When a covalent bond is formed between atoms of different electronegativity, the σ electrons of the bond get shifted towards the more electronegative atom of the bond that results in polar covalent bond. Such polarisation of σ bond which is the result of the polarisation of adjacent σ bond is referred to as the inductive effect. Let us see the inductive effect in chloropropane.
Example of I-effect
In this example, Cl being the most electronegative draws some electron density towards it in such a way that the carbon(1) gains some positive charge δ+ and the chlorine some negative charge δ-. In turn C(1), which has developed partial positive charge δ+ draws some electron density towards it from the adjacent C-C bond. Consequently, C(2) develops some positive charge δδ+. Similarly, C(3) develops some positive charge δδδ+ due to the presence of partial positive charge δδ+ on C(2).
Note : The order of inductive effect is δδδ+ < δδ+ < δ+ i.e. Inductive effect weakens steadily with increasing distance from the substituent (electron-withdrawing or electron-donating group).
Inductive effect becomes vanishingly small after three bonds.
There are two types of inductive effects
If the substituent attached to the end of the carbon chain is electron-withdrawing, the effect is called −I-Effect.
The above example of CH3CH2CH2Cl is actually -I-effect.
−I-Effect of some of the atoms and groups in decreasing order is :
−NO2 > −CN > −COOH > −F > −Cl > −Br > −I
If the substituent attached to the end of the carbon chain is electron-donating, the effect is called +I-Effect.
Example of +I-effect
+I-Effect of some of the atoms or groups in the decreasing order is:
(CH3)3C− > (CH3)C− > CH3CH2− > CH3
Electromeric effect is shown by compounds containing multiple bonds. Electromeric effect involves complete transfer of a shared pair of π electrons within the same molecule when exposed to an attacking reagent.
Electromeric effect is a temporary effect and remains as long as the attacking reagent is present, as soon as the reagent is removed, the molecule reverts back to its original position. There are two types of electromeric effects :
In positive electromeric effect, the π-electrons of the multiple bond are transferred to that atom to which the reagent gets attached.
Example of +E-effect
In negative electromeric effect, the π-electrons of the multiple bond are transferred to that atom to which the reagent does not get attached.
Example of -E-effect