Wohl-Ziegler reaction
| Wohl-Ziegler bromination | |
|---|---|
| Named after |
Alfred Wohl Karl Ziegler |
| Reaction type | Substitution reaction |
| Identifiers | |
| Organic Chemistry Portal | wohl-ziegler-reaction |
| RSC ontology ID | RXNO:0000225 |
The Wohl–Ziegler reaction is a chemical reaction that involves the allylic or benzylic bromination of hydrocarbons using an N-bromoimide and a radical initiator.
Best yields are achieved with N-bromosuccinimide in carbon tetrachloride solvent. Several reviews have been published.
In a typical setup a stoichiometric amount of N-bromosuccinimide solution and a small quantity of initiator are added to a solution of the substrate in CCl4, and the reaction mixture is stirred and heated to the boiling point. Initiation of the reaction is indicated by more vigorous boiling; sometimes the heat source may need to be removed. Once all N-bromosuccinimide (which is denser than the solvent) has been converted to succinimide (which floats on top) the reaction has finished.
The mechanism by which the Wohl-Ziegler reaction proceeds was proposed by Paul Goldfinger in 1953, and his reaction mechanism is one of two proposed pathways through which aliphatic, allylic, and benzylic bromination with N-bromosuccinimide (NBS) occurs. It has been shown that the Goldfinger mechanism is the proper mechanism as opposed to the previously accepted mechanism proposed by George Bloomfield, which, though consistent during selectivity studies, turned out to be overly simplistic.
The generation of NBS radicals depicted in the Bloomfield mechanism has been shown to be far more difficult than imagined when it was proposed, which is why it has failed as a proper model throughout the years; however, evidence suggests that the Bloomfield mechanism is still acceptable for the oxidation of alcohols using NBS. It should be noted that in the Goldfinger mechanism, the purpose of the NBS is simply to maintain a very low concentration of molecular bromine, while in the Bloomfield mechanism, its purpose is the generation of the initial radical used in the reaction, which again can be quite a difficult process. This is because it requires a special consideration for the behavior of the NBS radical; the only way it can possibly function as proposed in Bloomfield's mechanism is if the dissociation energy for the N-Br bond in NBS is smaller than that for Br2, and much evidence has been seen to suggest contrary behavior. Goldfinger's proposed mechanism does not require any special considerations, as all radical species are behaving normally, and it is partly because of this that his mechanism is regarded as correct.
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