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Among the following cationic species, which ones are resonance-stabilised carbocations? (A) benzyl carbocation (B) allyl carbocation (C) cyclohexenylmethyl (homoallyl-type) carbocation (D) carbocation conjugated with an enone system. Select the correct choice.

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

Correct answer: (A), (B) and (D) only

Solution

Resonance stabilisation of a carbocation requires the empty p orbital of the positive carbon to be directly adjacent to a pi bond (or aromatic ring/lone pair) so the charge can delocalise. (A) benzyl and (B) allyl cations are classic resonance-stabilised cations. (D) a cation conjugated to an enone (allylic to a C=C that is part of the conjugated system) is also resonance stabilised. (C) cyclohexenylmethyl is a homoallylic-type cation where the positive carbon is not directly conjugated with the double bond (only through-space/sigma participation, not normal resonance), so it is not counted as a standard resonance-stabilised cation. Hence (A), (B) and (D).

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