Which one of the carbocations from the following is most stable?

(a) –CH=CH–CH₂–O–CH₃ (allylic, adjacent to ether)
(b) –CH=CH–O–CH₃ (allylic vinyl ether type)
(c) –CH=CH–O–C(=O)–CH₃ (allylic, adjacen

Question

Which one of the carbocations from the following is most stable?

(a) –CH=CH–CH₂–O–CH₃ (allylic, adjacent to ether)
(b) –CH=CH–O–CH₃ (allylic vinyl ether type)
(c) –CH=CH–O–C(=O)–CH₃ (allylic, adjacent to ester)
(d) –CH=CH–F (allylic, adjacent to F)

Canvas Classes · Accepted Answer

Carbocation stability — key factors: Resonance  Hyperconjugation  Inductive effects.

Option (b):

This is an allylic carbocation where the oxygen of –OCH₃ is directly conjugated with the double bond:

The oxygen lone pair delocalises into the carbocation through the π system → oxocarbenium resonance → maximum stabilisation.

Comparison:
- (a): –OCH₃ is separated by –CH₂– (no direct conjugation with +)
- (c): Ester oxygen is less electron-donating (carbonyl withdraws electrons)
- (d): F is el

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