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 negative (-M) when the substituent is an electron-withdrawing group and the effect is positive (+M) when based on resonance and the substituent is an electron releasing group.
- Examples of -M substituents: acetyl (IUPAC ethanoyl) - nitrile - nitro
- Examples of +M substituents: alcohol - amine-benzene
The net electron flow from or to the substituent is determined also by the inductive effect. The mesomeric effect as a result of p-orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the electronegativity of the atoms and their topology in the molecule (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. "Mesomerism" in this context is often encountered in German and French literature but in English literature the term "resonance" dominates.
Read more about Mesomeric Effect: Mesomerism in Conjugated Systems
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“Before the effect one believes in different causes than one does after the effect.”
—Friedrich Nietzsche (1844–1900)