Question

Mesomeric effect involve:
A) delocalization of $\pi -$electrons
B) delocalization of $\sigma -$electrons
C) partial displacement of electrons
D) delocalization of $\pi$ and $\sigma$ electrons

Answer 1

The mesomeric effect is due to polarity produced in a molecule. Polarity formed in a molecule is due to the separation of charges which can be produced in a molecule by the delocalization of electrons.

Complete answer:

1. The mesomeric effect can be described as a qualitative approach which elaborates electron-withdrawing or releasing properties of substituents present in the molecule.
2. The delocalization of electrons happens between adjacent atoms in a molecule which has a conjugated system that leads to the transfer of electrons through the conjugated system.
3. Delocalization of electrons which leads to the partial charges at the ends of the conjugated system gives the molecule mesomeric effect.
4. For the delocalization of electrons, there must be the presence of a conjugated system.
5. Conjugated system forms due to the presence of $\pi$ bonds on adjacent atoms or the presence of lone pair on atoms adjacent to lone pair.
6. $\sigma$ bonds can not be delocalized as they are formed by sharing of electrons of two atoms and they are the primary structural elements of bonding of a molecule. Hence, options B and C are not correct.
7. This effect is a permanent effect that leads to changes in molecular chemical and physical properties.
   Hence, option C is incorrect.
8. The Mesomeric effect is due to the delocalization of the $\pi$.

Hence, option A is correct.

Additional information:
The mesomeric effect has its two types $+ M$ effect and $- M$ effect. These are differentiated on the basis of the movement of electrons in a molecule with respect to the functional group present in it.

Note:
The mesomeric effect is a permanent effect that is present in the chemical compound which has at least one or more double bonds or lone pairs on adjacent atoms that are present alternatively to single bonds. For the delocalization of electrons, the molecule must have a conjugated system. It is an effect arisen due to the overlap of p-orbitals.