HYPER CONJUGATION

1. The relative stability of various classes of carbonium ions may be explained by the number of no-bond resonance structures that can be written for them.

2. Such structures are arrived at by shifting the bonding electrons from an adjacent C - H bond to the electron deficient carbon.

3. In this way, The positive charge originally on carbon is dispersed to the hydrogen.

4. This manner of electron release by assuming no bond character in the adjacent C - H bond is called hyper conjugation or No-Bond.

Resonance:-

The more hyper conjugation structure (No-Bond Resonance structures) that can be written for a species, the more stable species for example:-

1. Ethyl carbonium ion is stabilized by threehyper conjugation structures:-

2. Isopropyl carbonium ion is stabilized by six hyper conjugation structures.

3. t-Butyl carbonium ion is stabilized by nine hyper conjugation structures.

Thus, the following order of stability holds:-

                             

                              (CH₃)₃ C⁺   >  (CH₃)₂  ⁺CH  >  CH₃ ⁺CH₂ >  ⁺CH₃

In general, resonance effects (Mesomeric effects) are more important than hyper conjugation effects.