Give a brief account of resonance effect or mesomeric effect.

It is a process of permanent displacement of electrons from one part of the conjugated  to the other reacting centres of low and high electrons density due to the phenomenon of resonance and is called effect (R-effect) or mesomeric effect (M-effect). The resonance effect is transmitted through -electrons and operates in unsaturated system particularly those containing conjugated systems. For this reason, it is also called conjugative effect. Mesomeric effect operates:
(i) Through conjugative mechanism of electron displacement such as

(ii) When any atom contains a lone pair of electrons and the system is conjugated.

Types of resonance or mesomeric effect. The resonance effect (R) or mesomeric effect (M) are two types:
(i) Groups having electron withdrawing resonance effect. Groups such as 
 etc tend to withdraw the electrons from the multiple bonds through resonance and are said to have -R or -M effect. For example,

(ii) Groups having electron-releasing resonance effect. Groups such as

etc. which can release electrons through resonance are said to have +R or +M effect. For example.