ORGANIC CHEMISTRY CHM 207

CHAPTER 2:ALKANES

NOR AKMALAZURA JANI

TOPICSTOPICS Nomenclature Structure Reactions:

- free radicals substitution - combustion

Industrial source and uses of aliphatic hydrocarbons- petroleum and natural gas- petroleum fractions: cracking and reforming and

their uses.

• General formula: CnH2n+2, where n = 1, 2, ….

• Only single covalent bonds are present

• Known as saturated hydrocarbons because contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present.

• Can be assumed to be sp3-hydridized

Structures of the first four alkanes

Homologous Series

Definition: A series of compounds in which each member differs from the next by a specific number and kind of atoms.

Alkanes: Differ only at number of (CH2) Series of compounds that has the same

functional group.

INITIAL NAMES OF THE HOMOLOGOUS SERIES

Number of carbon atoms, n Name 1 Meth2 Eth3 Prop4 But5 Pent6 Hex7 Hept8 Oct9 Non

10 Dec

• The general formula of an alkyl group is CnH2n+1.

• Alkyl groups are used to name organic compounds.

NAMING ALKANES

R= CnH2n+1 (any alkyl group)

R = CH3 — methyl group

R = CH3CH2 — ethyl group

• The letter “R” is often used in formulas to represent any of the possible alkyl groups.

IUPAC RULES International Union of Pure and Applied Chemistry

Consider all alkyl groups attached to it as branch chains or substituents that have replaced hydrogen atoms of the parent hydrocarbon. If two chains of equal length are found, use the chain that has the larger number of substituents attached to it.

The alkane’s name consists of the parent compound’s name prefixed by the names of the alkyl groups attached to it.

RULE 1. Select the longest continuous chain of carbon atoms as the parent compound.

This structure has 2 chains.

CH3 CH2 CH CH2 CH2 CH3

CH3

This chain has 6 carbon atoms.

1 2 3 4 5 6

CH3 CH2 CH CH2 CH2 CH3

CH3

This chain has 4 carbon atoms.

1 2 3

4

This is the longest continuous chain.

CH3 CH2 CH CH2 CH2 CH3

CH3

Select this chain as the parent compound.

1 2 3 4 5 6

CH3 CH2 CH CH2 CH2 CH3

CH3

It is a branch chain and can be considered to have replaced a hydrogen on the parent compound.

1 2 3 4 5 6

This is a methyl group.

CH3 CH2 CH CH2 CH2 CH3

CH3

1 2 4 5 63

The name of the compound is 3-methylhexane.3

3

– If the first subsitutent from each end is on the same-numbered carbon, go to the next substituent to determine which end of the the chain to start numbering.

RULE 2. Number the carbon atoms in the parent carbon chain starting from the end closest to the first carbon atom that has an alkyl group substituted for a hydrogen atom.

If the chain is numbered left to right, the isopropyl group is on carbon 5.

isopropyl group

1 2 53 7 864

If the chain is numbered right to left, the isopropyl group is on carbon 4.

isopropyl group

8 7 46 2 135

Use right to left numbering so that the isopropyl group is on the lowest numbered carbon.

4-isopropyloctane

RULE 3. Name each alkyl group and designate its position on the parent carbon chain by a number (e.g., 2-methyl means group attached to C-2).

5 4 2 13

2-isopropyl pentane

2,3-dimethylpentane

5 4 2 13

The methyl group appears twice

RULE 4. When the same alkyl-group branch chain appears more than once, indicate this repetition by a prefix (di-, tri-, tetra- and so forth) written in front of the alkyl group name (e.g. dimethyl indicates two methyl groups).

–The numbers indicating the alkyl-group positions are separated by a command and followed by a hyphen and are placed in front of thename (e.g., 2,3-dimethyl).

RULE 5. When several different alkyl groups are attached to the parent compound, list them in alphabetical order (e.g. ethyl before methyl in 3-ethyl-4-methyloctane). Prefixes are not included in alphabetical ordering (ethyl comes before dimethyl).

methyl

ethyl

1 2 4 53 6 7 83 4

3-ethyl-4-methyloctane

• Alkanes can have many different types of substituents.

