Application Of Inductive And Resonance Effect

Application Of Inductive And Resonance Effect

The inductive and resonance effect can be effectively employed to explain the stability of carbocations, free radicals and carbanions and the acidic and basic strengths of various organic compounds. Let us take them separately.

Stability of Carbocations, Free Radicals and Carbanions

Let us first compare the stabilities of following carbocations,

1. Methyl Carbonium ion Primary (1°) Carbonium ion Secondary (2°) Carbonium ion Tertiary (3°) Carbonium ion
2. A primary (1°) carbocation is the one in which the carbon bearing the positive charge is attached to one carbon.
3. A secondary (2°) carbocation is the one in which the carbon bearing the positive charge is attached to two carbons.
4. A tertiary (3°) carbocation is the one in which the carbon bearing the positive charge is attached to three carbons.
5. According to the laws of physics, the stability of a charged system is increased by the dispersal of charge. Any factor, which tries to spread out the positive chare of the electron deficient carbon of carbocation must stabilize it.
6. In the present case, it can be seen that greater the number of alkyl groups attached to the positive carbon of the carbocation, the more stable the carbocation will be. The reason is that electron donating groups (+I groups, alkyl groups in this case) partially compensates for the electron deficiency of the positive carbon of the carbocation. Thus, the order of stability of carbocations would be
7. Tertiary > Secondary > Primary > Methyl Carbocation
8. The presence of electron withdrawing group (−I group) would decrease the stability of carbocation.
9. Similarly, the stability of a free radical can be increased by the presence of +I groups and decreased by the presence of −I groups. Thus, the order of stability of free radicals would be

3° free radical 2° free radical 1° free radical methyl free radical

In carbanions, the presence of electron donating groups (+I groups) would destabilize it while presence of electron withdrawing groups (−I groups) would stabilize it. Thus the order of stability of carboanions would be

methyl carbanion 1° carbanion 2° carbanion 3° carbanion