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Question: Which of the following have +M effect (\({e^ - }\) donating mesomeric effect) ? This question has ...

Which of the following have +M effect (e−{e^ - } donating mesomeric effect) ?
This question has multiple correct options
(A) NO2N{O_2}
(B) COOHCOOH
(C) NH2N{H_2}
(D) SRSR

Explanation

Solution

The mesomeric effect in chemistry is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two Pi(π)Pi(\pi ) bonds or between a pi bond and lone pair of electrons present on an adjacent atom.

Complete answer:
There are two types of mesomeric effect:
a) +M+M effect (e−{e^ - } donating effect)
b) −M-M effect (e−{e^ - } withdrawing effect)
1. +M+M effect (e−{e^ - } donating effect) – When the electrons or the π\pi electrons are transferred from a particular group towards a conjugate system, this increasing the electron density of the conjugate system then such a phenomenon is known as (+M)(+M) effect e−{e^ - } donating mesomeric effect. For the +M+M effect the group should have a lone pair or negative charge.
Group showing +M+M effect –
−NH,−NH2−NHR,−NR,−O,−OH,−OR,−F,−Cl,−SH,−SR- NH, - N{H_2} - NHR, - NR, - O, - OH, - OR, - F, - Cl, - SH, - SR etc.
2. −M-M effect (e−{e^ - } donating effect) – When pi-bond electrons are transferred from the conjugate system to a particular group thus the electron density of the conjugate system is discovered, then the phenomenon is known as negative isomeric effect (−M)( - M).
Group showing −M - M effect –
−NO2,−CN,−COX,−SO3H,−CHO,−CONH2,−COR,−COOH,−COOR- N{O_2}, - CN, - COX, - S{O_3}H, - CHO, - CON{H_2}, - COR, - COOH, - COOR etc.
Therefore from the above information
−NO2- N{O_2} and −COOH- COOH are electron withdrawing groups


Resonating structures of nitrobenzene

Resonating structure of Benzoic acid

From above structures, it is clear that −NO2- N{O_2} and −COOH- COOH withdraw electrons from the ring. So they show a −M- M effect.
−NH2- N{H_2} and −SR - SR are electron donating groups –

Resonating structures of Aniline

Resonating structure of Thiophenol
From the above structure, it is clear that −NH2 - N{H_2} and −SH - SH donate electrons in the ring. So, they show +M + M effect.

Hence, the correct answers are (C) and (D) (−NH2,−SR)( - N{H_2}, - SR).

Note: Molecules which have double or triple bond show −M - M effect whereas molecules which have lone pair on the directly bonded atom show +M + M effect