• For example:

CHCH3

NO2

CH

Br

CH3 CH3

1 2 3 4 5

3-bromo-2-nitropentane

CYCLIC HYDROCARBONSA hydrocarbon that contains carbon atoms joined to

form a ring.Cycloalkanes – all carbons of the ring are saturated

NOMENCLATURE OF CYCLOALKANES

Similar to that alkanes. For examples:

CH3

CH3

CH2CH3methylcyclopentane

=1234

5 6

1-ethyl-3-methylcyclohexane

When the acyclic portion of the molecule contains more carbon atoms than the cyclic portion (or when it contains an important fuctional group), the cyclic portion is named as a cycloalkyl substituent.

Example:

5-cyclobutyl-1-pentyne4-cyclopropyl-3-methyloctane

CCH CH2 CH2 CH2

CYCLIC HYDROCARBONS

ISOMERISATION

Structural isomers:

- Molecules that have the same molecular formula, but different structure

Three isomers of pentane (C5H12)

STRUCTURE ISOMERS FOR ALKANES

NAME MOLECULAR FORMULA TOTAL OF ISOMERS

Methane CH4 1

Ethane C2H6 1

Propane C3H8 1

Butane C4H10 2

Pentane C5H12 3

Hexane C6H14 5

Heptane C7H16 9

Octane C8H18 18

Nonane C9H20 35

Decane C10H22 75

Solubilities and densitiesBoiling pointsMelting points

PHYSICAL PROPERTIES OF ALKANES

SOLUBILITIES AND DENSITIES OF ALKANES

1) Solubilities:

The C-H bond having only a very weak dipole moment.

Alkanes are weak polar molecules and considered as non-polar molecules.

Soluble in non-polar solvents such as benzene and weak non-polar organic solvents such as dimethyl ether (CH3-O-CH3).

Insoluble in water:- alkanes are non-polar and do not form hydrogen bonds with water molecules.- described as ‘hydrophobic’ (water hating).

2) Densities:

- alkanes have densities around 0.7 g/mL, compared to density of water (1.0 g/mL).

- alkanes is less dense than water and insoluble in water.

- water combined with alkanes will form two phase with the alkanes on the top.

oil

water

BOILING POINTS AND MELTING POINTS OF ALKANES

Effect of relative molecular mass on boiling point.- the first four chain alkanes are gases.- alkanes from C5H12 to C18H38 are liquids at room temperature because their melting point are lower than 28oC (301K).- alkanes above C18H38 are solids at room temperature.- the boiling points of straight chain alkanes increase steadily with relative molecular mass (due to increasing forces of attraction between molecules).

* A larger molecule, with greater surface area and greater van der Waals attractions, boils at higher temperature *

BOILING POINTS OF ALKANES

Boiling points of the straight chain of alkanes

Effect of branching on boiling point.

- Branched chain alkanes boils at a lower temperature (more volatile) than the straight chain alkane with the same number of carbon atoms.

- Examples: hexane boils at 68.7oC, 3-methylpentane (one branch) boils at 63.3oC and 2,3-dimethylbutane (two branches) boils at 58oC.

- Reason: the branched chain alkanes are more compact (nearly spherical), have smaller surface area, smaller van der Waals forces of attraction and boils at lower temperature.

The melting points increase with increasing of molecular weight.

Alkanes with even numbers of carbon atoms pack better into a solid structure, so higher temperatures are needed to melt them (high melting point).

Alkanes with odd numbers of carbon atoms do not pack as well, and melt at lower temperatures (low melting points).

Branched chain alkanes melts at a higher temperature than n-alkanes (straight alkanes) with same numbers of carbon atoms. - Reason: 3D-structure of branched alkanes are more compact, pack more easily into solid structure and melt at higher temperatures.

MELTING POINTS OF ALKANES

CH CH2

H3C

H3CCH2 CH3

bp 60oC

mp -154oC

CH CHH3C

H3C

CH3

CH3

C CH2

CH3

CH3

H3C CH3

bp 58oC

mp -135oCbp 50oC

mp -98oC

Formula: C6H14

Shape of the molecule become more highly branched and compact

Melting points increase, boiling points decrease

UNREACTIVITY OF ALKANES

• Alkanes is chemically inert to most reagents.

• For example, acids, alkalis, and oxidising agents such as potassium manganate (VII) or potassium dichromate (VI) do not react with alkanes.

• Alkanes reacts with oxygen and halogens in suitable conditions.

