Hell-Volhard-Zelinsky reaction

The Hell-Volhard-Zelinsky involves the halogenation of carboxylic acids at the alpha carbon of the carbonyl group.  The basic reaction reacts as such:

The first step in the mechanism of this reaction involves the substitution of a bromide on the hydroxyl(OH) group of the carboxylic acid.  The catalyst PBr3 causes this bromide to replace the OH group which causes the formation of the carboxylic acid bromide.  The present acyl bromide is then tautomerized to form an enol which reacts with the Br2 which creates the second placement of a bromide on the compound.  The following depicts the mechanism of this reaction:

An example of a synthesis where this Hell-Volhard-Zelinsky mechanism is used is in the formation of dimethylketene.  In this synthesis the starting reactant is isobutyric acid which then reacts with PBr2 to form the product of propanoyl bromide or a-bromoisobutyryl bromide.  This product is then reacted with zinc and some heat and results in the overall final product of dimethylketene.  The Hell-Volhard-Zelinsky mechanism is used in the first step of the reaction.  The steps in this reaction are:

The Hell-Volhard-Zelinsky reaction of a halogenation of carboxylic acids at the alpha carbon is named after three chemists.  Their names are Carl Magnus von Hell ( German chemist), Jacob Volhard (German chemist) and Nikolay Zelinsky who was a Russian chemist.  This reaction is being discussed in the Organic Chemistry 2 class at Campbellsville University.


Sources:
1.  "Hell-Volhard-Zelinsky Halogenation." Wikipedia, the Free Encyclopedia. Web. 21 Apr. 2011. http://en.wikipedia.org/wiki/Hell-Volhard-Zelinsky_halogenation.
2.  Organic Syntheses Website. Web. 21 Apr. 2011. http://orgsyn.org/orgsyn/default.asp?formgroup=basenpe_form_group.