Which one of the following species attacks the benzene ring in the above reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(Cl^+\)
Chlorination of benzene in the presence of anhydrous \( \ce{AlCl3} \) is an electrophilic substitution. The π electron cloud of the benzene ring is attacked by an electrophile (an electron-seeking species), and a hydrogen atom of the ring is replaced:
\( \displaystyle \ce{C6H6 + Cl2} \xrightarrow{\text{anhyd. }\ce{AlCl3}} \ce{C6H5Cl + HCl} \)
Chlorine alone is not electrophilic enough. The Lewis acid \( \ce{AlCl3} \) accepts a chloride ion from \( \ce{Cl2} \) and releases the chloronium ion, Cl+:
\( \displaystyle \ce{Cl2 + AlCl3 -> Cl+ + [AlCl4]-} \)
\( \ce{Cl^{+}} \) attacks the ring and forms a carbocation (arenium ion). \( \ce{[AlCl4]^{-}} \) then removes a proton from it, which restores the aromatic ring and gives back the catalyst:
\( \displaystyle \ce{C6H6 + Cl+ -> [C6H6Cl]+} \)
\( \displaystyle \ce{[C6H6Cl]+ + [AlCl4]- -> C6H5Cl + HCl + AlCl3} \)
So the species that attacks the benzene ring is Cl+.
Why the other options are incorrect
Option (1) \(Cl^-\) — Incorrect. It is electron rich (a nucleophile), so it is repelled by the π electrons of benzene.
Option (3) \(AlCl_3\) — Incorrect. It is the Lewis acid catalyst that generates the electrophile; it is recovered at the end.
Option (4) \([AlCl_4]^-\) — Incorrect. It acts as a base in the last step, removing \( \ce{H^{+}} \) from the intermediate; it does not attack the ring.