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

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Sep 17, 2015

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

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#bb "-M effect"#

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

-M effect
(from en.wikipedia.org)

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"#.

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