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#1 2011-07-09 12:24:12

orgopete
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Why can bromine radicals give addition or allylic bromination?

This is a very good question. The addition and allylic bromination reaction occur from a first formed bromine radical. The radical adds, but presumably this addition is a reversible reaction. In the presence of HBr, the radical is quenched by the HBr and reforms a bromine radical.

If the same reaction were to take place in which the HBr concentration is low, the initial addition reverses and a slower hydrogen abstraction takes place with generation of HBr. If the allylic bromination reaction is catalyzed by NBS, the most common catalyst, the HBr reacts with the NBS to make succinimide and Br2. The allylic radical can undergo a propagation step to form an allylic bromide and another bromine radical.

The following is excerpted from A Handbook of Organic Chemistry Mechanisms, available at http://www.curvedarrowpress.com/ or Amazon.com
     http://www.curvedarrow.com/chem/ZZ32EF61D3.jpg

A similar explanation is present in the Appendix of A Guide to Organic Chemistry Mechanisms, http://www.curvedarrowpress.com, or Amazon.com

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