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In which of the following pairs is carbocation (I) LESS stable than carbocation (II)?

1. C6H5-CH2(+) (I), CH2=CH-CH2(+) (II)
2. (CH3)3C(+) (I), CH3-CH2(+) (II)
3. CH2=CH(+) (I), CH3-CH2(+) (II)
4. H3C-CH2(+) (I), CH2=CH(+) (II)

Correct answer: CH2=CH(+) (I), CH3-CH2(+) (II)

Solution

We need the case where (I) is the LESS stable of the two. Order of carbocation stability: benzyl ~ allyl (resonance) > 3deg > 2deg > 1deg alkyl > vinyl/aryl (sp, very unstable). Option with benzyl (I) vs allyl (II): benzyl is more stable than allyl, so I > II - wrong direction. Option with tert-butyl (I) vs ethyl (II): a 3deg cation is far more stable than a 1deg ethyl, so I > II - wrong. Option with ethyl (I) vs vinyl (II): the 1deg alkyl ethyl is more stable than the vinyl cation, so I > II - wrong. Only when vinyl is (I) and ethyl is (II) is (I) genuinely less stable than (II): the vinyl cation (positive charge on an sp carbon, no hyperconjugation/resonance) is markedly less stable than the ethyl cation, which is stabilised by hyperconjugation from three C-H bonds.

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