Identify the substrate

The compound is $(-)-1$-chloro-$1$-phenylethane, whose structure is: $\mathrm{C_6H_5-CH(Cl)-CH_3}$

The carbon bearing chlorine is a chiral benzylic carbon.

Meaning of racemisation

Racemisation means that an optically active compound is converted into an equal mixture of both enantiomers.

This usually happens when the chiral center passes through a planar intermediate, so that attack can occur from either side.

Role of $SbCl_5$

$SbCl_5$ is a strong Lewis acid. It can coordinate with the chloride ion and help the $C-Cl$ bond break heterolytically.

$\mathrm{C_6H_5-CH(Cl)-CH_3 \xrightarrow[\text{with } SbCl_5]{} C_6H_5-CH^+-CH_3 + Cl^-}$

The chloride is stabilized by interaction with $SbCl_5$.

Nature of the intermediate formed

The species formed is a benzylic carbocation: $\mathrm{C_6H_5-CH^+-CH_3}$

This carbocation is stabilized by resonance with the benzene ring.

Also, the carbocation carbon is $sp^2$ hybridized and planar.

Why racemisation occurs

Since the carbocation is planar, chloride (or return of nucleophile) can attack from either face with nearly equal probability, producing both enantiomers.

Therefore, the optically active compound slowly racemises.

Check options

• A: carbene — incorrect; no neutral divalent carbon is formed.
• B: carbocation — correct; racemisation occurs via a planar benzylic carbocation.
• C: free radical — incorrect; radicals do not explain this Lewis-acid-induced ionization/racemisation here.
• D: carbanion — incorrect; not formed under these conditions.

Final answer

The racemisation is due to the formation of a carbocation.