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What is the mechanism of halogenations of alkanes?

Answer
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Hint: Halogenation of alkanes means the substitution of a halogen atom(s) by the removal of one or more hydrogen atoms in the alkane. The mechanism of halogenations occurs in three steps: chain initiation, chain propagation, and chain termination.

Complete step by step answer:
When alkane is treated with a suitable halogen in the presence of ultraviolet light or by heating the reaction mixture to 520-670 K, haloalkane is produced.
For example, chlorination of methane. The reaction is given below:
CH4methane+Cl2hvCH3Clchloromethane+HCl
CH3Clchloromethane+Cl2hvCH2Cl2dichloromethane+HCl
CH2Cl2dichloromethane+Cl2hvCHCl3trichloromethane+HCl
CHCl3trichloromethane+Cl2hvCCl4tetrachloromethane+HCl

Mechanism of halogenations: The halogenations occur in three steps and it follows a free-radical mechanism.

(a)- Chain initiation: When a mixture of CH4and Cl2 is heated at 520-670 K in dark or is subjected to UV light at room temperature, Cl2 absorbs energy and undergoes homolytic fission which produces chlorine-free radicals.
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(b)- Chain propagation: There are two steps in propagation. In the first reaction, the Cl attacks the CH4 molecule and abstracts a hydrogen atom forming CH3 and a molecule of HClas shown in reaction. In the second reaction, CH3 thus produced reacts further with a molecule of Cl2 forming a molecule of methyl chloride and another Cl. These reactions continue until the formation of CCl4.
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CH3Cl+ClCH2Cl+HCl
CH2Cl+Cl2CH2Cl2+Cl
CH2Cl2+ClCHCl2+HCl
CHCl2+Cl2CHCl3+Cl
CHCl3+ClCCl3+HCl
CCl3+Cl2CCl4+Cl

(c)- Chain termination: The chain reactions till now formed to have two types of free radicals combine to form molecules. The reaction is given below:
Cl+ClClCl
CH3+CH3CH3CH3
CH3+ClCH3Cl
 So, by following these steps the halogenations of an alkane are done.

Note: The order of reactivity of the halogens towards the halogenations reaction of alkanes as follows: F2 > Cl2 > Br2 > I2.The iodination reaction is reversible as follows:
CH4+I2CH3I+HI.