Hyperconjugation

Hyperconjugation

$\bullet$ It is delocalisation of sigma electrons.

$\bullet$ Also known as sigma-pi – conjugation or no bond resonance

$\bullet$ Hyperconjugation is a permanent effect

Occurrence

Alkene, alkynes

Free radicals (saturated type) carbonium ions (saturated type)

Condition

Presence of $\alpha$–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 $\alpha$-Hydrogen. Hence, in above examples structures i,ii,iii,iv are hyperconjugate structures (4-structures).

Effects of hyperconjugation

Bond Length:

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.

Dipole moment:

Since hyperconjugation causes these development of charges, it also affects the dipole moment of the molecule.

Illustration 1. Arrange the following species in increasing order of dipole moment.

Solution:

In I, there is addition of vector

In II, there will neither be addition nor subtraction of vector

In III, there is subtraction of vector

In IV, the vectors almost cancel each other. So the increasing order of dipole moment is IV < III < II < I.

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 a-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

${(C{H}_3)}_3\stackrel{\bullet }{C}>{(C{H}_3)}_2\stackrel{\bullet }{C}H>C{H}_3\stackrel{\bullet }{C}{H}_2>\stackrel{\bullet }{C}{H}_3$

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.

Illustration 2. Which among the following is most acidic?

Solution: As the inductive effect of chlorine decreases with distance (a) will be most acidic because the carboxylate ion which results after the loss of H⁺ can be stabilized by electron withdrawing nature of chlorine (-I effect) which is strongest in (a) where chlorine atom is at $\alpha$ - carbon.

Illustration 3. Which among the following is most basic?

(a) CH₃CH₂NH₂

(b) CH₃CH = NH

(c) CH₃ $-$ C $\equiv$ N

Solution: (a) is most basic because here nitrogen is sp³ hybridized i.e. p – character is greater than s – character (75% p and 25% s – character), orbital holding lone pair is more elongated than spherical, the hold of nitrogen nucleus over these electrons is less resulting in more basicity.

Illustration 4. Compare the acidic strength of the following

CH_3­­COOH ClCH₂COOH Cl₂HCCOOH CCl₃COOH

Solution:

Increasing acidic strength due to increasing number of ($-$I) effect group.