Hyper Conjugation

•  When sigma electrons are in conjugation with pi- bond then this conjugation is known to be ‘Hyper conjugation’.
• Generally used to compare stability.

Required Conditions:

1. Compound should have at least one sp2 hybridized carbon atom of alkene or alkyl carbocation or alkyl free radical.
2. Alpha carbon with respect to sp2 hybrid carbon must be sp3 and it should have at least one alpha-Hydrogen.

If both the conditions are fulfilled then only “Hyper conjugation” occurs otherwise not.

Types of Hyper conjugation:

• π conjugation
• Positive charge conjugation
• Free radical conjugation

Ex 1: Let’s consider an example of CH₃ – CH = CH₂Alpha-carbon is the carbon attached to the sp2 – hybrid carbon.

Alpha-Hydrogen is the Hydrogen attached to the alpha-carbon atom.

Hence, \(\alpha \,\,H-\underset{\alpha \,\,\,H}{\overset{\alpha \,\,\,H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,=C{{H}_{2}}\)

This compound has three alpha-hydrogens

Type 1: Pi-Conjugation

1. \(C{{H}_{3}}-\overset{\alpha }{\mathop{C{{H}_{2}}}}\,-CH=C{{H}_{2}}\)

            Two alpha hydrogens in the above compound.

2. \(\overset{\alpha }{\mathop{C{{H}_{3}}}}\,-CH=CH-\overset{\alpha }{\mathop{C{{H}_{2}}}}\,-C{{H}_{3}}\)

Type 2 : Positive Charge conjugation

1. \(\overset{\alpha }{\mathop{C{{H}_{3}}}}\,-\overset{\oplus }{\mathop{C{{H}_{3}}}}\,\)
   Three alpha-hydrogens in the above compound.

2. \(\overset{\alpha }{\mathop{C{{H}_{3}}}}\,-\underset{\oplus }{\mathop{CH}}\,-\overset{\alpha }{\mathop{C{{H}_{3}}}}\,\)
   Six alpha-hydrogens in the above compound.

Type 3: Odd electron conjugation

1. \(\overset{\alpha }{\mathop{C{{H}_{3}}}}\,-\overset{\bullet }{\mathop{C{{H}_{2}}}}\,\)

Three alpha-hydrogens in the above compound.

2. \(C{{H}_{3}}-\overset{2\alpha }{\mathop{C{{H}_{2}}}}\,-\overset{\bullet }{\mathop{C}}\,H-\overset{3\alpha }{\mathop{C{{H}_{3}}}}\,\)

Five alpha hydrogens in the above compound.

Applications:

Hyper conjugation is generally used to compare stability of alkene.

No. of Hyperconjugative structures = 1 + No. of alpha hydrogens.

No. of alpha hydrogens is proportional to the No. of Hyperconjugative

Structures:

No. of α – H  µ Stability

2. Stability of carbocations:

\(\underset{0\alpha \,-H}{\overset{\oplus }{\mathop{\underset{{}}{\overset{{}}{\mathop{C{{H}_{3}}}}}\,}}}\,\,<\,\underset{3\alpha \,-H}{\overset{\oplus }{\mathop{\underset{{}}{\overset{{}}{\mathop{C{{H}_{2}}}}}\,}}}\,-\,C{{H}_{3}}\,<\,\underset{6\alpha \,-H}{\overset{+}{\mathop{\underset{{}}{\overset{{}}{\mathop{C{{H}_{3}}-\,CH\,-\,C{{H}_{3}}}}}\,}}}\,\)

3. Stability of free radicals:

\(\underset{0\alpha \,-H}{\overset{\bullet }{\mathop{\underset{{}}{\overset{{}}{\mathop{C{{H}_{3}}}}}\,}}}\,\,<\,\underset{3\alpha \,-H}{\overset{\bullet }{\mathop{\underset{{}}{\overset{{}}{\mathop{C{{H}_{2}}}}}\,}}}\,-\,C{{H}_{3}}\,<\,\underset{9\alpha \,-H}{\overset{\bullet }{\mathop{\underset{{}}{\mathop{\underset{C{{H}_{3}}}{\mathop{\underset{|}{\overset{{}}{\mathop{C{{H}_{3}}-\,CH\,-\,C{{H}_{3}}}}}\,}}\,}}\,}}}\,\,\)

Reverse Hyper conjugation:-

Only possible if a halogen is in conjugation with pi- bonds.

\(\alpha \,\,Cl-\underset{\alpha }{\mathop{\underset{{}}{\mathop{\underset{Cl}{\overset{Cl}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,}}\,}}\,-CH=C{{H}_{2}}\)

Three alpha chlorines are in conjugation with the pi bond whenever 3 halogens are in conjugation with pi bond and hence it is known as reverse conjugation.