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ChemistryMediumClass 12
Hyperconjugation — Stability of carbocations, alkenes
GOC (General Organic Chemistry)
8
JEE Qs
8%
Hard
75
min
Always systematically identify alpha-hydrogens and prioritize resonance > hyperconjugation > inductive effect when comparing stability of intermediates and alkenes.
🧮 Key Formulas
Number of alpha-hydrogens ∝ Stability of carbocations
Number of alpha-hydrogens ∝ Stability of alkenes
✅ Key Points for JEE
- 1Hyperconjugation involves the delocalization of σ-electrons (from C-H bonds) into an adjacent empty p-orbital (in carbocations) or a π-orbital (in alkenes and alkyl benzenes).
- 2It requires at least one hydrogen atom on a carbon atom (alpha-carbon) directly attached to an sp² hybridized carbon (alkene) or an electron-deficient carbon (carbocation, free radical).
- 3The greater the number of alpha-hydrogens, the more extensive the hyperconjugation, leading to increased stability for carbocations and alkenes.
- 4Often referred to as 'no-bond resonance' or the Baker-Nathan effect, as it involves the partial formation of a double bond without a formal bond in the contributing resonance-like structures.
- 5Hyperconjugation's stabilizing effect is generally weaker than the resonance effect but stronger than the inductive effect in most organic systems.
⚠️ Common Mistakes
- ✕Incorrectly identifying alpha-carbons or alpha-hydrogens by counting hydrogens on carbons not directly adjacent to the sp² or carbocationic center.
- ✕Confusing hyperconjugation with the inductive effect; hyperconjugation is a delocalization of sigma electrons, while the inductive effect is a polarization through sigma bonds.
- ✕Overlooking hyperconjugation when comparing stability of carbocations or alkenes, especially in the absence of dominant resonance effects.
NCERT Chapters
- Class 11 Chemistry Ch 12: Organic Chemistry - Some Basic Principles and Techniques