

step as follows:

Formation of cyclic bromonium ion as intermediate is possible because bromine is of large size having lone pairs to be bonded to both the carbon atoms simultaneously.

2. In the second step the cyclic transition state is attacked from the opposite side (anti addtion) by the nucleophilie (BrΘ) where by the three membered ring opens up (as +vly charged Br is very electronegative and readily accepts and electron pair) giving trans vicinal dibromide as a product.

• Overall addition of halogen to the alkene follows trans stereoselctivity.

• The reaction is stereospecific as particular stereoismeric form of starting material gives a specific stereoisomeric form of product, for example.

• Evidence for the formation of cyclic transition state, is the absence of any rearranged

products.

Halohydrin: Halogenation of alkene in aqueous solution rather than CCl4, the major product formed is halo alcohol called as halohydrin.

15

• Apart from Brθ there is another nucleophile (solvent itself) is available to attack the bromonium ion.

It is a net addition of hypohalous acid to the alkene double bond, e.g. HO-Br hypobromus acid.

• In case of unsymmetrical alkenes addition of the nucleophile takes place on more substituted carbon atom.

For example:

Oxymercuration and Demercuration • The reaction sequence amount to hydration of alkene • In the overall reaction water is added to the alkene double bond by syn addition

according to markovnikov’s rule.

• The reaction does not show any rearrangement (no formation of carbocation intermediate).

Oxymercuration: Alkenes react with mercuric acetate in aqueous solution to give addition product in which acetoxymercuri group and hydroxy group derived from water, add to the double bond.

Mechanism: Electrophile of Hg(OAc)2 is Hg⊕ OAc that adds to double bond to form mercurinium ion. This ion then reacts with nucleophile (H2O) at more branched carbon atom.

Demercuration: In this step HgOAc is replaced by H. It is a reduction process where the above product is converted into alcohol by treatment with reducing agent, sodium borohydride (NaBH4) in presence of aq NaOH.

16

• The oxymercuration adducts are not isolated but treated directly with basis slution of

NaBH4 in same reaction vessel. Example.

Hydroboration – oxidation • The overall reaction amounts to syn addition of H2O on alkene double bond using

antimarkovnikov’s rule. • The reaction is free of rearrangement, as it does not involve carbocation intermediate.

Mechanism 1. Borane adds regioselectively to alkenes so that boron (electrondeficent) becomes bonded

to less branched carbon of double bond (polarization of double bond takes place such that the more substituted carbon gets the positive polarity).

2. Because borane has three B-H bonds so each borane can add to three molecules to yield trialkyl borane (R3B).

3. The organoborane is then converted into alcohol with hydrogen perxodie H2O2 and

aqueous NaOH.

Ozonolysis

• The reaction of an alkene with ozone to yield products of double bond cleavage is called ozonolysis.

• It is method to locate the position of a double bond in an alkene.

17

Mechanism

• In reductive ozonolysis zinc is added which reduces H2O2 to H2O and thus H2O2 is not present to oxidize the aldehyde found.

• Net transformation resulting from the ozonolysis of alkene followed by reduction with

Zn/H2O or dimethyl sulphide (CH3)2S is the replacement of >C=C< with two >C=O, O=C< groups.

Example

Hydroxylation

• It is the addition of two –OH groups to the double bond of an alkene.

• Addition can be from the same side of double bond (Syn Hydroxylation) or opposite sides (tans hydroxylation).

Syn Hydroxylation

• Osmium tertraoxide adds to the both the carbon atoms of the double bonds to yield cyclic osmic estecs.

18

• This cyclic structure undergoes hydrolytic cleavage at O-Os bond to yield diols.

• The reaction is stereo selective. For e.g. cis 2 – butene gives meso diol. • The drawback of the reaction is the toxicity of OsO4 and expense of the reaction. • The reaction also effectively takes place (in the same manner) with alkaline

permanganate it via stereo selective syn addition and formation of cyclic cis permanganic esters.

