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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.
- (C) and (D) only
- (A), (B) and (D) only
- (A) and (B) only
- (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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