Mesomeric effect
The
mesomeric effect or
resonance effect in
chemistry is a property of
substituents or
functional groups 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 positive (
+M) when the substituent is an electron-withdrawing group and the effect is negative (
-M) when based on resonance the substituent is an electron releasing group.
*Examples of +M substituents:
acetyl (IUPAC ethanoyl) -
nitrile -
nitro*Examples of -M substituents:
methyl -
alcohol -
amineThe net electron flow from or to the substituent is determined also by the
inductive effect. It's important to note that the mesomeric effect as a result of p - orbital overlap (resonance) has absolutely no effect on this inductive effect, as this effect is purely to do with the
electronegativity of the atoms and their structural chemistry (which atoms are connected to which).
The concepts of mesomeric effect,
mesomerism and
mesomer were introduced by
Ingold in 1938 as an alternative to the
Pauling's synonymous concept of
resonance [If It's Resonance, What Is Resonating? Kerber, Robert C. . J. Chem. Educ. 2006 83 223. Abstract]. Mesomerism is in this context is often encountered in German and French literature but in English literature the term resonance dominates.
* IUPAC
Gold Book definition*
Important publications in organic chemistry
