Carboxylic acids having an a-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

What happens when phenyl acetic acid reacts with chlorine molecule in the presence of red phosphorus?

α-Chlorophenyl acetic acid 

The correct answer is option 1. α-Chlorophenyl acetic acid.

Out of the options provided, the most likely product when phenyl acetic acid reacts with chlorine molecule in the presence of red phosphorus is α-Chlorophenyl acetic acid.

The reaction you described is known as the Hell-Volhard-Zelinsky halogenation. This reaction specifically targets the alpha carbon (α-carbon) adjacent to the carbonyl group (C=O) in carboxylic acids containing an alpha hydrogen (H).

The reaction specifically targets the alpha carbon due to the combined effects of:

Acidity of the alpha hydrogen: The alpha hydrogen is slightly more acidic than other hydrogens in the molecule due to the electron-withdrawing effect of the nearby carbonyl group. This makes it more susceptible to abstraction by the chlorine radical.

Stability of the carbocation intermediate: The carbocation formed after alpha hydrogen abstraction is more stable due to resonance interaction with the carbonyl group.

Therefore, the chlorine atom will be attached to the alpha carbon of the phenyl ring, resulting in α-chlorophenyl acetic acid.

The other options (β-chlorophenyl acetic acid, m-chlorophenyl acetic acid, and p-chlorophenyl acetic acid) are not the primary products of this reaction. While the reaction might involve some side reactions, the major product will be the alpha-substituted derivative.