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

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Sep 17, 2015

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

#### Explanation:

$\boldsymbol{\text{-M effect}}$

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

(from en.wikipedia.org)

The molecule therefore has a δ^- charge on $\text{O}$ and a δ^+ charge on $\text{C-3}$.

Since the electrons have moved away from the rest of the molecule and towards the $\text{C=O}$ group, the effect is called a $\boldsymbol{\text{-M effect}}$.

Other $\text{–M}$ substituents are $\text{–COR}$, $\text{-CN}$, and ${\text{-NO}}_{2}$.

$\boldsymbol{\text{+M effect}}$

If the π electrons move away from the group and towards the rest of the molecule, the effect is called a $\boldsymbol{\text{+M effect}}$.

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

Other $\text{+M}$ substituents are $\text{–OH}$, $\text{-OR}$, $\text{-OCOR}$, ${\text{-NR}}_{2}$, and $\text{–NHCOR}$.

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