The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect.
The C-Cl bond in the butyl chloride, CH3-CH2-CH2-CH2-Cl is polarized due to electronegativity difference. The electrons are withdrawn by the chlorine atom. Thus the first carbon atom gets partial positive charge. In turn, this carbon atom drags electron density partially from the next carbon, which also gets partial positive charge. Thus the inductive effect is transmitted through the carbon chain.
The electron withdrawing or releasing effect attributed to a substituent through delocalization of p or π electrons, which can be visualized by drawing various canonical forms, is known as mesomeric effect or resonance effect.
Negative resonance or mesomeric effect (-M or -R): It is shown by substituents or groups that withdraw electrons by delocalization mechanism from rest of the molecule and are denoted by -M or -R. The electron density on rest of the molecular entity is decreased due to this effect.
E.g. -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H etc.
Positive resonance or mesomeric effect (+M or +R): The groups show positive mesomeric effect when they release electrons to the rest of the molecule by delocalization. These groups are denoted by +M or +R. Due to this effect, the electron density on rest of the molecular entity is increased.
E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc.
It is a temporary effect which takes place between two atoms joined by a multiple bond, i.e. a double or a triple bond. This occurs at the requirements of the attacking reagent and involves instantaneous transfer of a shared pair of electrons of the multiple bonds to one of the linked atoms. However, when the attacking reagent is removed, the molecule acquires its original electronic condition.
The phenomenon of movement of electrons from one atom to another in multibonded atoms at the demand of attacking reagent is called electromeric effect and is denoted as E effect.
