Which one of the carbocations from the following is most stable?
General Organic Chemistry (GOC) · Class 11 · JEE Main Previous Year Question
Which one of the carbocations from the following is most stable?
- a
–CH=CH–CH₂–O–CH₃
- b✓
–CH=CH–O–CH₃
- c
–CH=CH–O–C(=O)–CH₃
- d
–CH=CH–F
–CH=CH–O–CH₃
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 electronegative; –I effect destabilises carbocation
Most stable: (b)
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