Hell-Volhard Zelinsky Reaction

Hell-Volhard Zelinsky Reaction is an organic reaction by which the carboxylic acids are halogenated at the α-carbon atom. This reaction is named after the 3 German-chemists namely: Carl Magnus Von Hell,  Jacob Volhard and Nikolay Zelinsky.  

The basic reaction is shown below:

The reaction is initiated by the addition of a catalytic amount of Phosphorus tribromide (PBr₃), after which one molar equivalent of Diatomic Bromine (Br₂ ) is added. Iodination and fluorination is not possible through this reaction.

Hell-Volhard Zelinsky Reaction Mechanism

This reaction takes place in the absence of a halogen carrier. Halogen carriers take molecules from other compounds to generate electrophiles.

• The Phosphorus tribromide (PBr₃) replaces the carboxylic OH with a bromide.
  This results in the formation of a Carboxylic acid Bromide.
• Next, Tautomerization takes place i.e. acyl bromide/ carboxylic acid bromide tautomerizes to Enol.
•  This Enol will readily react with Br₂ to brominate at the α-position which results in the formation of α-bromoAcyl bromide.

If the aqueous solution is neutral to slightly acidic then, the hydrolysis of the α-bromoAcyl bromide takes place spontaneously, which yields the α-bromo carboxylic acid.

If a little nucleophilic solvent is present then the α-bromo acyl bromide  with the carboxylic acids yields the α-bromo carboxylic acid and again generates the acyl bromide intermediate.

Given below is the mechanism of the the exchange between an alkanoyl bromide and a carboxylic acid.

The α-bromoalkanoyl bromide has a strongly electrophilic carbonyl carbon due to the electron-withdrawing effects of the two bromides.

Applications

• Used in the α- bromination of carboxylic acids.
• It is used in the synthesis of amino acids through the displacement of halogens with the help of ammonia (NH₃), Given below is an example.

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