Hyperconjugation
The "No-Bond Resonance" or Baker-Nathan Effect explaining molecular stability.
Hyperconjugation is a permanent electronic effect that involves the delocalization of $\sigma$-electrons of a C-H bond of an alkyl group directly attached to an atom of an unsaturated system or to an atom with an unshared p-orbital.
1. Conditions for Hyperconjugation
For hyperconjugation to occur, the molecule must satisfy the following:
- There must be an $sp^2$ hybridized carbon (either in a double bond, carbocation, or free radical).
- There must be at least one $\alpha$-Carbon attached to the $sp^2$ carbon.
- The $\alpha$-Carbon must be $sp^3$ hybridized and possess at least one $\alpha$-Hydrogen.
Note: The greater the number of $\alpha$-hydrogens, the greater the number of hyperconjugative structures, and thus, the greater the stability.
2. Mechanism (Orbital Overlap)
It involves the overlap of a $\sigma$-orbital (of the C-H bond) with an adjacent vacant p-orbital (in carbocations) or a $\pi^*$-orbital (in alkenes).
In the resonating structures, there is no bond between the $\alpha$-Carbon and the $\alpha$-Hydrogen ion ($H^+$), yet the proton remains bound by electrostatic attraction.
3. Applications of Hyperconjugation
A. Stability of Carbocations
Alkyl groups stabilize carbocations by dispersing the positive charge through hyperconjugation.
Order: $3^\circ > 2^\circ > 1^\circ > \text{Methyl}$
- Tert-butyl ($3^\circ$): 9 $\alpha$-H (Most Stable)
- Isopropyl ($2^\circ$): 6 $\alpha$-H
- Ethyl ($1^\circ$): 3 $\alpha$-H
- Methyl ($CH_3^+$): 0 $\alpha$-H (Least Stable)
B. Stability of Alkenes
More substituted alkenes are more stable (Saytzeff Rule) due to a higher number of hyperconjugative structures.
Example: Compare But-2-ene and But-1-ene.
6 $\alpha$-Hydrogens $\rightarrow$ More Stable
2 $\alpha$-Hydrogens $\rightarrow$ Less Stable
C. Stability of Free Radicals
Similar to carbocations, the stability order is:
D. Bond Length
Hyperconjugation introduces partial double bond character to single bonds and partial single bond character to double bonds.
- Example: In Propene ($CH_3-CH=CH_2$), the C-C single bond is shorter (1.48 Å) than normal (1.54 Å) due to partial double bond character.
4. Comparison with Other Effects
| Effect | Electrons Involved | Strength Order |
|---|---|---|
| Resonance (M) | $\pi$ or lone pair | Strongest |
| Hyperconjugation (H) | $\sigma$ and $p$ (or $\pi$) | Moderate |
| Inductive (I) | $\sigma$ only (displacement) | Weakest |
*General priority order: Resonance > Hyperconjugation > Inductive (Exceptions exist).
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