
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:
Mechanism of halogenations: The halogenations occur in three steps and it follows a free-radical mechanism.
(a)- Chain initiation: When a mixture of and is heated at 520-670 K in dark or is subjected to UV light at room temperature, absorbs energy and undergoes homolytic fission which produces chlorine-free radicals.
(b)- Chain propagation: There are two steps in propagation. In the first reaction, the attacks the molecule and abstracts a hydrogen atom forming and a molecule of as shown in reaction. In the second reaction, thus produced reacts further with a molecule of forming a molecule of methyl chloride and another . These reactions continue until the formation of .
(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:
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: > > > .The iodination reaction is reversible as follows:
.
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:
Mechanism of halogenations: The halogenations occur in three steps and it follows a free-radical mechanism.
(a)- Chain initiation: When a mixture of

(b)- Chain propagation: There are two steps in propagation. In the first reaction, the

(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:
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:
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