Question

what is +M and -M effect

Answer 1

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

Explanation:

-M effect

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

-M effect

The molecule therefore has a δ− charge on Oand 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 -M effect.

Other –M substituents are –COR, -CN, and -NO2.

+M effect

If the π electrons move away from the group and towards the rest of the molecule, the effect is called a +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, -NR2, and –NHCOR.