Question

How does `C-C` coupling arise in the radical halogenation of toluene by chlorine gas?

Answer 1

I confess I am not really sure what you ask.

Explanation:

Free radical halogenation TYPICALLY gives `C-C` coupled products in the reaction mixture; i.e. for the free-radical halogenation of toluene...........

`X-X + hnu rarr 2dotX`

Radicals are exceptionally reactive species, because in their elementary reactions, they generate ANOTHER radical to continue, i.e. `"to propagate"` the reaction chain.

`dotX+H_4C rarr H-X + dotCH_3`

And the benzyl radical continues the chain.......

`X_2 + dotCH_3 rarr dot X+XCH_3`

The radical continues until most of the unreacted methane/toluene has substituted to give benzyl halides. At the start of the reaction, clearly the unreacted hydrocarbon is the species in excess, and thus most likely to react with a radical. Towards the end of the of the reaction, the process `"terminates"` when 2 radical species couple, i.e.:

`2dotXrarrX_2`

or `dotX + H_2dotCPh rarr XH_2CPh`

or `2H_2dotC-PhrarrPh-CH_2CH_2-Ph`

likewise, `H_3dotC + dotCH_3 rarr H_3C-CH_3`.

The presence of such `C-C` coupled products in the product mixture is good evidence of the intermediacy of radical species. `C-C` bond formation reactions are exceptionally rare and difficult to accomplish. Nevertheless, the major product is the hydrocarbyl halide.