[davidenarb]

Hi !

A free radical bromination can take place ONLY (why is that?) at the benzylic position producing a benzylic halide in the presence of NBS and heat.

In the figure bellow, we have the resonance structures of a benzylic system.

When an methylbenzene react with NBS and heat, why do we have ONLY one product, which is the product at the benzylic position, and the remaining 3 other products are not produced since we have 4 resonance structures?

Is it because of this reason? : "if the other allylic products are produced  the aromaticity of the benzene will be destroyed"

I have a second question: the bromination won't occur if the benzylic position lacks proton, right?

[discodermolide]

The benzylic radical and the benzylic cation also are quite stable, and with the correct substituents they can be isolated.
The resonance contribution, as you have shown, from the other radicals is small as you correctly say addition there will initially destroy the aromatic system. If you do this reaction on a large enough scale (kg) you can sometimes see the ring brominated products in very small quantities (<0.5%).

[davidenarb]

The benzylic radical and the benzylic cation also are quite stable, and with the correct substituents they can be isolated.
The resonance contribution, as you have shown, from the other radicals is small as you correctly say addition there will initially destroy the aromatic system. If you do this reaction on a large enough scale (kg) you can sometimes see the ring brominated products in very small quantities (<0.5%).

Thank you for your confirmation!!!

Just to ask another quick question: the bromination won't occur if the benzylic position doesn't have any proton, correct?

[discodermolide]

You need something that can form a radical. If it's like t-Butylbenzene then you won't get benzylic bromination.
But you may form other radicals which can re-arrange to give a multitude of products.

[davidenarb]

You need something that can form a radical. If it's like t-Butylbenzene then you won't get benzylic bromination.
But you may form other radicals which can re-arrange to give a multitude of products.

Thank you very much