Question

What are hyperconjugation and inductive effect?

Answer 1

1. Hyperconjugation is the delocalisation of $\mathrm{\sigma }$ electrons of $\mathrm{C}-\mathrm{H}$ bond of an alkyl group which is directly attached to an atom of an unsaturated system or to the atom with an unshared p orbital.
2. The $\mathrm{\sigma }$ electrons of $\mathrm{C}-\mathrm{H}$ bond enter into partial conjugation with the attached unsaturated system or with the unshared p orbital.
3. It is a permanent effect.
4. Hyperconjugation is also known as no bond resonance because there is no between the carbon and the hydrogen.
5. For example Ethyl cation(${\mathrm{CH}}_3{{\mathrm{CH}}_2}^{+}$) in which a positively charged carbon atom has an empty p orbital.
6. One of the $\mathrm{C}-\mathrm{H}$ bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the $\mathrm{C}-\mathrm{H}$ bond in-plane with this p orbital can then be delocalised into the empty p orbital.
7. This type of overlap stabilises the carbocation because electron density from the adjacent $\mathrm{\sigma }$ bond helps in dispersing the positive charge.
8. The free hydrogen shown is not free as it is bound to the p cloud.
9. The order of hyperconjugation is as follows.- $-{\mathrm{CH}}_3>-{\mathrm{CH}}_2{\mathrm{CH}}_3>-\mathrm{CH}{\left({\mathrm{CH}}_3\right)}_2>-\mathrm{C}{\left({\mathrm{CH}}_3\right)}_3$.
10. The greater the number of alkyl groups attached to the carbon atom, the greater the hyperconjugation.
11. The inductive effect is the ability of substituents to either withdraw or donate electron density to the attached carbon atom.
12. For example: ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Cl}$
13. Here, the $\mathrm{C}-\mathrm{Cl}$ bond induces polarity in the adjacent bonds.
14. Such polarisation of $\mathrm{\sigma }$ bond to the adjacent $\mathrm{\sigma }$ bond is referred to as the inductive effect.
15. It is also a permanent effect.