Question

Carbocation rearrangement of benzyl cation to tropylium cation

Answer 1

I've read that Tropylium carbocation is formed by the ring expansion of benzyl carbocation. But in benzyl carbocation, the CC atoms in the benzene ring are sp2sp2 hybridized i.e. 120∘120∘ is the required angle for stability. And in the benzene ring of the benzyl carbocation, the angle is already 120 degrees. Therefore benzyl ring need not expand for stability. Therefore why does benzyl rearrange to give tropylium?

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Answer 2

• Tropylium cation is more stable than benzyl cation simply because it has seven resonating structures while benzyl cation has five. More the resonating structures , more the stability. Allyl carbocation is less stable than benzyl carbocation due to only two resonating structures.