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Wohl–Ziegler bromination
Type of chemical reaction From Wikipedia, the free encyclopedia
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The Wohl–Ziegler reaction[1][2] is a chemical reaction that involves the allylic or benzylic bromination of hydrocarbons using an N-bromosuccinimide and a radical initiator.[3]
Best yields are achieved with N-bromosuccinimide in carbon tetrachloride solvent. Several reviews have been published.[4][5]
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. Due to the high toxicity and ozone-depleting nature of carbon tetrachloride, trifluorotoluene has been proposed as an alternative solvent suitable for the Wohl–Ziegler bromination.[6]
The corresponding chlorination reaction cannot generally be achieved with N-chlorosuccinimide,[7] although more specialized reagents have been developed,[8] and the reaction can be achieved industrially with chlorine gas.[9]
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Mechanism
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The Wohl–Ziegler reaction proceeds through a mechanism first proposed by Paul Goldfinger in 1953.[10][11] An earlier mechanism proposed by George Bloomfield, though consistent with selectivity studies, proved overly simplistic.[10]
The key puzzle in mechanizing the Wohl–Ziegler reaction is the role of the succinimide moiety. Bloomfield's mechanism required direct NBS radicals.[12] But the N–Br bond has dissociation energy much larger than that for Br2,[11][13] and rarely homolyzes like Bloomfield expected.[10][11]
Goldfinger instead explains the necessity of succinimide through competing addition and substitution pathways.[13] These pathways apply to almost all radical reactions, and a generic depiction (including side-reactions 6 and 8) is as follows:[14]
Relative rate laws describing each pathway depend strongly on the molecular bromine concentration. The limiting cases of high and low concentration are:
- High bromine concentrations
- ra/rs = k2a/k2s(1 + k4a/k3a[Br2])
- Low bromine concentrations
- ra/rs = k2a/k2sk3a/k4a[Br2]
where ra/rs is the ratio of addition to substitution, and the k values correspond to the rate constant for the labeled reaction step.[13]
The desired bromination is the substitution product. As the above equations indicate, addition is suppressed as [Br2] decreases.[13] Goldfinger thus concludes that as NBS acts primarily as a bromine sink, promoting substitution through a very low Br2 concentration.[13][10]
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References
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