Description: The Hell-Vollhard-Zelinsky [HVZ] reaction is a means of converting a carboxylic acid to a brominated carboxylic acid (the bromine ends up on the alpha carbon of the carboxylic acid).
Notes: There is some variation in the use of reagents here; either phosphorus (P) or PBr3 can be used as the phosphorus reagent, and either Br2 or N-bromosuccinimide (NBS) can be used as the source of bromine. Water is added in the second step to convert the acid bromide into a carboxylic acid.
Notes: The first example shows the use of P and Br2 to give the final product. Phosphorus tribromide (PBr3) can be used in place of phosphorus (example 2) and NBS can be used in place of Br2 as a source of bromine (example 4). The purpose of water is to convert the final product into a carboxylic acid (see mechanism below).
Mechanism: This reaction progresses in three stages. In the first stage, the carboxylic acid is converted into an acid bromide. This reaction begins through attack of oxygen at phosphorus (Step 1, arrows A, B, C) followed by addition of bromide to the carbonyl carbon (Step 2, arrows D and E). Oxygen is then eliminated in the process of forming a C-O π bond (Step 3, arrows F, G, H) to give a protonated acid bromide, which can be deprotonated by bromide ion (Step 4, arrows I and J). The driving force for this reaction is formation of a strong P–O bond.
In the second phase of the mechanism, the alpha-carbon of the acid bromide is brominated. This occurs through protonation of the acid bromide (Step 1, arrows K and L) followed by formation of an enol (Step 2, arrows M, N, and O). The enol then reacts with bromine at the carbon (Step 3, arrows P, Q, and R). Deprotonation of the acid bromide (Step 4, arrows S and T) then give the neutral alpha-brominated acid bromide.
In the third phase, treatment of the acid bromide with water leads to hydrolysis of the acid bromide to give back the carboxylic acid. In the first step, addition of water (Step 1, arrows U and V) is followed by elimination of bromide ion (Step 2, arrows W and X) to give the protonated carboxylic acid, which is them deprotonated by excess water (Step 3, arrows Y and Z).
Notes: A similar process can be performed using Cl2 instead of Br2. This is still called the Hell-Vollhard-Zelinsky reaction.