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Which of these carbocations is the most stable?

1. (CH3)3C+ (tert-butyl cation)
2. Ph2C+Me (diphenyl methyl-substituted cation)
3. CH3-CH2+ (ethyl cation)
4. Cyclohexadienyl cation

Correct answer: Ph2C+Me (diphenyl methyl-substituted cation)

Solution

Ph2C+Me has the positive carbon flanked by two phenyl rings; the charge is delocalised over both aromatic rings by resonance, giving very strong stabilisation (benzylic times two). The tert-butyl cation is only stabilised by hyperconjugation/inductive effects, the ethyl cation is a poorly stabilised primary cation, and the simple cyclohexadienyl cation is far less delocalised than a doubly benzylic system. Hence Ph2C+Me is most stable.

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