Volhard Method

The Volhard method is a reaction in which the alpha hydrogen group of a carboxylic acid is replaced by a halogen. This reaction is a type of halogenation reaction. This reaction is used for the preparation of halogen derivatives of carboxylic acid. Here, we will discuss the mechanism of the Volhard method.

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What is the Volhard method reaction referred to as? The Volhard method is referred to as Hell Volhard Zelinsky Reaction. Only the carboxylic acid-containing alpha hydrogen group compounds can give this reaction. Carboxylic acids are the compounds containing the carboxyl function group. 

Volhard Method Procedure

In the Volhard method procedure of halogenation, the carboxylic acids having alpha hydrogen react with chlorine or bromine in the presence of a small amount of red phosphorus to give compounds in which alpha hydrogen atoms get replaced by halogen atoms. This reaction is called the Hell Volhard-Zelinsky reaction. 

The General Reaction of the Volhard Method Procedure is Shown below:

R-CH2COOH → RCHX-COOH (reaction takes place in the presence of halogen molecule, red phosphorus, and water molecule).

R-CH2COOH is a carboxylic acid

RCHX-COOH is alpha halocarboxylic acid.

In the above reaction, a halogen molecule can be chlorine or bromine.

CH3COOH + Cl2 + red.P + H2O → ClCH2COOH Cl2 + red.P + H2O→ Cl2CHCOOH Cl2 + red.P + H2O → Cl3CCOOH

CH3CH2COOH + Br2 + red.P + H2O → CH3CHBrCOOH +Br2 + red.P + H2O  → CH3C(Br)2COOH

The Function of Red Phosphorus

The function of red phosphorus is to first combine with bromine to form PBr3. This then reacts with a carboxylic acid to form a corresponding acid bromide which enolizes to a larger extent than the acid. Thus, it brings out alpha bromination readily. 

P4 + 6Br2 → 4 PBr3

3CH3CH2COOH + PBr3 → 3CH3CH2COBr + H3PO3

 The halogen atom in monosubstituted acid can be easily replaced by a suitable atom or group to form a variety of compounds. Thus, alpha- halo acids are important synthetic intermediates. 

For Example,

(1) Action with aqueous Potassium Hydroxide

CH2(Cl)- COOH + KOH (aq) → CH2 (OH)- COOH + KCl

CH2 (OH)- COOH is alpha-hydroxy acetic acid also known as glycolic acid.

(2) Reaction with Alcoholic Ammonia

CH2 (Cl)- COOH + NH3 → CH2 (NH2)- COOH + HCl

CH2 (NH2)- COOH is an alpha-amino acetic acid also known as glycine.

Glycine or alpha-amino acetic acid exists in a dipolar form (NH3+CH2COO-).

(3.) Action with Potassium Cyanide

CH2(Cl)-COOH + KCN  CH2(COOH)-COOH

Condition for the Hell Volhard Zelinsky Reaction Mechanism

The Hell Volhard Zelinsky Reaction does not occur under normal conditions. It requires a little severe environment. The optimum temperature for such a reaction to takes place is 373 K. The rate of this reaction is low. Therefore, it is a slow reaction. This reaction takes place in the presence of red phosphorus. This reagent acts as a catalyst for the reaction. It combines with the halogen molecule (chlorine and bromine) and forms phosphorus trihalide. This phosphorus trihalide further halogenates the carboxylic acid and forms halogenated carboxyl derivatives.

Back Titration Method

The back titration method is a titration process used to determine the concentration of the analyte using an excess reagent. The excess amount of reagent is the known concentration. This known concentration of the excess reagent helps in the determination of the concentration of the analyte. By using this titration process we can also find the strength of the analyte. 

This type of titration method is used in the following reactions:

• When the acid or base component acts as an insoluble agent in the solution.

• When it is hard to find out the endpoint of the solution of the analyte.

• When the rate of reaction is very slow. 

• It is used in the determination of carboxylic acid concentration using a halogen molecule (it can be chlorine or bromine) as an excess reagent. The rate of reaction for this type of halogenation reaction is too slow. Therefore, it is hard to find out the endpoint of this reaction.

Did You Know?

• The name of Hell Volhard Zelinsky represents the name of three chemists.

• The carboxylic acid without alpha hydrogen does not give this reaction.

• The climax part of this reaction is tautomerization.