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From the carbocations below, identify the most stable one and the least stable one, in that order: (I) cycloheptatrienyl cation (tropylium ion) (II) CH2=CH-CH2+ (allyl cation) (III) C6H5-CH2+ (benzyl cation) (IV) CH3-CH+-CH3 (isopropyl cation)

1. II, I
2. III, IV
3. I, II
4. I, IV

Correct answer: I, IV

Solution

Tropylium is aromatic (6 pi electrons, Huckel) and the most stable. Isopropyl is only a secondary cation with no resonance, the least stable; allyl and benzyl are resonance-stabilized.

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