Reactions of Alkanes
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.
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.
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.
Propagation step
Propagation step consists of two substeps
- The free radical of chlorine (formed in initiation step) attacks the CH4 molecule and removes a hydrogen atom
from CH4 forming ∙CH3 and HCl.
- ∙CH3 thus produced, reacts with a molecule of Cl2 which results in the formation of methyl chloride.
- This reaction continues until all the hydrogen atoms of methane are replaced by halogen atoms.
Termination step
The chain reaction may terminate if two of the same or different free radicals combine among themselves without producing new free radicals.
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.
The general formula for combustion is :
Combustion of ethane :
Incomplete Combustion of Alkanes
If combustion is carried out with insufficient amount of air or O2, CO is usually formed.
Carbon black, which is used in the manufacture of ink, paints, polishes etc., is prepared by incomplete combustion.
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.
Alkanes usually resist oxidation with oxidising agents such as KMnO4, K2Cr2O7 etc. However, alkanes containing tertiary hydrogen can be oxidised to corresponding alcohols.
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.
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.
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.
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.