Similarly, the stability of a radical is explained where there is an overlap between the p orbital occupied by the odd electron and σ orbital of the alkyl group (hyperconjugation). In terms of resonance theory, the ethyl radical e.g. is a hybrid of the four structures.
The order of inductive effect is tert -butyl > isopropyl >ethyl > methyl whereas the rate of the above reaction follows the order: methyl > ethyl > isopropyl > tert butyl, i.e., when these alkyl groups are attached to an and unsaturated system the order of +I effect is reversed. This effect is known as hyperconjugation effect or no-bond resonance. It can be explained on the basis of hyperconjugative forms, contributing to the overall electron-donating effect for the substituent having hydrogen attached to it, methyl group showing maximum effect because of the maximum number of hydrogen. Since there is no bond between the carbon and hydrogen in the canonical forms, this phenomenon is called no-bond resonance. It may be shown as follows.
This hyperconjugative contributing form of propene shows that hyperconjugation occurs through σ bonded hydrogen atoms present on the carbon adjacent to the doubly bonded carbon, i.e., α-hydrogen atoms. Therefore, larger the number of such α hydrogen atoms, more is the number of such hyperconjugative structure. Thus the hyperconjugation effect decreases in the following order:
-CH3 > -CH2CH3 > -CH(CH3)2 > -C(CH3)3
The heat of hydrogenation of alkyl-substituted ethylenes decreases with the degree of substitution is explained on the basis of hyperconjugation or no-bond resonance.
Importance of hyperconjugation effect
Both resonance and hyperconjugation are the results of the delocalization of electrons involving π-π conjugation and -π conjugation respectively. The conjugation effect due to hyperconjugation is weaker then resonance.
(i) Stability of carbocations and free radicals: The order of stability of carbocations and free radicals can be explained on the basis of hyperconjugation. The order of stability of carbocations and free radical is 30 > 20 > 1o
(ii) Stability of alkenes: The alkenes which have a maximum number of alkyl groups attached to the doubly bonded carbon atom are more stable and can be explained on the basis of hyperconjugation.
More the number of alkyl groups more is the number of hyperconjugation structures that can be written for each one of the alkenes. For 2-butene trans-form is more stable than the cis form due to electronic repulsion present in a cis-arrangement where both methyl groups are on the same side, i.e., close to each other.
(iii) Directive influence of alkyl group: The Ortho-para directing influence of alkyl groups can be explained on the basis of hyperconjugation. For example, toluene can be depicted as a hybrid of the following contributing forms.
