Hyperconjugation

Hyperconjugation is an important electronic effect in organic chemistry that involves the delocalization of electrons in sigma (σ) bonds to stabilize adjacent unsaturated systems like carbocations, alkenes, and free radicals. It is sometimes referred to as no bond resonance or the Baker-Nathan effect. Let’s understand in detail.

1.0What is Hyperconjugation?

Hyperconjugation refers to the delocalization of electrons from a sigma bond (typically C-H or C-C), to an adjacent empty or partially filled p-orbital or π-system. This interaction provides additional stabilization to the molecule by allowing the electrons from the sigma bond to be shared over multiple atoms.

2.0Mechanism of Hyperconjugation

• In hyperconjugation, the C-H sigma bond from an alkyl group (like CH₃, CH₂, or CH) can interact with the adjacent empty p-orbital (in carbocations), or π-bond (in alkenes).
• The sigma electrons from the C-H bond are delocalized over the neighboring atoms, effectively spreading out the electron density, which lowers the energy of the system and stabilizes it.

Hyperconjugation is similar to resonance, but instead of involving π-electrons, it involves σ-electrons (typically from C-H bonds).

3.0Illustration of Hyperconjugation

1. In Alkenes:

• Consider propene (CH₃-CH=CH₂). The C-H sigma bond of the methyl group (CH₃) adjacent to the double bond can overlap with the π-bond of the alkene.
• This interaction stabilizes the alkene by delocalizing electron density from the C-H bond into the π-bond of the double bond.

2. In Carbocations:

• In carbocations, hyperconjugation plays a key role in their stability.
• Example: In the tert-butyl carbocation (C⁺-(CH₃)₃), the empty p-orbital of the central carbon interacts with the adjacent C-H sigma bonds of the methyl groups. The more C-H bonds there are available for hyperconjugation, the greater the stabilization.
• This explains why tertiary carbocations (with more alkyl groups) are more stable than primary carbocations.

4.0Types of Systems that Exhibit Hyperconjugation

1. Alkenes: The interaction between the C-H bond of an alkyl group and the adjacent π-bond stabilizes the alkene.

• Example: In propene (CH₃-CH=CH₂), the C-H sigma bond of the methyl group interacts with the π-bond of the double bond, stabilizing the molecule.

2. Carbocations: Hyperconjugation stabilizes carbocations by donating electron density from adjacent C-H sigma bonds to the empty p-orbital on the positively charged carbon.

• Example: The tert-butyl carbocation (C⁺-(CH₃)₃) is more stable than the ethyl carbocation because it has more C-H bonds available for hyperconjugation.

3. Free Radicals: In free radicals, hyperconjugation stabilizes the unpaired electron by delocalizing electron density from adjacent C-H bonds.

• Example: In the tert-butyl radical (CH₃)₃C•, the unpaired electron is stabilized by hyperconjugation from the adjacent C-H bonds.

5.0Examples of Hyperconjugation

Structure	Number of alpha hydrogens	Total number of Hyperconjugative structure	Number of Hyperconjugative structure (involving C–H bond)
CH3–CH=CH2	3	4	3
CH3–CH2–CH=CH2	2	3	2
CH3–CH=CH–CH3	6	7	6
	3	4	3
	9	10	9

6.0Applications of Hyperconjugation

Hyperconjugation is important in understanding the stability and properties of various organic molecules. Its effects are observed in several chemical phenomena, including the stability of alkenes, carbocations, free radicals, and bond lengths. Below are the main applications of hyperconjugation:

1. Stability of Alkene

• The more hyperconjugative structures a molecule has, the more stable the alkene becomes. The stability increases with the number of alkyl groups attached to the double bond due to the +I effect (electron-donating) and hyperconjugation.
• Rule: More alkylated alkenes are more stable due to a higher number of α-hydrogens available for hyperconjugation.
• Example:
• • CH₃-CH=CH₂ (propene) > CH₂=CH₂ (ethylene)
  • In propene, the 3 α-hydrogens from the methyl group provide hyperconjugation, making it more stable than ethylene, which has no α-hydrogens.
• General Stability Order of Alkenes:

2. Heat of Hydrogenation:

• The more α-hydrogens an alkene has, the more stable it is. This leads to a lower heat of hydrogenation, as hyperconjugation stabilizes the alkene, reducing the energy required to hydrogenate it.
• Example:
• • Heat of Hydrogenation decreases in the following order: CH₂=CH₂ (ethylene) > CH₃-CH=CH₂ (propene) > CH₃-CH=CH-CH₃ (2-butene).
  • Explanation: Ethylene, with no α-hydrogens, has the highest heat of hydrogenation, while 2-butene, with multiple α-hydrogens, has a lower heat of hydrogenation, reflecting greater stability due to hyperconjugation.

3. Bond Length:

• Hyperconjugation affects the bond lengths in alkenes and other molecules. It can result in bonds that are slightly longer or shorter than expected based on normal single or double bond character.
• Examples:

• The bond length of C(II)–C(III) in propene is slightly shorter than a normal C-C single bond because of partial double-bond character induced by hyperconjugation.
• The bond length of C(II)–C(I) in propene is slightly longer than a normal C=C double bond due to electron delocalization from hyperconjugation.

4. Dipole Moment:

• Since hyperconjugation can cause partial charge separation in a molecule, it affects the dipole moment. More extensive hyperconjugation can lead to a larger dipole moment in the molecule.
• Example:
• • CH₂=CH₂ (ethylene) has a lower dipole moment than CH₃-CH=CH₂ (propene) because the methyl group in propene donates electron density via hyperconjugation, creating a greater dipole moment.

5. Stability of Carbocations:

• Carbocations are stabilized by hyperconjugation. The more alkyl groups attached to the carbocation, the greater the stabilization due to hyperconjugation from adjacent C-H bonds.
• Rule: Tertiary carbocations (3°) are more stable than secondary (2°) and primary (1°) carbocations due to the availability of more α-hydrogens for hyperconjugation.
• Example:
• • Tert-butyl cation (C⁺-(CH₃)₃) is more stable than ethyl cation (C⁺-CH₂-CH₃) because the former has more α-hydrogens available for hyperconjugation.

6. Stability of Free Radicals:

• Hyperconjugation stabilizes free radicals. Similar to carbocations, free radicals are stabilized by adjacent alkyl groups that can provide hyperconjugation through their α-hydrogens.
• Example:
• • Tertiary free radicals are more stable than secondary and primary radicals because they have more α-hydrogens to participate in hyperconjugation.
• Stability Order of Free Radicals: Tertiary (3°) > Secondary (2°) > Primary (1°).

7.0Comparison with Resonance