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Reactions of Alkanes

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Alkanes contain only C−C and C−H σ-bonds. Since σ-bonds are quite strong bonds, alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions :

Substitution reactions

In substitution reactions, a hydrogen of a hydrocarbon is replaced by an atom or a group of atoms. For example, hydrogen is replaced by a halogen in halogenation.

Halogenation

When the mixture of hydrocarbon and halogen is heated at 520-670 K in dark or is subjected to ultraviolet light at room temperature, the free radical substitution reaction takes place.

Preparation of haloalkanes from hydrocarbons by free radical halogenation

The reactivity of hydrogen towards free radical substitution is 3° > 2° > 1°. For example, when butane is used in free radical substitution reaction, the products we get are 1-Chlorobutane and 2-Chlorobutane. 2-Chlorobutane being 2° is the major product.

Preparation of 2-chlorobutane from hydrocarbons by free radical halogenation

The rate of reaction of alkanes with halogens follows the order : F2 > Cl2 > Br2 > I2. Fluorination of alkanes is too vigorous to be controlled under normal conditions while iodination is very slow and a reversible reaction.

Mechanism of free radical substitution reaction

Free radical substitution reaction is a chain reaction. Let's take the example of CH4 and Cl2 to understand the mechanism. The free radical substitution reaction consists of the following steps :

Initiation step

When a mixture of CH4 and Cl2 is heated at 520-670 K or is subjected to ultraviolet light at room temperature, Cl2 absorbs energy and undergoes homolytic fission.

Mechanism of free radical substitution : initiation step
Propagation step

Propagation step consists of two substeps

  1. The free radical of chlorine (formed in initiation step) attacks the CH4 molecule and removes a hydrogen atom from CH4 forming CH3 and HCl.
    Mechanism of free radical substitution : Propagation step, substep 1
  2. CH3 thus produced, reacts with a molecule of Cl2 which results in the formation of methyl chloride.
    Mechanism of free radical substitution : Propagation step, substep 2
  3. This reaction continues until all the hydrogen atoms of methane are replaced by halogen atoms.
    Formation of Chain reaction in free radical substitution
Termination step

The chain reaction may terminate if two of the same or different free radicals combine among themselves without producing new free radicals.

Free radical substitution termination step : Cl radicals form Cl2
Free radical substitution termination step : CH3 radicals form CH3-CH3
Free radical substitution termination step : Cl and CH3 radicals form CH3Cl

Oxidation of Alkanes

Some important oxidation reactions of alkanes are given below :

Combustion of Alkanes

Alkanes on heating in the presence of air or O2 produce CO2 and H2O with the evolution of large amount of heat.

Reactions of alkanes, combustion

The general formula for combustion is :

General formula for combustion

Combustion of ethane :

Combustion of ethane

Incomplete Combustion of Alkanes

If combustion is carried out with insufficient amount of air or O2, CO is usually formed.

incomplete combustion of alkanes

Carbon black, which is used in the manufacture of ink, paints, polishes etc., is prepared by incomplete combustion.

Incomplete combustion, formation of carbon black

Controlled Oxidation of Alkanes

Alkanes on heating with a regulated supply of air or O2 in the presence of suitable catalyst give different products under different conditions.

Controlled oxidation of alkanes
Reactions of alkanes, controlled oxidation
Controlled oxidation of alkanes

Alkanes usually resist oxidation with oxidising agents such as KMnO4, K2Cr2O7 etc. However, alkanes containing tertiary hydrogen can be oxidised to corresponding alcohols.

Oxidation of Alkanes using potassium permangnate or potassium dichromate

Isomerisation of Alkanes

Straight chain alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas are converted to their branched chain isomers.

Isomerisation of alkanes

Aromatization of Alkanes

Straight chain alkanes containing six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of a catalyst consisting of oxides of chromium, vanadium and molybdenum supported over alumina get dehydrogenated and converted into benzene and its homologues.

Aromatization of alkanes

Pyrolysis of Alkanes

Higher alkanes on heating to higher temperature decompose into lower hydrocarbons (alkanes, alkenes etc.). Such a decomposition is known as pyrolysis or cracking.

Pyrolysis of alkanes

 

Revision includes earlier concepts

Let's do some revision. The reactions used in the answers are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.

Conversion : Prepare chloroethane from ethene.

Conversion : Prepare ethanoic acid from methyl bromide.

Remember these reactions. We will need them to do conversions in later chapters.

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