• Drawback of the use of permangnate for this reaction is that the diol formed is susceptible to further oxidation.

Antihydroxylation • Peroxy acids, RCOOOH oxidise alkene by adding oxygen atom across the double bond to

form epoxide, which are stable and can be isolated. • Epoxides undergo nucleophillic attack under acid or base catalyzed conditions from the

opposite side of the oxygen bridge forming the product with inversion of configuration. • Overall reaction is a stereo selective antihydroxylation.

19

Oxidative cleavage by hot alk KmnO4 • Terminal CH2 group of alkenes is completely oxidized to CO2 and H2O. • Disubstituted atom of the doble bond becomes >C=O of a ketone. • Monosubstituted atom of the double bond becomes an aldehyde which is subsequently

oxidized to the salt of carboxylic acid. For example

Free radical substitution at allylic position

• Alkene with an alkyl substituent for example methyl group in propylene (CH3-CH=CH2) alters the reactivity of the alkene double bond.

• For such a substrate there are two sites of attack undergoing different type of reaction under different condition of reaction.

• Free radical reaction e.g. halogenation are favoured under influence of U.V. light or high temperature and generally in gas phase analogus to substitution reactions of alkanes.

Addition of halogens on alkene like structure on the site of the substrate molecule is favoured at low temperature and in absence of light and generally in liquid phase. Therefore

Mechanism for radical reaction

• Halogen atom does add but at high temperatures, it is expelled before the second step of free radical addition can occur.

20

• Allylic hydrogen is abstracted forming a resonance stabilized allylic radical. Hence the reaction proceeds at a fast rate in the forward direction followed by the attack of halogen to yield the allyl substitution.

• Hydrogens attached to doubly bonded carbon are called vinylic hydrogen ( ) and are difficult to be abstracted as ease of H atom abstraction is allylic >3°>2°>1°>CH4 vinylic. This is due to the fact that stability of free radicals formed is allyl > 30 > 20 > 10 > CH3 > vinylic.

• Allylic bromination can be carried out using N-bromosuccinamide (NBs) in CCl4. It functions by providing constant low concentration of bromine. As each molecule of HBr is formed by haiogenation, NBS converts it to Br2.

• At low concentration of bromine, probability of finding the bromide ion in the vicinity,

after the formation of brominium ion, is very low. Addition slows down and competes with allylic substitution.

• Also in the non polar medium, there are no polar molecules to solvate (and thus stablize) the bromide ion formed. The bromide ion uses a bromine molecule as a substitute.

• As a result the rate equation in non polar solvent is second order with respect to bromine

Rate = K[>C=C<] (Br2]2. And low concentration, Br2 has more pronounced effect in slowing the rate of addition, increasing the tendency for substitution.

Polymerization of Alkenes Large number of monomer molecules with double or triple bond joins together to form high molecular weight molecules called polymers. These are of two types addition polymerization and condensation polymerization. Ionic Polymerization Initiation Addition polymerization proceeds by Propagation

Radical Polymerization termination

Ionic Polymerization: Ionic catalyst like lewis acids, eg. H2SO4, HF, AlCl3 etc. catalyse the reaction. For example. (i) Initiation.

21

(ii) Propagation

(iii) Termination

Free Radical polymerization: This is brought about by the catalyst that generate free radicals eg. organic or inorganic peroxides or salts of the peracids eg. Benzoyl peroxide.

Mechanism Initiation:

Propagation:

Termination

i) By combination of two chains.

ii) By disproportionation.

Industrial Application of ethylene & Propene • Ethylene and propene rank fourth and ninth, respectively in all industrially produced

chemicals. • Both are considered to be petroleum products though they are not obtained directly from

crude oil. • Polymerization is the major end use of ethylene and propene which gives polyethylene,

polypropylene, as the polymeric end products having wide industrial and commercial applications.

• Ethylene is a starting material for the manufacture of ethylene glycol, HO-CH2-CH2-OH, which is the main ingredient of automotive antifreeze and also starting material in the production of polyesters.

