Topscore 1 Ochem question

[sherry225]

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Hey guys,

For the last question on Topscore 1(#100) where a toluene reacted with Br2 in a radical reaction. The answer has the Br attached to the methyl instead of the benzene ring. I thought radical reactions always do an attack at the hydrogen on the carbob atom that can form the most stable free radical. So the most substituted carbon is a better choice. With this reasoning, I pick the answer that has the Br attached to benzene ring.........(but of course, it's wrong) Am I missing something in my reasoning?


Thanks for the help!

 

[jheidenr]

The benzene ring is resonance stabilized and very unreactive under most conditions. That is why the radical Br* takes the hydrogen from the methyl group instead of the aromatic hydrogen.

 

[sherry225]

even if the methyl group is electron donating, that won't make the benzene ring more reactive?

 

[jheidenr]

Yes. The only way to halogenate a benzene via electrophilic aromatic substitution would require the presence of a Lewis acid. In the case of bromination, you need Br2 and FeBr3.

 

[sherry225]

Thanks so much!