Carbocation StabilityJEE Main

Concept

A carbocation is a carbon with only six electrons — desperately electron-poor. Anything that feeds electron density into the empty p-orbital stabilises it: σ(C–H) donation from neighbouring bonds (hyperconjugation), alkyl +I induction, and best of all resonance delocalisation (allyl, benzyl). Hence the ladder: 3° > 2° > 1° > CH₃⁺.

Key formula

Derivation

Hyperconjugation: each α C–H σ-bond overlaps the empty p-orbital — 'no-bond resonance'. t-butyl has 9 such H's, ethyl 3, methyl 0.

Resonance beats hyperconjugation: allyl spreads + over two carbons, benzyl over four positions of the ring. This ladder controls SN1 rates, E1 pathways, Markovnikov addition and most rearrangements.

Scenarios to explore

• Carbocation Stability — Hyperconjugation & resonance — the 3° > 2° > 1° ladder.

Real-world applications

• Markovnikov's rule = protonate to give the MORE stable carbocation.
• SN1/E1 solvolysis rates: 3° ≫ 2° ≫ 1°.
• 1,2-hydride & methyl shifts always run toward the more stable cation.

JEE exam tips

• Count α-H's for quick hyperconjugation ranking: 9 (t-Bu) > 6 (iPr) > 3 (Et) > 0 (Me).
• Benzyl > allyl (4 resonance structures vs 2).
• Rearrangement radar: 2° cation next to a quaternary carbon WILL shift to 3°.

Common mistakes

• Counting β-hydrogens for hyperconjugation (need α to the C⁺, i.e. on adjacent carbons).
• Ranking allyl below 2° — resonance makes it ≈ 3°.
• Forgetting vinyl & phenyl cations are DEstabilised (empty orbital in sp² plane, no overlap).

Exam traps to avoid

• Tropylium C₇H₇⁺ is aromatic — the most stable common carbocation.
• −M groups (NO₂) adjacent to C⁺ are killers; +M groups (OMe) at the right position rescue it.