• Ethylene is hydrated to give industrial ethanol and is used alongwith benzene to produce styrene.

• Ethylene is used as a fruit ripener.

22

• Propene is the key compound in the production of phenol which is used in adhesives and acetone a commercially important solvent.

• 2 Methyl propene is used to prepare octane isomers that are important components in high octane gasoline.

Cycloalkenes Nomenclature: this class of cyclic unsaturated hydrocarbon takes the name of the corresponding open chain alkene preceded by the word cyclo. eg.

Methods of formation: 1. From cyclic ketones: which in turn may be prepared by distillation of calcium or barium salt

of dicarboxylic acids.

2. Catalytic reduction: Six membered cycloalkanes are prepared from catalytic reduction of benzene and its derivatives under pressure using nickel. eg.

3. Dehydration of cyclo alkyl bromide with ethanolic potassium hydroxide. eg.

4. From esters of dicarboxylic acids (Dieckmann reaction): These undergo intramolecular condensation when treated with sodium to form β-keto ester. β-keto ester on hydrolysis give cyclic ketones which on reduction and further dehydration yields cyclo alkenes.

5. From 1,3-dienes: Under the influence of heat and light, conjugated dienes undergo isomerization (electrocyclic reaction) to form cycloalkenes. For example

With substituted dienes it give isomeric (cis or trans) cyclo butenes.

23

6. By Diel’s Alder reactions: [4 + 2] cyclo additions of 1, 3 butadienes or derivatives with alkene gives cycloalkenes.

Conformations Cyclo alkenes with 3 or 4 carbon atoms are unstable, cyclopentene is a planar stable structure. However, cyclohexene, attains stability by assuming half chair conformations due to the presence of double bond [atoms 6, 1, 2, and 3 lie in one plane].

Reactions Most of the reactions of a stable cycloalkene e.g. cyclopentene are similar to that of open chain alkene compounds.

1. Addition of Bromine: Cyclo alkenes undergo anti addition of Br2.

2. Hydrogenation: Hydrogen undergoes syn addition to form saturated compound.

3. Hydroxylation:

• Peroxy formic acid leads to the anti-addition forming trans diol. • Cis-addition leading to formation of cis diols takes place by hydroxylation with cold dil

KMnO4 solution or OsO4.

24

4.

Dienes Dienes are the compounds which contain two double bonds.

Types of dienes 1. Conjugate diene: A diene were the two double bonds are separated by one single bond i.e. it

has alternate double bond and single bond.

2. Isolated diene: A diene where the double bond is separated by more than one single bond. Each double bond behaves independent of the other and each of them shows the reaction of a normal alkene.

3. Cumulated diene (Allenes): A diene where one carbon atom has two carbon – carbon double bonds i.e. double bonds are adjacent to each other.

Nomenclature • Naming of the dienes taken place similar to the corresponding alkene except the name ends

as ‘adiene’. • Longest carbon chain containing maximum number of double bonds is chosen as parent

hydrocarbon. • First carbon atom of each double bond is given the lowest number. e.g.

• Stability: Heats of hydrogenation (∆Hn) for various dienes are, for example. 1, 2 –Pentadiene = – 70 KCal/mol (cumulated) 1, 3 – Pentadiene = – 54 KCal/mol (conjugated) 1, 4 - Pentadiene = – 61 KCal /mol (Isolated)

25

Indicate that decreasing orders of stability of dienes are conjugated > isolated > cumulated as less negative the ∆Hh, the more stable is diene.

Allenes: • These are hydrocarbons with cumulated double bonds. • Simplest compound is allene or propadiene. (CH2 = C = CH2). • The structure of allene consist of a middle carbon atom which is SP hybridized. It is bonded

to each of the adjacent carbon atoms by a sigma bond. One of its P orbital say Py AO overlapps with the Py AO of the adjacent and carbon atom in xy lane. Its Pz orbital overlapps with Pz orbital of the carbon at the other end giving a π bond in xz plane.

