Write a note on mesomeric effect.

Explanation

''The electron pair displacement caused by an atom or group of atoms along a chain by a conjugative mechanism is chain by a conjugative mechanism is called the Mesomeric effect of that atom or group ''.
Salient features of mesomeric effect (M) :
(a) This is a permanent effect operating in the ground state of the molecule.
(b) Lone pairs and `pi` electrons are involved and operate through conjugative mechanism of electron displacement.
(c) It influences the physical properties, reaction rates etc.
Groups which tend to increase the electron density of the rest of the molecule are said to have `+M` effect. Such groups tend to possess lone pairs of electrons.
Ex : `H_(2)N-underset(|)C=underset(|)C-`. Hence `-NH_(2)` group has `+M` effect.
Groups that decrease the electron density of the rest of the molecule are said to have `-M` effect. Unsaturated groups having polar character have `-M` effect.

Here `C=O` group decreases the electron density of the remaining molecule. It has `-M` effect.