Sir,

What is +M effect and - M effect. What are its consequences?

Dear anurag,

The mesomeric effect or resonance effect  is a property of substituents  in a chemical compound. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant 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 (IUPAC ethanoyl) 
• Examples of +M substituents: alcohol - amine

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