• Plane of the two π orbitals of the central carbon atom is perpendicular to each other.

Methods of Preparation : 1. By heating 1, 2, 3 – tribromopropane with potassium hydroxide and then treating the

resulting 2, 3 – dibromopropene with zinc dust in methanol solution.

2. By treating cyclopropane derivative formed from an alkene and bromoform and alkali with magnesium in the ether.

Reactions: 1. Allenes react with sulphuric acid to form acetone.

2. It reacts with bromine to form 1,2,2,3 – tetrabromopropane.

3. It reacts with sodium in dry ether to produce sodium salt of propyne.

1, 3 – Butadiene (CH2 = CH – CH = CH2) • It is the simplest member of group of compounds with conjugated double bond.

26

• Structure: Double bond lies on the two adjacent carbon atoms.Both these adjacent doubly bonded carbon atom have p orbital in the same plane and tend to undergo sideways overlapp with the p-orbital of the end carbon atoms to give the extended π system. This results in greater stability due to increased resonance. It is a resonance hybrid of the following canonical forms:

• C – C single bond in conjugated dienes are shorter than C-C single bond in isolated dienes. Shortening of bonds results in greater orbital overlapp and increased bond energy, making the molecule more stable.

• 1, 3 – Butadiene shows two conformations due to rotation possible about C – C single bond.

• s refers to the fact that there is geometrical isomerism with respect to single bond. Transoid form is more stable conformation owing to greater steric repulsion in cisoid form.

Methods of preparation: 1. By catalytic dehydrogenation of butane.

27

2. By allylic bromination of butene followed by dehydrohalogenation of the alkenyl halide.

3. By dimerization of acetylene and then the hydrogenation of the product forms.

4. By acid catalysed double dehydration of 1, 4 – butanediol.

5. By dehydrohalogenation of dihalides.

Reactions: 1. Addition of Halogen acids (HX):

• It is an electrophillic addition. • It undergoes two kinds of addition – 1, 2 addition and 1, 4 addition. • The electrophile (H+) adds to either C = C to forms an allylic carbocation. • The positive charge of the carbocation is deloclized to C-2(A) and C-4(B) resulting in

greater stability. • The nucleophillic Br- adds to C-2 to give 1, 2 – addition product and to C-4 to give 1, 4 –

addition product.

• 1, 2 addition product predominates at low temperature (-80°C) while 1, 4 product is major product formed at higher temperature (40°C).

Reason At low temperature the amount of energy available is limited. As a result, activation energy for 1,4 addition product is too great to be overcome since the limited energy is unable to cross the energy barrier. The energy of activation for the 1,2 addition product is not as high so there is enough energy available at that low temperature to overcome the energy barrier. This results in 1,2 addition product as a major product though thermodynamically it is not more stable product. This reaction is kinetically controlled. On the other hand at the higher temperature(40°C), the amount of energy available allows both the activation energy barriers to be overcome and both the products can be easily produced. At this point thermodynamically more stable product is to be formed which according to Zeitsev, “the more substituted alkane is more stable”. The reaction is thermodynamically controlled.

28

• At higher temperature 1, 2 addition product rearranges to give mainly 1, 4 product which is thermodynamically more stable as it is a more substituted alkene.

2. Addition of Halogens.

• Addition of halogens give a mixture of two dibromocompounds by 1, 2 and 1, 4 addition of Br2 in presence of CCl4.

3. Addition of hydrogen:

4. Diel’s Alder reaction: 1, 3 – Butadiene reacts with a dienophile (diene loving) i.e. alkene or an alkyne to form a cyclic adduct.

5. Polymerization: 1, 3 – Butadiene undergoes radical induced or electrophile or nucleophile induced polymerization to give polybutadiene (i.e. Bunna Rubber).

• The derivtive isoprene ( ) on similar polymerization yieds synthetic rubber.

