mesomeric effect

Resonance & Mesomeric Effect:






There are many organic molecules which can not be represented by a single lewis structure. In turn, they are assigned more than one structure called canonical forms or contributing of resonating structures. The phenomenon exhibited by such compounds is called resonance. For example, 1, 3 – butadiene has following resonance structure.


The two essential conditions are


There must be conjugation in the molecule. Conjugation is defined as the presence of alternate double and single bonds in the compound like 


The part of the molecules having conjugation must be essentially planar or nearly planar. The first condition of conjugation is not only confined to the one mentioned above but some other systems are also categorized under conjugation. These are




Resonance (mesomeric) effect is of two types.

(i) If the atom or group of atoms is giving electrons through resonance, it is called +R or +M effect. For example,



















Other groups that shows +M effect are -NHR, -NR2, -OH, -OR, -NHCOR, -Cl, -Br,-I etc.










(ii) If the atom or group of atoms is withdrawing electrons through resonance, it is called ¾R or ¾M effect. For example,














(-M effect of -NO2 group)The -NO2 group in nitrobenzene has -M effect.

Other groups showing -M effect are -CN, -CHO, -COR, -CO2H, -CO2R, -CONH2, -SO3H, -COCl etc.