The solution of (+)-2-chloro-2-phenylethane in toluene racemizes slowly in the presence of small amount of SbCl₅; due to the formation of

carbocation

The correct answer is option 4. carbocation.

SbCl₅ is a Lewis acid, which means that it can accept a pair of electrons. When it reacts with (+)-2-chloro-2-phenylethane, it accepts the electron pair from the chlorine atom, forming a carbocation. The carbocation is a planar molecule, so the chloride ion can attack from either side, resulting in a racemic mixture.

The other options are incorrect.

Carbanion: A carbanion is a negatively charged carbon atom with three bonds. It is not formed in this reaction.

Carbene: A carbene is a neutral carbon atom with two unbonded electrons. It is not formed in this reaction.

Free radical: A free radical is an atom or molecule with an unpaired electron. It is not formed in this reaction.

Therefore, the carbocation is formed in the reaction of (+)-2-chloro-2-phenylethane with SbCl5, resulting in racemization.