Hell – Volhard – Zelinsky reaction is halogenation of carboxylic acids at the carbon. It can also be called HVZ Reaction. It is a type of substitution reaction. It is named after German Chemists Carl Magnus von Hell, Jacob Volhard and Soviet Chemist Nikolay Zelinsky.
In Hell – Volhard – Zelinsky reaction, carboxylic acid with \[\alpha \] – hydrogen is converted into \[\alpha \]-halo carboxylic acid by using red phosphorus, halogens and water. The reaction is given below –
Hydrogen atom bonded with COO group or hydrogen of carboxylic acid is more acidic than \[\alpha \]- hydrogen attached to carbon. So, in 1st step removal of more acidic hydrogen atom takes place. For this catalyst red phosphorus reacts with halogen (Cl, Br) and gives phosphorus trihalide or phosphorous pentahalide (we have taken phosphorus trihalide). Now phosphorus trihalide reacts with the carboxylic acid compound and halogen of phosphorus trihalide replaces the hydroxyl group (which has a more acidic hydrogen atom). Reactions are given below –
2P + 3X2 🡪 2PX3 (Where X can be either Cl or Br)
Now tautomerism takes place in acyl halide. In which transfer of proton within the compound takes place. Oxygen atom attracts pi electrons towards itself, so oxygen gets negative charge and carbon atom has a free valency so bonded electrons of -hydrogen goes to carbon atom and α-hydrogen gets released as H+ or proton. Now this proton gets attached to the negatively charged oxygen atom. Thus, acyl halide changes into its enol form. Reaction is given below –
Now this enol form reacts with X2 and the hydrogen atom of the hydroxyl group gets removed as HX. Reaction is given below –
Now water reacts with this compound and forms desired product - halo carboxylic acid by removing HX. Reaction if given below –
The Hell-Volhard-Zelinsky reaction is used in the preparation of alanine.
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