Name: Structure of Benzene
Uploaded: 2019-07-19
Duration: 3 min 53 s
Description: Benzene is the most commonly known aromatic compound and is a significant part of gasoline. In this video, we will study the structure and properties of benz...

Question

Write a note on mesomeric effect.

Accepted Answer

The polarity developed between atoms of a conjugated system by the electron transfer or pi–bond electron transfer is known as the Mesomeric effect. In simple terms, we can describe mesomeric effect occurs when π electrons move away from or towards a substituent group in a conjugated orbital system.

The mesomeric effect can be subdivided into two types:

1) +M effect

2) -M effect

1) +M effect
When the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugated system then such a phenomenon is known as (+M) effect or positive mesomeric effect.

Conditions
For the +M effect, the group should have either a lone pair of electrons or should have a negative charge.
The +M effect gives negative charge to the conjugate system or it can be said that the electron density increases on the conjugate system due to this. These conjugate systems show more reactivity towards electrophiles and less reactivity towards a nucleophile.

Group showing +M effect

$- NH, - {NH}_{2}, - NHR, - {NR}_{2}, - O, - OH, - OR, - F, - Cl, - O - COR, - NHCOR, - SH, - SR etc .$

2) -M effect
When the pi-bond electrons are transferred from the conjugate system to a particular group thus the electron density of the conjugate system is decreased, then this phenomenon is known as negative mesomeric (–M) effect.

Conditions
For –M effect, the group should have either a positive charge or should have a vacant orbital.
–M effect makes the compound more reactive towards a nucleophile as it decreases the electron density in the conjugate system and at the same time it is less reactive towards electrophile due to same reasons.
Groups showing -M effect

The group which shows –M effect include;

$- {NO}_{2}, - CN, - COX, - {SO}_{3} H, - CHO, - {CONH}_{2}, - COR, - COOH, - COOR etc .$

Write a note on mesomeric effect.

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