Question

Write a short note on Mesomeric effect.

Answer 1

Mesomeric effect

• The mesomeric effect is the polarity developed between atoms in a conjugated system via electron transfer or $\mathrm{\pi }$–bond electron transfer.
• The mesomeric effect is separated into two categories: +M and -M effects.

+M effect

• The group must have either a lone pair of electrons or a negative charge to produce the +M effect.
• The +M effect causes the conjugate system to have a negative charge, or the electron density to rise in the conjugate system.
• These conjugate complexes have a higher electrophile reactivity and a lower nucleophile reactivity.
• Group showing +M effect: $–\mathrm{NH},–{\mathrm{NH}}_2,–\mathrm{NHR},–{\mathrm{NR}}_2,–\mathrm{O},–\mathrm{OH},–\mathrm{OR},–\mathrm{F},–\mathrm{Cl},$ etc.

–M effect

• The group must have either a positive charge or an empty orbital for the –M effect to occur.
• The –M effect makes a molecule more reactive to a nucleophile by lowering the electron density in the conjugate system, but also makes it less reactive to an electrophile.
• Group shows –M effect: $–{\mathrm{NO}}_2,–\mathrm{CN},–\mathrm{COX},–\mathrm{COR},–\mathrm{COOH},–\mathrm{COOR}$ etc.