What are the steps of radical halogenation of alkanes?

1 Answer
anor277
Nov 7, 2016

i. Initiation. ii. Propagation. iii. Termination

Explanation:

Initiation:

Cl2+hν→2Cl⋅

The hν represents UV light, the radical halide can abstract an hydrogen to make the carbon radical:

Cl⋅+H3C−CH3→⋅CH2−CH3+H−Cl

So we have made a ethyl radical, which serves to propagate the reaction by subsequent reaction with UNREACTED halogen. This serves to propagate the reaction:

Propagation:

H3C−CH2⋅+Cl−Cl→ClCH2−CH3+Cl⋅

And thus by making a C−Cl bond, another radical species is generated, which thus serves to propagate the reaction. Thus the radical halogenations tend to take off.

Termination:

The reaction is terminated when two radicals couple to close down the chain mechanism:

H3C−CH2⋅+⋅Cl→H3C−CH2−Cl

OR Cl⋅+⋅Cl→Cl−Cl

OR THE COUPLING OF TWO ETHYL RADICALS:

H3C−CH2⋅+⋅H2C−CH3→H3C−CH2CH2CH3

The appearance of small quantities of butane in the product mix is excellent evidence for the intermediacy of ethyl radicals in the reaction mechanism.

The net reaction is:

C2H6+Cl2→H3C−CH2Cl+HCl

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