Question

How to convert benzene to chlorobenzene?

Answer 1

Conversion of Benzene into Chloronebenzene:

The Benzene is first treated with Chlorine in presence of Ferric(III) chloride${\mathrm{FeCl}}_3$ to produce Chlorobenzene and Hydrochloric acid:

Mechanism:-

Step 1: Generation of electrophile$\mathbf{\left(}{\mathbf{Cl}}^{\mathbf{+}}\mathbf{\right)}$

The catalyst Lewis acid which is Ferric(III) chloride $\left({\mathrm{FeCl}}_3\right)$ helps in the generation of an electrophile $\left({\mathrm{Cl}}^{+}\right)$ that is caused by polarisation of the chlorine molecule.

$\stackrel{\delta +}{\mathrm{Cl}}-\stackrel{\delta -}{\mathrm{Cl}}+{\mathrm{FeCl}}_3\to {\mathrm{FeCl}}_4^{-}+\underset{\mathrm{Chloronium}\mathrm{ion}}{\stackrel{+}{\mathrm{Cl}}}$

Step 2: Formation of the carbocation

The electrophile $\left({\mathrm{Cl}}^{+}\right)$ formed then attacks the benzene ring to form an intermediate carbocation which is resonance stabilised. This is a slow step.

For the above shown resonating structures, the resonance hybrid can be represented as:

Step 3: Formation of Chlorobenzene

The proton now gets transferred from the carbocation to form the resulting Chlorobenzene.