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Which of the following carbocations is the least stable?

1. CH3-CH2(+)
2. CH3-CH2-CH2(+)
3. (CH3)2CH-CH2(+)
4. (CH3)3C-CH2(+)

Correct answer: CH3-CH2(+)

Solution

All listed cations are primary carbocations (positive charge on a -CH2 group). Stability of primary cations still rises with the size/branching of the attached alkyl chain (more hyperconjugative and inductive donation). The ethyl cation CH3-CH2(+) has the fewest electron-donating alkyl groups adjacent, so it is the least stable.

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