Abstract Sheet

Important Concepts from Chapter 12

Halogenation occurs on carbons attached to a double bond (i.e. at the allylic position) under certain
conditions.

C

C

C

H

H

H

Allylic hydrogens - can be replaced by halogens

Two methods of allylic halogenation were discussed:
1. Cl2and heat gives chlorination
2. NBS (N-bromosuccinimide) gives bromination

Examples:

Cl

Cl2
400oC

and

Br

NBS
CCl4, hv

Important points to remember concerning allylic halogenation:

1. A radical mechanism is involved. The steps are the same for any radical mechanism: Chain
Initiation, Chain Propagation and Chain Termination
The steps are:1. Initiation forms X.(where X is either Br or Cl)
2. Chain propagation involves:
a. Abstraction by the halogen radical (X.) of an allylic Hydrogen
(because this is the weakest C-H bond) to give an allylic radical
and
b. Abstraction by the allylic radical of X from X2to give the product
(an allylic halide) and X.to carry on the chain

2. A delocalized allylic radical intermediate is formed - the resonance structures of this intermediate
tells us which atoms have radical character and therefore are capable of abstracting a halogen atom
from X2.

The Bottom Line:
You must know the mechanism of allylic halogenation and be able to predict all the possible
products.

The steps involved in predicting allthe products of allylic halogenation are:
1. Identify all the allylic hydrogens of the molecule.
2. Abstract the allylic hydrogens to form allylic radicals, then draw all their resonance structures.
3. Put halogen atoms at all carbons which have a radical on them.