Hyperconjugation

It is an another kind of delocalisation called hyperconjugation  which is a special type  of resonance. It is attributed to σ-π  orbital  overlap  against theπ-π  overlap  in butadiene or benzene of benzene. In the methyl cation CH₃⁺, however, the C-H bonds lie in the nodal plane of the vacant 2p_z-orbital and prevent an overlap with it. The σ bond that provides the bonding electrons is a C-H bond. The best way to depict hyperconjugation with Lewis structures is with resonance structures. The double-bond character is reflected in the bond lengths of the carbon-carbon bonds in the tert-butyl cation which is considerably shorter (1.442 Angstrom) than the carbon-carbon single bond in propene (1.501 Angstrom). 

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 this 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  show 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 are 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 delocalisation 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 > 1^o

(ii) Stability of alkenes: The alkenes which has maximum number of alkyl groups attached to the doubly bonded carbon atom  is 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 alkene. For 2-butene  trans form  is more stable than the cis form  due to electronic repulsion present in 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.