Mesomeric effect

The mesomeric effect or resonance effect in resonance structures and is symbolized by the letter M. The mesomeric effect is negative (-M) when the substituent is an electron-withdrawing group and the effect is positive (+M) when based on resonance the substituent is an electron releasing group.

• Examples of -M substituents: acetyl (UPAC ethanoyl) - nitrile - nitro
• Examples of +M substituents: alcohol - amine