• Mechanisms 1. Radical induced

2. Electrophile induced

3. Nucleophile induced

Alkynes General formula: CnH2n-2 The isomeric compounds with same general formula are, dienes cycloalkanes and bicyclics. Structure

• Each Carbon uses sp hybrid orbitals to form two sigma bonds, leaving two pure p orbitals on each carbon. Each of the pure p orbitals is used in forming two pi bonds, both of which align perpendicular to each other. The pi bonds gets mixed up and all the 4 π e- are cylindrically distributed around the two sp hybridized carbon atoms e.g. CH ≡ CH.

29

• sp hybrid orbitals are linear, and molecules posses cylindrical symmetry, ruling out cis-trans stereoisomerism.

Nomenclature

• The ending “ane” of the corresponding alkane is replaced by suffix ‘yne’.

• If more than one position in the molecule is occupied by triple bond, then assign minimum possible number to the carbon atom bearing the triple bond.

• Substituents are indicated by numbers.

• If a molecule contains more than one triple bond the suffix ‘ane’ is replaced by diyne, triyne etc.

Methods of preparation 1. Hydrolysis of carbides.

• Both the carbides are ionic. • In CaC2 the anion is ΘC ≡ CΘ therefore acetylene is final product. • In MgC3 the anion exist as 3-C – C ≡ CΘ, hence produce propyne. • Aluminium carbide (Al4C3) and beryllium carbide (Be2C) do not form any alkyne an

hydrolysis as their anion exist as C4-. They form methane.

2. From Acetylene.

• Two hydrogens of acetylene are acidic so can be replaced by a strong base forming sodium acetylide.

• Sodium acetylide reacts with primary alkyl halide to form 1 alkynes by nucleophillic substitution (SN2).

30

• Secondary alkyl halides undergo competitive elimination alongwith substitution reaction in presence of acetylide ion (HC ≡ C-) which act as strong base. Therefore, poor yield of product is obtained.

• Tertiary alkyl halides do not undergo substitution, but only elimination forming alkene as the only product.

3. Dehydrohalogenenation of Dihalides

• This is a 1, 2 elimination reaction. • First step of dehydrohalogenation can be done using a strong base alch. KOH. • In second step loss of another HBr molecule is difficult as they are attached to more

electronegative Sp2 hybrid carbon atom. Therefore, a more stronger base NaNH2 is employed.

4. Dehalogenation of Tetrahalides

Properties Physical Properties

• Low polarity. • Insoluble in water but soluble in organic solvents of low polarity like ether, benzene etc.

31

• Melting point and Boiling point increase with increasing number of carbon atoms. • Unsymmetrical alkynes have a dipole moment greater than that of alkenes while

symmetrical alkynes have zero dipole moment. Acidity of terminal hydrogen “The more s character in the orbital used by the carbon bonded to the hydrogen, the more acidic is the hydrogen ”. this is due to the fact that electronegativity of sp hybridized carbon atom is greater than that for sp2 hybridized carbon atom which is greater than sp3 hybridized atom. Therefore the attraction for electrons by hybridized carbons will be sp > sp2 > sp3. • sp hybridized carbon atom pulls the electron from the ≡ Csp – H bond towards itself, as a

result the loss of H+ becomes easy and therefore the hydrogen attached to an sp hybridized carbon is acidic in nature.

• ≡ C – H bond in acetylene has considerable ionic character due to resonance.

HC ≡ C – H ↔ H – C ≡ CΘH⊕ ↔ H⊕CΘ ≡ C – H ↔ H⊕CΘ ≡ CΘH⊕

Some of the typical reactions shown due to acidic nature of alkynes are:-

1. A solution of sodium amide in liq. ammonia readily converts acetylene and other terminal alkyne into the corresponding carbanions.

RC ≡ CH + Na+NH2

- R – C ≡ C-Na+ + NH3.

2. Alkynes are deprotonated by alkyl lithium compounds, which act as base.

CH3(CH2)3 C ≡ CH + n – C4 H9- Li+ CH3(CH2)3 ≡ C-Li+ + n-C4 H10

Since alkynes are stronger acid than alkane, equilibrium lies to the right.

