Allylic Halogenation

Allylic Halogenation

Recall that when alkenes are subjected to high concentrations of diatomic halogens (Br₂ and Cl₂) addition normally occurs.  However when the halogen concentration is low and a radical initiator (hv or ROOR) is present, substitution at an allylic hydrogen occurs.

Notice that both C1 and C3 were each brominated and we obtain a 50:50 mix of each product.  In this case it doesn't make a difference sinc ethey are the same product, but this could be problematic in cases were distinctly different allylic positions are possible.

This reaction proceeds by a radical mechanism with three different steps 1) Initiation, propagation and termination.

Role of NBS

The role of NBS in allylic brominations is to provide a bromine atom (radical) which occurs in the presense of a radical initiator (hv or ROOR).  In the absence of a radical initiator and in the presence f small amounts of HBr (present in NBS) NBS generates diatomic bromine molecule. 

Practice Problems

1)  Complete the following reactions.

2)  Predict the product of the allylic bromination reaction of  sec-butylbenzene. (Hint: How are benzylic hydrogens similar to allylic hydrogens?)

3)  Cyclopentene undergoes radical allylic bromination. Write out the complete mechanism including reactants, intermediates and products.

4) What conditions are required for allylic halogenation to be successful? Why does allylic bromination occur instead of  bromine addition to alkene?

5) Explain why allylic bromination of 2-pentene is not a very effective strategy in a synthesis?