Sat January 29, 2011 By: Vineet Sharma

Will u please explain what is mesomeric effect? what is the main difference between mesomeric and resonance effect?

Expert Reply
Sun January 30, 2011

Dear Student

The mesomeric effect or resonance effect is a property of substituents or functional groups in a chemical compound. This effect is an electron redistribution that occurs via a pi orbital, quite often via conjugated systems. 

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.

  •  -M substituents: acetyl (IUPAC ethanoyl) - nitrile - nitro, etc.
  •  +M substituents: alcohol - amine, etc.

A good example of this effect is seen in the carbonyl group:

The properties of the carbonyl are not properly explained by the classical image (far left), nor by the dipole (centre), which represents the total relocation of the pi electrons; the "real" structure is a combination of these, shown on the far right.  There is an inductive effect (indicated by the arrow) in the "real" structure, but it is smaller than the mesomeric effect. This is because the sigma electrons are less readily polarizable than pi electrons, hence they are not shifted as far.

The mesomeric effect can be transmitted along conjugated systems if, for example, a carbonyl group is conjugated with a C=C bond:

Therefore, as above, there is an electron deficiency at the carbon next to oxygen, and also at the one indicated in the C3 position.  Mesomeric effects are much better transmitted through bonds than the inductive effect;in the C3position, the effect of the the carbonyl mesomeric influence is still noticeable.


We hope that clarifies your query.




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