Hell-Volhard Zelinsky reaction is used for the formation of

The correct option is α-halocarboxylic acids.

The Hell-Volhard-Zelinsky (HVZ) reaction is a classic organic chemical reaction used to selectively halogenate carboxylic acids at the alpha (α) carbon atom. The α-carbon is the carbon atom directly adjacent to the carboxylic acid (-COOH) functional group.

Reaction Reagents:
The reaction is carried out using a halogen (such as bromine, Br₂, or chlorine, Cl₂) in the presence of a catalytic amount of phosphorus (typically red phosphorus, P), followed by hydrolysis with water.

Step-by-Step Mechanism:
1. In-situ Generation of Phosphorus Trihalide: The phosphorus catalyst reacts with the halogen to form a phosphorus trihalide (PX₃) intermediate:

$$2P+3X_2\to 2P{X}_3$$

2. Conversion to Acyl Halide: The carboxylic acid reacts with PX₃ to form an acyl halide. Acyl halides undergo enolization much more readily than carboxylic acids.
3. Halogenation at the α-Carbon: The acyl halide tautomerizes into its enol form. The enol double bond acts as a nucleophile and attacks the halogen (X₂), resulting in the substitution of a hydrogen atom at the α-carbon with a halogen atom to form an α-halo acyl halide.
4. Hydrolysis: Water is added to hydrolyze the α-halo acyl halide intermediate, converting the acyl halide group back into a carboxylic acid group, which yields the final α-halocarboxylic acid.