Hell–Volhard–Zelinsky halogenation
Hell–Volhard–Zelinsky halogenation | |
---|---|
Named after | Carl Magnus von Hell Jacob Volhard Nikolay Zelinsky |
Reaction type | Substitution reaction |
Identifiers | |
Organic Chemistry Portal | hell-volhard-zelinsky-reaction |
The Hell–Volhard–Zelinsky halogenation reaction is a chemical transformation that involves the halogenation of a carboxylic acid at the
An example of the Hell–Volhard–Zelinsky reaction can be seen in the preparation of alanine from propionic acid. In the first step, a combination of bromine and phosphorus tribromide (catalyst) is used in the Hell–Volhard–Zelinsky reaction to prepare 2-bromopropanoic acid,[5] which in the second step is converted to a racemic mixture of the amino acid product by ammonolysis.[6][7]
Mechanism[edit]
The reaction is initiated by addition of a catalytic amount of PBr3, after which one molar equivalent of Br2 is added.
PBr3 replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. The acyl bromide tautomerizes to an enol, which reacts with the Br2 to brominate at the
In neutral to slightly acidic aqueous solution, hydrolysis of the
If little nucleophilic solvent is present, reaction of the
The mechanism for the exchange between an alkanoyl bromide and a carboxylic acid is below.
The
By quenching the reaction with an alcohol, instead of water, the
See also[edit]
References[edit]
- ^ von Hell, Carl Magnus (1881). "Ueber eine neue Bromirungsmethode organischer Säuren" [About a new bromination method for organic acids]. Berichte (in German). 14: 891–893. doi:10.1002/cber.188101401187.
- ^ Volhard, Jacob (1887). "Ueber Darstellung
α -bromirter Säuren" [On the Representation ofα -Brominated Acids]. Annalen der Chemie (in German). 242 (1–2): 141–163. doi:10.1002/jlac.18872420107. - ^ Zelinsky, Nikolay (1887). "Ueber eine bequeme Darstellungsweise von
α -Brompropionsäureester" [About a convenient representation of an ester ofα -bromopropionic acid]. Berichte (in German). 20: 2026. doi:10.1002/cber.188702001452. - ^ Allen, C. Freeman; Kalm, Max J. (1958). "2-Methylenedodecanoic Acid". Organic Syntheses. 38: 47. doi:10.15227/orgsyn.038.0047; Collected Volumes, vol. 4, p. 616.
- ^ Marvel, C. S.; du Vigneaud, V. (1931). "
α -Bromoisovaleric acid". Organic Syntheses. 11: 20. doi:10.15227/orgsyn.011.0020; Collected Volumes, vol. 2, p. 93. - ^ Tobie, Walter C.; Ayres, Gilbert B. (1937). "Synthesis of d,l-Alanine in Improved Yield from
α -Bromopropionic Acid and Aqueous Ammonia". Journal of the American Chemical Society. 59 (5): 950. doi:10.1021/ja01284a510. - ^ Tobie, Walter C.; Ayres, Gilbert B. (1941). "dl-Alanine". Organic Syntheses. doi:10.15227/orgsyn.009.0004; Collected Volumes, vol. 1, p. 21.