Question

How do you write a stepwise mechanism that shows how a small amount of `CH_3CH_3` could form during the bromination of #CH_4?

Answer 1

Bromination proceeds by a radical mechanism, in which `Br*` radicals are invoked. So let's see.

Explanation:

`"Initiation:"` `Br_2 +hnu rarr2Br*`

`"Propagation:"` `Br* +CH_4 rarr*CH_3 + H-Br`

`Br_2 +*CH_3 rarr BrCH_3 + Br*`

`"Termination:"` `Br* + Br* rarr Br_2`, i.e. the coupling of 2 radicals could occur, or the coupling of hydrocarbyl radicals made in the propagation step:

`H_3C*+*CH_3 rarr H_3C-CH_3`

And thus the coupling of 2 methyl radicals results in `C-C` bond formation.

Overall, the bromination rxn is:

`CH_4 + Br_2 rarr CH_3Br + H-Br`

The discovery of ethane in the reaction mixture is good evidence for the radical reaction as shown.