Wohl–Ziegler bromination

The Wohl-Ziegler reaction (after Alfred Wohl and Karl Ziegler ) represents a radical bromination of alkenes into allylic dar. Since the bromination of an alkene to electrophilic addition leads to the double bond, only small amounts of bromine may be present. This is achieved by a comproportionation with N-bromosuccinimide ( NBS). The reaction proceeds according to the mechanism of free radical substitution, and is carried out in nonpolar solvents, for example, tetrachloromethane. The solvent has the advantage that the resulting succinimide is not soluble therein, and thus can be easily separated. The reaction is carried out in polar solvents, there is a normal addition of bromine.

Overall reaction by the example of the cyclohexene:

The reaction can be initiated either by means of a free radical initiator or by UV radiation. As a free radical initiator may be used azobisisobutyronitrile (AIBN), dibenzoyl peroxide ( BPO ) and tert -butyl hydroperoxide. NBS usually contains traces of bromine, thereby slightly brownish color of the salt takes place. This bromine molecules are important for the start of the reaction, which is why they should not be removed by recrystallization.

Reaction mechanism

Reaction mechanism on the example of the cyclohexene.

Start reaction:

1) decomposition of AIBN

2) formation of the bromine radical

Kettenpropagation:

1) H - abstraction at the reactant

2) formation of bromine and succinimide by comproportionation of bromine

3) Formation of the product