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Wohl–Ziegler bromination

Type of chemical reaction

Type of chemical reaction

Karl Ziegler The Wohl–Ziegler reaction is a chemical reaction that involves the allylic or benzylic bromination of hydrocarbons using an N-bromosuccinimide and a radical initiator.

:[[Image:NBS allylic bromination scheme.svg|400px|The Wohl-Ziegler reaction]]

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. 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.

The corresponding chlorination reaction cannot generally be achieved with *N-*chlorosuccinimide, although more specialized reagents have been developed, and the reaction can be achieved industrially with chlorine gas.

Mechanism

The Wohl–Ziegler reaction proceeds through a mechanism first proposed by Paul Goldfinger in 1953. An earlier mechanism proposed by George Bloomfield, though consistent with selectivity studies, proved overly simplistic.

:[[Image:Goldfinger and Bloomfield Mechanisms.png|600px|Goldfinger and Bloomfield mechanisms]]

The key puzzle in mechanizing the Wohl–Ziegler reaction is the role of the succinimide moiety. Bloomfield's mechanism required direct NBS radicals. But the N–Br bond has dissociation energy much larger than that for Br2,

Goldfinger instead explains the necessity of succinimide through competing addition and substitution pathways. :[[Image:Addition vs. Substitution.png|600px|Dissects competing reaction pathways for radical mechanisms]] 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 : ;Low bromine concentrations : where is the ratio of addition to substitution, and the k values correspond to the rate constant for the labeled reaction step.

The desired bromination is the substitution product. As the above equations indicate, addition is suppressed as [Br2] decreases.

References

References

  1. [[Alfred Wohl]]. (1919). "Bromierung ungesättigter Verbindungen mit N-Brom-acetamid, ein Beitrag zur Lehre vom Verlauf chemischer Vorgänge". [[Berichte der deutschen chemischen Gesellschaft]].
  2. [[Karl Ziegler. (1942). "Die Synthese des Cantharidins". [[Justus Liebig's Annalen der Chemie]].
  3. (1963). "4-Bromo-2-heptene".
  4. C. Djerassi. (1948). "Brominations with N-Bromosuccinimide and Related Compounds. The Wohl–Ziegler Reaction". [[Chem. Rev.]].
  5. (1959). "Neuere Methoden der präparativen organischen Chemie II 14. N-Bromsuccinimid, Eigenschaften und Reaktionsweisen Studien zum Ablauf der Substitution XV". [[Angew. Chem.]].
  6. (June 2009). "Benzylic Brominations with N-Bromosuccinimide in (Trifluoromethyl)benzene". Synthesis.
  7. Djerassi, Carl.. (1948-10-01). "Brominations with N-Bromosuccinimide and Related Compounds. The Wohl-Ziegler Reaction.". Chemical Reviews.
  8. (1967). "Olefinreaktionen, I. Chlorierung in Allyl-Stellung". Justus Liebigs Annalen der Chemie.
  9. (2000). "Allyl Compounds". American Cancer Society.
  10. (1970). "N-Bromosuccinimide. Mechanisms of Allylic Bromination and Related Reactions". J. Am. Chem. Soc..
  11. (1944). "Rubber, Polyisoprenes, and Allied Compounds. Part VI. The Mechanism of Halogen-substitution Reactions, and the Additive Halogenation of Rubber and Dihydromyrcene". J. Am. Chem. Soc..
  12. (1974). "Free Radical Chemistry: Structure and Mechanism". Cambridge University Press.
  13. Neuman, R.C.. (1992). "Organic Chemistry". Robert C. Neuman, Jr..
  14. (1953). "Laws of Addition and Substitution in Atomic Reactions of Halogens". Nature.
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