• Why alkanes has low reactivity?- lack of electron-deficient or electron-rich sites on the alkanes molecules.- polar molecules, positive and negative ions (such as H+ and OH-), do not react with alkanes because the C-H bond is weak polar and C-C bond is non-polar.

• COMBUSTION- Alkanes burn in a plentiful supply of air or oxygen to produce water and CO2 only.

REACTIONS OF ALKANES

- for example:

C3H8 + 5 O2 → 3CO + 4H2O

- In a very limited supply of air, alkanes burn to form carbon as one of the product.

C2H6 + 3 O2 → 2C + 3H2O

2

- In a limited supply of air, combustion of alkanes produces carbon monoxide and water

C2H6 + 5 O2 → 2CO + 3H2O

2

• HALOGENATION OF ALKANES

- At RT, alkanes do not react with chlorine or bromine in the dark.

- if the mixture of alkanes and chlorine or bromine is heated at high temparature (300-400oC), or irradiated by ultraviolet light, the hydrogen atoms in the alkanes are successively replaced by chlorine or bromine atoms to produce a mixture of products (halogenated alkanes).

REACTIONS OF ALKANES

• Equation 1: reaction with limited supply of chlorine and excess of methane. Tha major product is chloromethane.

• Equation 2: reaction with the excess of chlorine. The major product is tetrachloromethane.

CH4 + Cl2 CH3Cl + HCl

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

(1)

(2)

Equations for the reactions of methane with chlorine:

* Bromine reacts with alkanes in the same way of chlorine, but iodine do not react well with alkanes *

• This reaction is called a substitution reaction (an atom or a group atom in an organic compound is replaced by another atom or a group of atoms).

• Involves a halogen - called halogenation• If the halogen is chlorine – called chlorination.• If the halogen is bromine – called bromination.

* Condition of reaction: light or heat (high temperature) or ultraviolet radiation (provides energy that is absorbed by reactant molecules to produce free radicals).

MECHANISM OF FREE RADICAL MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONSSUBSTITUTION REACTIONS

1) INITIATION STEP- homolytic fission

Cl Cl Cl Clhv

chlorine atoms(free radicals)

MECHANISM OF FREE RADICAL MECHANISM OF FREE RADICAL SUBSTITUTION REACTIONSSUBSTITUTION REACTIONS

hv = radiation energy= movement of single electron (radical)

ΔH = +242 kJMol-1

2) PROPAGATION STEPS- Free radical species produce another free radical

species.- Free radicals is highly reactive.

methyl radical

Cl H C HH

HHCl + CH3

methane

Cl2methylradical

CH3Cl + ClCH3

chloromethane

- Methyl radical propagates a chain reaction as the methyl free radical then reacts with another chlorine molecule to form chloromethana and a chlorine free radical.

- Chlorine free radical produced then react with another methane molecule and the cycle is repeated.

3) TERMINATION STEPS- The reaction stops when two free radicals collide and combine.- highly exothermic.

Cl2CH3Cl

H3C CH3

CH3

Cl Cl

Cl

CH3 CH3

(by product)

- If a large excess of methane is used, CH3Cl is obtained as the main organic product.

- In excess of chlorine, the propagation steps may proceed with the reaction between a chlorine free radical with chloromethane to produce dichloromethane.

- The reaction may continue to produce trichloromethane and finally tetrachloromethane.

CH3Cl

Cl2CH2Cl

CH2Cl2

CHCl2 Cl2

CHCl3CCl3 Cl2 CCl4

CCl3

CHCl2

CH2Cl

CHCl3

CH2Cl2

HCl

HCl

HCl

Cl

Cl

dichloromethane

Cl

Cltrichloromethane

Cl

Cl

tetrachloromethane

Br Br

CH3CH3

Br2

Br

CH2CH3

BrBr Br2

CH3CH2 Br

CH3CH2 CH3CH2

CH2CH3

CH3CH2Br

CH3CH2Br

CH3CH2CH2CH3

Br Br

HBr

Br

hvinitiation step

propagation step

termination step

FREE RADICAL SUBSTITUTION REACTIONS OF ETHANEFREE RADICAL SUBSTITUTION REACTIONS OF ETHANE

Further propagation steps can take place until CBr3CBr3 is finally produced

• 2-bromopropane is the main product because:- it is easier for halogen free radical to abstract a hydrogen atom from a 2o carbon atom than 1o carbon atom.

