See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Electrophilic aromatic substitution (EAS) is a well-established reaction mechanism in which an electrophile replaces a hydrogen atom on an aromatic ring. Step 1 - Mechanism overview: In EAS, the electrophile (E+) attacks the pi electron cloud of the aromatic ring to form an intermediate. This intermediate is a carbocation in which the electrophile is covalently bonded (sigma bond) to one carbon of the ring, disrupting aromaticity. Step 2 - Identity of the intermediate: This intermediate is called a sigma complex (also known as an arenium ion or Wheland intermediate). It is a cyclohexadienyl cation where the electrophile is attached via a sigma bond, and the positive charge is delocalized over the remaining carbons of the ring. Step 3 - Completion of the reaction: In the second step, a base (often the conjugate base of the acid catalyst) removes a proton from the sp3 carbon bearing the electrophile, restoring aromaticity and yielding the substituted aromatic product. Step 4 - Why other options fail: (a) Free radical - free radical intermediates are involved in radical substitution reactions, not EAS. (c) Benzyne - benzyne intermediates arise in elimination-addition mechanisms typical of nucleophilic aromatic substitution under very harsh conditions, not EAS. (d) Carbene - carbenes are neutral species with a divalent carbon and are intermediates in reactions such as cyclopropanation, not EAS. Therefore, the correct answer is B.