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Which of the following stability orders is correct? (A) cyclohexadienyl cation > (CH3)3C+ > CH2=O+CH3 (oxocarbenium) > CH3+ (B) allylic cation > vinylic cation > tertiary carbocation (C) p-nitrobenzylic anion > p-bromobenzylic anion > benzyl anion > p-methylbenzylic anion (D) CH2=CH-C+(CH3)2 > CH2=CH-CH2• (radical) > CH3CH2CH2• (radical) > CH2=CH• (radical)

1. (A) only
2. (C) only
3. (B) and (D) only
4. (A), (C) and (D)

Correct answer: (A), (C) and (D)

Solution

(A) The order holds: a delocalised cyclohexadienyl (pentadienyl-type) cation and the lone-pair-stabilised oxocarbenium (CH2=O+CH3) are well stabilised, tert-butyl cation is stabilised by hyperconjugation, and CH3+ is least stable. (B) is incorrect because a vinylic cation (positive charge on sp carbon) is highly unstable - less stable than a tertiary cation - so 'vinylic cation > tertiary carbocation' is false. (C) For benzylic anions, electron-withdrawing groups (NO2 strongest, then Br) stabilise the negative charge, while electron-donating CH3 destabilises it, giving NO2 > Br > H > CH3. (D) The species are correctly ordered: an allylic tertiary cation is most stable, then allylic radical (resonance), then primary radical, then vinyl radical (least). Hence (A), (C) and (D) are correct.

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