Which species is involved in chlorination of benzene using Cl_2 in pr…

The correct answer is Option (2) → \(Cl^+\)

Chlorination of benzene is an electrophilic substitution. Benzene is an electron-rich ring, so it is attacked by an electron-seeking species (an electrophile), but the \( \ce{Cl2} \) molecule itself is not electrophilic enough. Anhydrous \( \ce{AlCl3} \), a Lewis acid, accepts a chloride ion from \( \ce{Cl2} \) and so generates the chloronium ion, \( \ce{Cl^{+}} \):

    \( \displaystyle \ce{Cl2 + AlCl3 -> Cl+ + AlCl4-} \)

The Cl⁺ ion attacks the ring and forms a resonance-stabilised carbocation (the arenium ion). The \( \ce{AlCl4-} \) ion then removes a proton from it, which restores the aromatic ring and regenerates the catalyst:

    \( \displaystyle \ce{C6H6 + Cl+ -> [C6H6Cl]+} \)

    \( \displaystyle \ce{[C6H6Cl]+ + AlCl4- -> C6H5Cl + HCl + AlCl3} \)

The net reaction:

    \( \displaystyle \ce{C6H6 + Cl2} \xrightarrow{\text{anhyd. }\ce{AlCl3}} \ce{C6H5Cl + HCl} \)

So the species that attacks benzene is Cl⁺.

Why the other options are incorrect

• Option (1) \(Cl^-\) — Incorrect. The chloride ion is electron-rich (a nucleophile); it is repelled by the pi electrons of benzene and cannot attack the ring.

• Option (3) \({AlCl_4}^+\) — Incorrect. There is no such species; aluminium (+3) with four chloride ions gives an ion of charge −1, not +1.

• Option (4) \({AlCl_4}^-\) — Incorrect. It is formed in the reaction, but it is the base that removes the proton in the last stage, not the electrophile that attacks the ring.