Question

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

Answer 1

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

Explanation:

`bb "-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 `"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.

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