Question

What is the halogenation of alkanes?

Answer 1

A way to introduce functionality into an otherwise unreactive alkyl chain.

Explanation:

Alkanes undergo radical halogenation, with the radicals generated by UV light:

`Br_2 + hnu rarr 2Br*`

`Br*` is a neutral bromine radical. Radical species are quite reactive in that in the act of reaction they generate another radical, that continue the chain of reactivity. UV light is known to produce radicals from bromine molecules. This is the propagation step.

`Br* + CH_4rarr H-Br + *CH_3`

The methyl radical is a very reactive species, and can react with bromine to give the product:

`H_3C* + Br_2 rarr H_3C-Br + *Br`

The termination step involves the coupling of two radicals:

`H_3C* + *Br rarr H_3C-Br`

Or two methyl radicals can couple:

`H_3C* + *CH_3 rarr H_3C-CH_3`

The observation of ethane where a `C-C` has been formed is good evidence to support the radical mechanism.

Overall the rxn is:

`CH_4 + Br_2 rarr CH_3Br + HBr`