RESONANCE EFFECT OR MESOMERIC EFFECT
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. It is symbolized by M or R.
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.
