Chemical Properties of Alkanes - Practice Questions & MCQ

Halogenation
When alkanes are treated with halogens in presence of light or at elevated temperatures, the hydrogen atoms of alkanes are successively replaced by halogen atoms. This process is known as halogenation. The rate of reaction of alkanes with halogen follows the following order:
F₂ > Cl₂ > Br₂ > I₂ 
This reaction is carried out with chlorine as fluorine is too violent to be controlled and iodine is too slow and reversible.
The mechanism of this reaction occurs by a free radical mechanism and it is done in three successive steps as follows:

1. Chain initiation step: 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.
   
2. Chain propagation step: 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.
   
3. Chain termination step: 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:
   • 
   • 
   • 

The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.

Nitration
Nitration is a substitution reaction in which a hydrogen atom of alkane is replaced by nitro(-NO₂) group. The reaction occurs as follows:

Lower members do not react with concentrated nitric acid at ordinary temperatures but long-chain members on heating with fuming nitric acid yield nitroalkanes. However, when a mixture of vapours of an alkane and nitric acid is heated at 673-773K, nitroalkane is formed readily. This is known as vapour phase nitration. By this process, lower, as well as higher alkanes, can be converted into nitroalkanes.

Sulphonation
The replacement of hydrogen atom by sulphonic cid group(-SO₃H) is known as sulphonation. Lower alkanes do not undergo sulphonation but higher members (from hexane onwards) are sulphonated slowly when treated with fuming sulphuric acid. at about 673K. The reaction occurs as follows:

For example:

However, lower members such as propane, butane, pentane, etc. react with SO₃ in vapour phase to form sulphonic acids.

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.

Due to the production of large amount of heat, alkanes are used as fuels.

Catalytic oxidation
Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products. The reactions occur as follows:

Isomerisation
n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched-chain alkanes. Major products are given below. Some minor products are also possible which you can think over. Minor products are generally not reported in organic reactions.

Aromatization
n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as aromatization or reforming.

Pyrolysis
The higher alkanes split into lower alkanes when heated strongly at a high temperature in the absence of air. During pyrolysis, C-C bond breaks rather than C-H bonds as bond energy of C-H > C-C. Here product formation depends upon the structure of alkane, the extent of temperature and pressure and presence/absence of catalysts like SiO₂-Al₂O₃ etc. 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                   Heptane        Pentene