Chemical Properties

Alkanes are saturated hydrocarbons. They contain only strong C-C and C-H sigma bonds. Hence, alkanes are quit less reactive and are known as paraffins (parum-little affinis-reactivity or affinity). Alkanes are generally in towards acids, bases, oxidizing and reducing agents. However, they undergo the following reactions under certain conditions.

Substitution Reactions

Ø One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group.

Ø Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light.

Ø Lower alkanes do not undergo nitration and sulphonation reactions.

These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below:

Halogenation

It is found that the rate of reaction of alkanes with halogens is F₂ > Cl₂ > Br₂ > I₂. Rate of replacement of hydrogens of alkanes is : 3° > 2° > 1°. Fluorination is too violent to be controlled. Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like HIO₃ or HNO₃.

Halogenation is supposed to proceed via free radical chain mechanism involving three steps namely initiation, propagation and termination as given below:

Mechanism

(i) Initiation:

The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The Cl–Cl bond is weaker than the C–C and C–H bond and hence, is easiest to break.

(ii) Propagation:

Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.

The methyl radical thus obtained attacks the second molecule of chlorine to form CH₃ – Cl with the liberation of another chlorine free radical by homolysis of chlorine molecule.

The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible and may occur. Two such steps given below explain how more highly haloginated products are formed.

(iii) Termination:

The reaction stops after some time due to consumption of reactants and/or due to the following side reactions:

The possible chain terminating steps are:

Though in (c), CH₃ – Cl, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps to understand the reason for the formation of ethane as a byproduct during chlorination of methane.

Combustion

Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.

The general combustion equation for any alkane is:

Due to the evolution of large amount of heat during combustion, alkanes are used as fuels.

During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.

Pyrolysis

Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis.

For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.