Wohl-Ziegler process

Hydrogen atoms in the benzylic position can be replaced by elemental bromine as shown. This is not true for hydrogen atoms in the allylic position. The alkene reacts rapidly with molecular bromine via addition and allylic bromination is not observed (Figure 1.25, left). A chemoselective allylic bromination of alkenes succeeds only according to the Wohl-Ziegler process (Figure 1.25, right), that is, with A-bromosuccinimide (NBS). 

The difference is that in the Wohl-Ziegler process there is always a much lower Br2 concentration than in the reaction of cyclohexene with bromine itself. Figure 1.27 shows qualitatively how the Br2 concentration controls whether the combined effect of Br/Br2 on cyclohexene is an addition or a substitution. The critical factor is that the addition takes place via a reversible step and the substitution does not. During the addition, a bromocyclohexyl radical forms from cyclohexene and Br in an equilibrium reaction. This radical is intercepted by forming dibromocyclohexane only when a high concentration of Br2 is present. However, if the concentration of Br2 is low, this reaction does not take place. The bromocyclohexyl radical is then produced only in an unproductive equilibrium reaction. In this case, the irreversible substitution therefore determines the course of the reaction. 

Fig. 1.24. Mechanism for the allylic bromination of cyclohexene according to the Wohl-Ziegler process. 

The substitution of a hydrogen atom in the benzylic position by a bromine atom on reaction with N-bromosuccinimide in the presence of catalytic amounts of AIBN (2.37) is known as the Wohl-Ziegler process k... 

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