Does radical halogenation only occur in alkanes?

1 Answer
Aug 12, 2015

No, radical halogenation also occurs in alkenes.


But there are differences.

If you use #"Br"_2#, you get addition instead of substitution.

Instead, you use NBS (N-bromosuccinimide) in #"CCl"_4#.

NBS is insoluble in #"CCl"_4#, but the suspended solid reacts with trace amounts of #"HBr"# to produce a low concentration of bromine.

Then the bromine molecules are homolytically cleaved by light to produce the bromine radicals that initiate the reaction.

The second difference is that the substitution occurs almost exclusively at the allylic position.

Allylic bromination is the only practical method for laboratory free radical halogenation.

But allylic chlorination with low concentrations is important in industry, where the reaction is carried out at high temperature.