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Rank the stability of the following para-substituted benzylic carbocations in decreasing order: (1) p-hydroxy (p-OH) benzyl cation, (2) p-amino (p-NH2) benzyl cation, (3) p-methyl (p-CH3) benzyl cation, (4) p-tert-butyl (p-C(CH3)3) benzyl cation.

1. 2 > 1 > 4 > 3
2. 1 > 2 > 4 > 3
3. 3 > 4 > 2 > 1
4. 2 > 1 > 3 > 4

Correct answer: 2 > 1 > 3 > 4

Solution

Benzylic cations are stabilized by para substituents that push electron density onto the cationic center. Resonance donors (+M) are strongest: NH2 donates its lone pair better than OH because nitrogen is less electronegative and holds its lone pair more loosely, so cation 2 > cation 1. The alkyl groups stabilize only by weaker hyperconjugation/induction: a p-CH3 (cation 3) actually provides effective hyperconjugative donation; p-tert-butyl (cation 4) has no C-H on the attached carbon for direct hyperconjugation from that position and is a weaker net donor here. Hence stability order is 2 > 1 > 3 > 4.

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