What catalyst is used in the Hell-Volhard Zelinsky reaction?

In the Hell-Volhard-Zelinsky reaction, phosphorus tribromide is the catalyst utilized to introduce a bromine atom into the alpha position of carboxylic acids. This reaction allows for the bromination of carboxylic acids, and phosphorus tribromide serves an essential role as it helps convert the acid into an intermediate that is more reactive towards bromine. Upon heating, the reaction typically involves the formation of alpha-bromo acid products, which can undergo further reactions or substitutions.

Other phosphorus compounds mentioned, such as phosphorus triiodide and phosphorus trichloride, may have applications in other organic transformations but are not used in this particular reaction, leading to the conclusion that phosphorus tribromide is the specific catalyst required for facilitating the Hell-Volhard-Zelinsky reaction effectively.