Hell-Volhard-Zelinsky halogenation

The Hell-Volhard-Zelinsky halogenation mechanism halogeantes carboxylic acids at the alpha carbon. In this reaction it takes place in the absence of a halogen carrier. From here the reaction can be initiatied by the addition of a catalytic amount of either PCl3, PBr3, red phosphorous, or Br2. The standard mechanism of this is shown below:

 
This mechanism shows how PBr3 replaces the carboxylic OH with a bromide which results in the carboxylic acid bromide. At this point the acyl bromide can be tautomerize to an enol, which will react with the Br2 a second time at the alpha poisition. In addition to this, the reaction depends on the enol type character of carbonyl compounds as well as the product of the reaction, an alpha-bromocarboxylic acid can be converted by substitution reactions to either a alpha-hydroxy or a alpha-amino carboxylic acid.
An example that I found where the Hell-Volhard-Zelinsky halogenation process occured is shown below:

This reaction shows that using PBr3 with Br2 also known as an intermediate acid bromide is formed which undergoes enolization and bromination. 

Refernces:
1.Hell-Volhard-Zelinsky halogenation. Wikipedia the encyclopedia. 21 April 2011. http://en.wikipedia.org/wiki/Hell-Volhard-Zelinsky_halogenation.

2. Jie Jack Li. Hell-Volhard -Zelinsky halogenation. http://www.springerlink.com/content/t2ur113r36720270/.