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?

1 Answer
anor277
Sep 7, 2016

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.

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