3. Terminal alkynes give insoluble salts with numbers of heavy metal cations such as Ag+ and Cu+.

H – C ≡ CH + 2 [Ag(NH3)2]+ Ag - C ≡ C – Ag + 2 NH4

+

Silver acetylide (White ppt.) CH3 – C ≡ CH + [Cu(NH3)2]+ CH3 - C ≡ C. Cu + NH4

+ + NH3 Cuprous methyl acetylide (red ppt.) This is also a method for identification of terminal alkynes.

Chemical Properties • Being unsaturated and rich with four pi electrons alkynes under go electrophillic addition

reactions to form alkene derivatives with addition of one molecule and alkane derivative with addition of two molecules.

32

• Reactivity of CH ≡ CH < CH2 = CH2

Though the π e- density around the triple bond is higher than that of double bond yet it shows the decreased reactivity towards to electrophile due to the fact that bridged halonium ion of acetylene is more strained than the bridged halonium ion from ethylene.

• Since hydrogenation does not involve electrophillic attack alkynes are more reactive than alkanes.

Reactions Hydrogenation • Alkynes can be reduced directly to alkanes by addition of H2 in presence of Ni, Pt or Pd as

catalyst.

• Alkynes can be partially hydrogenerated to alkenes by using lindlars catalyst [Pd on CaCO3 + Pb(CH3COO)2], nickel boride or palladised charcol.

Addition of Halogen acids • Addition takes place in accordance to markovnikov’s rule. • Order of reactivity of alkynes towards addition is HI > HBr > HCl.

• Peroxides have same effect on addition of HBr to alkyne as that on alkene and reaction

follows the free radical mechanism (Refer to alkene for details)

33

Addition of Halogens: • Reaction is used as a test to detect unsaturation i.e. Br2 in CCl4 is reddish brown in colour

while the product obtained is colourless.

Addition of water • Alkyne when treated with 40% H2SO4 containing 1% HgSO4 (as a catalyst) adds water to the

pi bond in accordance to Markovnikov’s addition. • The reaction proceeds by forming enol as intermediate which tautomerizes to give more

stable carbonyl compound. • Acetylene forms aldehyde while higher analogues form ketones.

Addition of Boron hydrides • Diborane splits into two units of BH3 and addition takes place as per the markovnikov’s rule. • Addition takes place to form trialkenyl borane by substantiating all hydrogen of BH3 with

alkenyl group.

• Trialkenyl borane on hydrolysis gives alkenses.

• Oxidation of trialkenyl borane with H2O2 forms carbonyl compounds. Dimerization • Acetylene dimerises when trated with mixture of Cu2Cl2 and NH4 Cl to give vinyl acetylene.

34

• Dimer undergoes addtion reaction, preferably at triple bond and not the double bond inspite

the fact that alkynes are less reactive than alkane towards electrophilic addition reaction. This is due to the fact that one addition at triple bond forms 1, 4 – conjugated diene system which shows extra stability due to resonance.

Oxidation: • Alkaline KMnO4 oxidises alkynes by causing the cleavage of triple bond forming the salt of

carboxylic acids. Internal alkynes give the mixture of carboxylic acids on acidification while terminal alkynes give carboxylic acid and the terminal carbon is oxidized to CO2 and H2O.

Ozonolysis • Alkynes with O3 form ozonide which on hydrolysis with H2O (oxidative ozonlysis) gives a

mixture of two carboxylic acids. Ozonide when hydrolysed with Zn + H2O (reductive ozonolysis) a diketone is formed.

• Ethyne behaves differently

Polymerization 1. Acetylene with HCl in presence of Hg2+ as catalyst forms vinyl chloride which polymerizes

to give PVC.

35

2. Acetylene polymerizes to benzene when passed through a lot metallic tube.

Addition of hypohalous acid • Alkynes react with hypohalous acid in molor ratio 1 ; 2. • Addition is in accordance to Markovnikov’s rule.

36