Wohl-Ziegler reaction

The Wohl-Ziegler reaction (after Alfred Wohl and Karl Ziegler ) is a radical bromination of alkenes in the allyl position . Since the bromination of an alkene leads to electrophilic addition at the double bond, only small amounts of bromine may be present. This is achieved by a comproportionation with N -bromo succinimide (NBS). The reaction proceeds according to the mechanism of radical substitution and is carried out in non-polar solvents , for example carbon tetrachloride . The solvent has the advantage that the succinimide formed is not soluble in it and can therefore be easily separated off. If the reaction is carried out in polar solvents, normal bromine addition occurs.

Gross reaction using the example of cyclohexene :

The reaction can be initiated either with the help of a radical initiator or by UV radiation . As the radical initiator may azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO) or tert -butyl hydroperoxide are used. NBS usually contains traces of bromine, which is what causes the slightly brown color of the salt. These bromine molecules are important for starting the reaction, which is why they should not be removed by recrystallization .

Reaction mechanism

Reaction mechanism using the example of cyclohexene .

Start reaction:

As a starting reaction, the radical initiator (e.g. AIBN) splits the molecular bromine, which must be present in impurities in the NBS, into two bromine radicals.

Chain propagation:

1) H abstraction on the educt

2) Formation of bromine and succinimide by comproportioning the bromine

3) Formation of the product

literature

• Reinhard Brückner, reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods, spectrum textbook, Heidelberg / Berlin 1996 1st edition, ISBN 3-86025-363-8