CH

H H CH

R H CR

R H CR

R R

methyl radical primaryradical

secondaryradical

tertiaryradical

stability increases

CH3CH2CH3

Br

BrCH3CHCH3

CH3CH2CH2

Br2

Br2

CH3CHCH3

Br

CH3CH2CH2Br

Br

Br

2-bromopropane2o alkyl radical

1o alkyl radical 1-bromopropane

FREE RADICAL SUBSTITUTION REACTIONS OF PROPANEFREE RADICAL SUBSTITUTION REACTIONS OF PROPANE

MAIN PRODUCT

• Three major sources of alkanes are the fossil fuels which are natural gas, petroleum and coal.

i) Natural gas- consists of 90-95% methane, 5-10% ethane, and a mixture of other relatively low boiling alkanes, which are propane, butane and 2-methylpropane.

- used primarily as a fuel to heat buildings and generate electricitya as well as starting material for the production of fertilizers.

INDUSTRIAL SOURCE AND USES OF ALIPHATIC HYDROCARBON

ii) Petroleum

- Petroleum: a thick, viscous liquid mixture of literally thousands of compounds, mostly are hydrocarbons, formed from the decomposition of ancient marine plants and animals.

- Uses:a) fuel for automobiles, aircraft and train.b) provide most of the greases and lubricants required for the machinery of highly industrialized society.c) petroleum with natural gas provides 90% of organic raw materials for the synthesis and manufacture of synthetic fibers, plastics, detergents, drugs and dyes.

• Petroleum refining:

- The process whereby the petroleum is separated into its components along with the separation of impurities.

- The refining is done by fractional distillation. Each hydrocarbon component with its own boiling point separates out neatly when the petroleum is heated.

- The fractions are further treated to convert them into mixtures of more useful saleable products by various methods such as cracking and reforming.

Fractional distillation

• Crude oil enters a refinery and then goes to distillation units where it is heated to temperatures as high as 370 to 425 oC and separates into fractions.

• Volatile components (low boiling point) will come out first.• Common names of fractions and their uses:

i) Gases boiling below 20oC- taken off at the top of distillation column.- mixture of low-molecular-weight hydrocarbons, mainly propane, butane and 2-methylpropane.- substances can be liquefied under pressure at RT.- uses: liquefied petroleum gas (LPG) is a convenient source of gaseous fuel for home heating and cooking.

ii) Naphthas, bp 20-200 oC- mixture of C5 to C12 alkanes and cycloalkanes, small amount of benzene, toluene, xylene and aromatic hydrocarbons. - light naphtha fraction (bp 20-150oC) is a source of straight-run gasoline.- uses: motor fuel, source of raw materials for the organic chemical industry.

iii) Kerosene, bp175 to 275 oC- mixture of C9 to C15 hydrocarbons- uses: heat, jet fuel

iv) Fuel oil (diesel), bp 250 to 400 oC- mixture of C15 to C18 hydrocarbons- motor fuel

v) Lubricating oil and heavy fuel oil distill (above 350oC)- mixture of C16-C30 hydrocarbons- uses: heating, lubrication

vi) Asphalt- black, tarry residue remaining after removal of the other volatile fractions- C35 and above

Fractional distillation

i) Cracking: A process whereby a saturated hydrocarbon is

converted into an unsaturated hydrocarbon and hydrogen.

Ethane is cracked by heating in furnace at 800 to 900 oC.

CH3CH3 H2C CH2 H2800-900oC

thermal crackingethane ethene

Reforming process

ii) Catalytic reforming:

- A process for increasing the octane number of naphthas.

- It involves isomerization of alkanes, dehydrogenation of cyclohexanes to aromatic hydrocarbons, isomerization and dehydrogenation of alkylcyclopentanes, and dehydrocyclization of alkanes.

- Example:

CH3CH2CH2CH2CH2CH3 -H2 -H2

catalyst catalyst

hexane cyclohexane benzene

* The quality of gasoline as a fuel for internal combustion engines.

* Fuel that have high octane number, has very good antiknock properties (the fuel / air mixture burns smoothly in the combustion chamber).

* 2,2,4-trimethylpentane (isooctane) has very good antiknock properties with octane number 100 compared to heptane which has poor antiknock properties (octane number 0).

* If other fuel has octane number 90, means that the knock properties of the fuel is same as those mixture of 10% heptane and 90% isooctane.

Octane number / octane rating