There are few electronic effect in covalent bonding. These effect arises due to the nature of reacting species, condition of reactions and other different factors. Some of the common electronic effects are:
1. Inductive effect:
2. Electromeric effect
3. Mesomeric effect
1. Inductive effect: When there is a formation of covalent between two atoms having different electronegativities, the bonding electron pair shift towards the more electronegative atom resulting in a certain degree of polarity produced in this bond. This effect is called inductive effect. In other words, the polarity developed in a covalent compound due to the difference in electronegativities of a bonding atom is called inductive effect. It is also called as I effect.
In C-Cl, the bond is displaced slightly towards the chlorine atom as it has higher electronegativity. So, a slightly negative character is developed in chlorine atom while a positive character is developed in carbon atom. This illustrates the I-effect. In case of C-C, the bond is exactly in between the carbon atoms as they have equal electronegativities. So this bonding does not have I-effect.
There are two types of Inductive effect:
a. – I effect
b. + I effect:
a. – I effect: The atom or group of atom is said to have – I effect if it attacks the bonded electrons away from the carbon. Examples: -Cl, -Br, -NO2, -CN, -OH etc.
The decreasing order of I effect is given below:
-CN > NO2 > COOH > CHO > F > Cl > Br > I
b. + I effect: The atom or group of atom is said to have +I effect if it releases electron towards carbon. Examples: Alkyl group such as CH3, C2H5 etc.
Electromeric effect: The effect in which involves the complete transfer of bonded electrons to one of the atoms joined together by a multiple bond in presence of attacking agent is called electromeric effect. No electrometic effect takes place without the presence of attacking agent. This effect is observed in multiple bonded compounds. This is a temporary effect.
Mesomeric effect: The mesomeric effect is observed in conjugated compounds. The effect which refers to the polarity produced in the molecule as a result of interaction between two pi bonds or one pi bonds and a lone pair of electrons is called mesomeric effect. It is also a permanent effect.