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Hell Volhard Zelinsky Reaction

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The Hell-Volhard Zelinsky reaction is a method for forming $\alpha$-halo acid. This is a synthetically useful procedure because the $\alpha$-halo acids are useful starting materials for other reactions. For example, the addition of hydroxide ion leads to the replacement of the halogen with an -OH group. The reaction with ammonia replaces the halogen with -NH2. The reaction with cyanide ion, CN-, converts the halide to a nitrile. 

Hell Volhard Zelinsky Reaction

Carboxylic acids having $\alpha$-H atom, react with P and Cl2/I2, forming $\alpha$-halogen carboxylic acid. The reaction is called Hell Volhard Zelinsky reaction. 

Formation of Chloro Acetic Acid

Mechanism

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Unlike ketones, carboxylic acids do not undergo halogenation easily. For this, the reaction requires a catalyst red phosphorus. In presence of the phosphorus, the halogen forms phosphorus trihalide which converts the acid into an acid halide, which then undergoes halogenation at the $\alpha$-carbon atom through an enol form. This $\alpha$-haloacid halide reacts with a fresh molecule of acid into an acid halide by an interchange reaction and a chain of reaction ensues which carries on the reaction till completion. 

Hell Volhard Zelinsky Reaction Mechanism

The reaction can be stopped at the required stage, that is mono-, di-, or tri- substitution by adjusting the required amount of halogen.

Although carboxylic acids, simple esters and amides do not undergo acid or base catalyzed bromination because the respective enols and enolates do not form easily enough, the Hell-Volhard-Zelinsky bromination of carboxylic acids gives easy access to $\alpha$-bromo carboxylic acid derivatives.

A popular variation is a carboxylic acid is treated with bromine and a little red phosphorus. Some PBr3 is formed, which converts the carboxylic acid to the corresponding acyl bromide; the crucial stage probably entails enolization of the acyl bromide and reaction of its enol with bromine; acyl bromide interchange with unmodified carboxylic acid allows the process to be repeated until all the starting acid is consumed.

HVZR with Amino Acids

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$\alpha$-Halo acids can be convenirntly prepared by the Hell-Volhard Zelinsky (HVZ) reaction. Here carboxylic acids react with chlorine or bromine in presence of phosphorus or phosphorus halide to yield the $\alpha$-halo acid. Halogenations occur specifically at the $\alpha$-carbon because the reaction probably proceeds via enolisation of the acyl halide. The function of phosphorus halide is to convert a little of the acid into acid halide. 

HVZR with Amino Acids

Amino acids like glycine, valine, leucine, isoleucine and aspartic acid, serine and threonine can be synthesized using direct ammonolysis method.

Examples

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When a carboxylic acid that contains $\alpha$-hydrogens is treated with Cl2 or Br2 in the presence of phosphorous the $\alpha$-hydrogen atom is replaced by halogen atom. This reaction is known as HVZ reaction. 

HVZR with Amino Acids Examples

Applications

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This reaction is given only by aliphatic acids having $\alpha$-hydrogen. Aliphatic carboxylic acids react with chlorine or bromine in the presence of small amount of red phosphorus to give exclusively $\alpha$-halogenated acids.

Applications of this reaction is in preparation of various important substituted productas $\alpha$-halogen can be easily replaced by nucleophilic reagents. 

HVZR Applications