What is the $+ M$ $+M$ and $- M$ $-M$ effect? What are examples of electron releasing and electron withdrawing groups?

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Answer 1 · 2015-09-17T16:50:03

The mesomeric effect (or resonance effect) is the movement of π electrons toward or away from a substituent group.

$-M effect$ $bb "-M effect"$

For example, propenal has a mesomeric contributor in which the π electrons move towards the oxygen atom.

The molecule therefore has a $δ^-$ charge on $"O"$ and a $δ^+$ charge on $"C-3"$ .

Since the electrons have moved away from the rest of the molecule and towards the $"C=O"$ group, the effect is called a $bb "-M effect"$ .

Other $"–M"$ substituents are $"–COR"$ , $"-CN"$ , and $"-NO"_2$ .

$bb "+M effect"$

If the π electrons move away from the group and towards the rest of the molecule, the effect is called a $bb "+M effect"$ .

An example is the donation of electrons from an amino group into a benzene ring, putting $δ^-$ charges on the ortho and para positions.

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Other $"+M"$ substituents are $"–OH"$ , $"-OR"$ , $"-OCOR"$ , $"-NR"_2$ , and $"–NHCOR"$ .

What is the +M+M+M and -M−M-M effect? What are examples of electron releasing and electron withdrawing groups?