ALKANE

A saturated hydrocarbon with general formula:

Each carbon atoms undergoes sp3 hybridisation during bonding.

Nomenclature

CH3 ― CH3 ― CH3

CH3 ― CH ― CH ― CH3 ― CH3 ― CH3

CH3

CH3 CH3

CH3 ― CHCH ― CH2 ― C ― CH3

CH2 CH3 CH2 ― CH2 ― CH3

Physical Properties

Alkanes with C1 and C4 are gases at room temperature.

Reasoning:

The melting and boiling points of alkanes ____________ as the number carbon atoms increase.

Reasoning:

For any one set of branched chain isomers, the boiling point decreases as the branching increase.

Reasoning: The more highly branched molecules are more nearly spherical in shape; the surface area of contact is smaller. Therefore they cannot be packed together so closely resulting in a decrease in strength of the intermolecular V.D.W force.

All alkanes do not dissolve in water.

Reasoning;

All alkanes are practically less dense than water, the liquid and solid alkanes float on the surface of water.

Q1. Which of the following alkanes is a solid at temperature 298K?

A. CH3(CH2)16CH3 C. (CH3)2CHCH2CH(CH3)2

B. CH3(CH2)6CH3 D. (CH3)2CH(CH2)4CH3

Q2. Which of the following compounds has the highest boiling point?A. CH3 CH2C(CH3) C. CH3 CH2CH(CH3) CH2 CH3

B. CH3 CH2 (CH3) CH2CH2CH3 D. CH3 CH2 CH2CH2 CH2CH3

Q3. Which of the following properties of alkane decreases in numerical value when the number of carbon atoms in the homologue series increases?

A. density B. enthalpy of vapourisation C. number of isomer D. vapour pressure

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Natural source of alkanes:

1. Fractional distillation of crude oil

Alkanes Class Uses

CH4 ― C4H10 Refinery gas Fuel (cooking gas) and as a feedstock for petrochemicals

C5H12 ― C10H22 Gasoline Petrol for transport and as a feedstock for petrochemical

C10H12 ― C16H34 Kerosene Fuel for jet, for cracking into petrol

C12H26 ― C25H52 Diesel oil Fuel for transport, for cracking into petrol

C19H40 ― C40H82 Lubricant Lubricating oil, cosmetics creams,candles

Residue Polishing wax, bitumen on road

2. Cracking

Cracking is a process in which large alkane molecule is broken up into smaller alkane molecules in the absence of oxygen.

During the process of cracking small akane and alkene (unsaturated hydrocarbon ) molecules are produced.

The advantages of thermal cracking

The molecules in the residue can be cracked. The catalytic cracking only works on the distillate (liquid that has been distillate).

eg: C8H18 eg: C10 H22

Laboratory synthesis of alkanes

1. From alkenes by catalytic hydrogenation

― C = C ― ― C ― C ― (showing the reaction)

H H

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cracking

Thermal cracking (using heat alone ≈ 1000oC)

This process proceeds via carbonium ion mechanism.

Catalytic cracking (using catalyst and heat ≈ 400oC)

Catalyst: a mixture of silica and aluminium (zeolite)

This process proceeds via free radical mechanism.

H2

Ni, 150 oC

Ni

CH3CH = CH2 + H2 CH3CH2CH3 ( a balanced equation)

eg 1 : CH3CH = CHCH = CH2

eg 2:

note: 1 mole of double bond required 1 mole of H2 for complete hydrogenation.

2. Decarboxylation

Carboxylic acids and salts of carboxylic acids can be decarboxylated by heating with soda lime.

O

R ― C ― OH R ― H

or

O

R ― C ― ONa R ― H

eg1: CH3COOH eg2: 2- methylbutanoic acid

eg3: sodium benzoate eg4: potassium propanoate

note : After decarboxylation, the product has one carbon atom less than the initial reactant.

Chemical Properties

The alkanes are not reactive towards the polar reagents. The C ― H bond is only slightly polar because of the small difference in electronegativity values between carbon atom and hydrogen atom. Therefore, the alkanes molecules have no distinguished electrical charge resulting in not able to attract charged species like H+, OH- amd MnO4

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Alkanes react with reagents which are non-polar like O2 and halogen

1. Combustion.

(a) Complete combustion:

(b) Incomplete combustion:

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150 oC

soda lime

heat

soda lime

heat

2. Halogenation

R ― H + Cl2 RCl + HCl ( free radical substitution)

Observation: white fumes (HCl) evolved.

Mechanism:

note : the stability of free radical 3o free radical 2o free radical 1o

Petrol and the Octane number

Hydrocarbon C5 – C12 are the major components in petrol

Each hydrocarbon components in petrol is used as fuel to move the engines.

Fuel which can undergo smooth combustion is said to be efficient fuel.

Fuel which cannot undergo smooth combustion will give knocking to the engine. Knocking will lead to a loss of power, inefficiency use in fuel and an increase in wear.

Octane number is a measure of the petrol grade.

Octane number varies in the range from 0 to 100. Higher the octane number, higher is the quality of the petrol.

2,2,4-trimethylpentane resembles the best fuel( although now there are some other better still with octane number more 100 ), gives the least knocking to the engines and is graded with octane number of 100. In the other hand, heptanes is graded with octane number of 0 which is considered as a bad fuel.

Branched – chain alkanes burn more smoothly in engine than straight-chain alkanes, therefore they are graded with higher octane number.

Octane number of a petrol can be upgraded by

i) adding small amount of anti-knocking agen such as tetraethylllead or tetramethyllead.

ii) adding more branched-chain hydrocarbons

iii) adding of aromatic hydrocarbons, such as benzene.

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The leaded petrol will cause deposition of lead oxide in the cylinder wall after combustion, therefore small amount of 1,2- dibromoethane is added to the leaded petrol so that to convert the lead compounds to the volatile lead bromide, which are swept out of the engine as the exhaust gases.

However the use of lead in petrol to raise octane number is now being phased out in most countries for two reasons both related to the pollution of environment by lead discharged through exhausts..

I) lead is poison that accumulated in the bodies of human and other animals.

II) exhaust lead inhibits the action of catalysts in catalytic converters. (catalyst poison).

Octane number of unleaded petrol is raised by adding more branched-chain hydrocarbons or aromatic hydrocarbons, which is known to be carcinogenics.

The effect of hydrocarbons on the environment

The pollution caused by the vehicle emissions and the solution to the vehicle pollution seem to be the hot topics of discussion for scientist.

the major exhaust gases from vehicles are

nitrogen ( air consists of 78% of nitrogen )

carbon dioxide ( gives the green house effect that causes the global warming )

Carbon monoxide( a poisonous gas )

unburnt hydrocarbons ( carcinogen )

oxides of nitrogen such as NO and NO2 ( poisonous gases, cause the acid rain and photochemical smog )

Catalytic converters

Catalysts can be used to reduced exhaust emissions. A mixture of Platinum and Rhodium in catalytic converter is used to catalyse the reactions of the exhaust gases to remove pollutants. Catalytic converter catalyses the oxidation of the poisonous carbon monoxide to carbon dioxide and water. It also catalyses the reaction of axides of nitrogen to nitrogen and oxygen.

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