Hyperconjugation
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Hyperconjugation
IUPAC & GOC of Class 12
- It is delocalisation of sigma electrons.
- Also known as sigma-pi – conjugation or no bond resonance
- Hyperconjugation is a permanent effect
Occurrence
Alkene, alkynes
Free radicals (saturated type) carbonium ions (saturated type)
Condition
Presence of α–H with respect to double bond, triple bond carbon containing positive charge (in carbonium ion) or unpaired electron (in free radicals)
Example
Note:
Number of hyperconjugative structures = number of α-Hydrogen. Hence, in above examples structures i,ii,iii,iv are hyperconjugate structures (4-structures).
Effects of hyperconjugation
Bond Length:
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Like resonance, hyperconjugation also affects bond lengths because during the process the single bond in a compound acquires some double bond character and vice-versa. E.g. C—C bond length in propene is 1.488 Å as compared to 1.334Å in ethylene. |
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Dipole moment:
Since hyperconjugation causes these development of charges, it also affects the dipole moment of the molecule.
Stability of carbonium Ions:
The order of stability of carbonium ions is as follows
Tertiary
• Secondary
• Primary
Above order of stability can be explained by hyperconjugation. In general greater the number of hydrogen atoms attached to α-carbon atoms, the more hyperconjugative forms can be written and thus greater will be the stability of carbonium ions.
Stability of Free radicals:
Stability of Free radicals can also be explained as that of carbonium ion
Directive influence of methyl group:
The o,p-directing influence of the methyl group in methyl benzene is attributed partly to inductive and partly to hyperconjugation effect.
(orientation influence of the methyl group due to +I effect )
(Orientation influence of methyl group due to hyperconjugation)
The role of hyperconjugation in o,p,-directing influence of methyl group is evidenced by the part that nitration of p-iso propyl toluene and p-tert-butyl toluene from the product in which —NO2 group is introduced in the ortho position with respect to methyl group and not to isopropyl or t-butyl group although the latter groups are more electron donating than Methyl groups
i.e., The substitution takes place contrary to inductive effect. Actually this constitutes an example where hyperconjugation overpowers inductive effect.
