Aromatic Hydrocarbons

Question

Which of the following correctly describes the mechanism by which benzene undergoes reaction with bromine in the presence of FeBr₃?

RankGuru · Accepted Answer

Benzene has a delocalized π electron cloud above and below the ring, making it nucleophilic toward electrophiles. FeBr₃ (Lewis acid) activates Br₂ by accepting a lone pair from one bromine atom: Br₂ + FeBr₃ → Br⁺···FeBr₄⁻. This generates a potent electrophile (Br⁺ equivalent). The π electrons of benzene attack this electrophile to form a carbocation intermediate called a sigma complex (arenium ion or Wheland intermediate), in which aromaticity is temporarily lost. Loss of a proton from the sigma complex restores aromaticity and regenerates the catalyst FeBr₃, yielding bromobenzene. Aromaticity is restored by elimination (not addition), making this electrophilic aromatic substitution (EAS), not electrophilic addition (which would destroy aromaticity permanently).

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Aromatic Hydrocarbons

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About This Question

Solution

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