Which one of the following species attacks the benzene ring in the above reaction?

\(Cl^+\)

The correct answer is option 2. \(Cl^+\).

The reaction given is the Halogenation reaction.

Benzene does not undergo rapid chlorination or bromination with \(Cl_2\) or \(Br_2\). When it does undergo a reaction with halogens, it occurs via substitution instead of addition; a C-H bond on the aromatic ring breaks, and a C-X bond forms (where X is a halogen). This occurs via electrophilic aromatic substitution, with the rate-limiting step being an attack on the halogen electrophile by the aromatic ring. This generates a carbocation intermediate, which is quickly deprotonated to re-generate the aromatic ring. Halogenation can be made much more rapid by using a Lewis acid such as \(AlCl_3\) or \(FeCl_3\). This accepts a lone pair from the halogen, making the halogen an even better electrophile.

The coordination of the Lewis acid to one of the chlorines converts it into an even better leaving group, with the net effect of weakening the \(Cl-Cl\) bond. Attack on the terminal \(Cl\) by a nucleophile results in the loss of \([Cl-FeCl_3]^–\) , an even better leaving group than \(Cl^–\) .