Question

What are the +M and –M effect? What are the examples of electron releasing and electron withdrawing groups?

Answer 1

We have to know that +M and –M are electromeric effect they can also be denoted by –E and +E. the only difference lies in the resonance and electromeric effect resonance is temporary while in mesomeric resonance is permanent that means permanent sharing of pi electrons.

Complete answer:
The term, though a bit outdated, is important for understanding changes in electronic density in a molecule in the presence of other species. This also involves movement of electrons but in this case due to some external agent. For example if a positive charge like H+ is brought near a double bond (say $C{H_2} = C{H_2}$), the double bond which is electron rich, the bond is polarized towards the proton, which can be shown as follows: the given below is showing +E effect.

This shifting of electrons or polarization of the covalent bond is termed as Electromeric effect. This case is called +E, as the polarization occurs due to the presence of a positive charge. A –E effect can be seen when some negatively charged species like OH– attacks a double bond: the given below is showing –E effect.

Electron releasing groups: $- C{H_3}, - OH$
Electron withdrawing groups: $- N{O_2}, - CN$

Note:
We have to know that the mesomeric effect in chemistry is a property of substituent or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom.