Question

Identify electron withdrawing group in resonance among the following.
A.\[ - {\text{COOH}}\]
B.\[ - {\text{CONHC}}{{\text{H}}_3}\]
C.\[ - {\text{COCl}}\]
D.\[ - {\text{CN}}\]
E.\[ - {\text{O}} - {\text{CH}} = {\text{C}}{{\text{H}}_2}\]

Answer 1

Hint: An electron withdrawing group with \[ - {\text{R}}\] effect takes electrons from the chain it is attached to through resonance. When pi electrons are present on an adjacent atom with respect to the group that is going to get attached then it will act as an electron withdrawing group.

Complete step by step solution:
Resonance is the delocalization of the pi electron. Resonance occurs in various ways. One of the ways of doing resonance is the presence of pi conjugation that is there is presence of alternate pi bonds.
An electron group will act as a donating group through resonance, when it has a lone pair of electrons and there is some pi bond system on the adjacent atom.
For example the group given in the option E is electron donating group. The compound will show the \[{\text{ + R}}\] effect. The oxygen will attract electrons towards it being the electronegative element.
All the carbonyl functional groups act as electron withdrawing groups. This is due to the presence of electrophilic carbon. The oxygen being more electronegative takes electron density from carbon makes it electron deficient and hence it takes electrons showing \[ - {\text{R}}\] effect. Hence option A, B, C will act as an electron withdrawing group.
\[ - {\text{CN}}\] also takes electron resonance through and shows \[ - {\text{R}}\] effect.

Note: It is not necessary that a group that shows \[ - {\text{R}}\] will also show \[ - {\text{I}}\] effect. For example \[ - {\text{O}} - {\text{CH}} = {\text{C}}{{\text{H}}_2}\] shows \[{\text{ + R}}\] effect by donating electron through resonance, but is a \[ - {\text{I}}\] group and takes electron density through single bond due to the more electro-negativity of oxygen. Resonance is all about the movement of pi bonds. One of the most important things in resonance is the p orbital should be in the same plane. If the plane distorts then resonance won’t happen.