Hyperconjugation

• 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:

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.