The key difference between EDG and EWG is that the EDG (stands for Electron Donating Groups) can increase the electron density of a conjugated pi system whereas the EWG (stands for Electron Withdrawing Groups) decreases the electron density of a conjugated pi system.
EDG and EWG are electrophilic aromatic directing groups. Both of these are forms of substituents that we can find in organic compounds.
CONTENTS
1. Overview and Key Difference
2. What is EDG
3. What is EWG
4. Side by Side Comparison – EDG vs EWG in Tabular Form
5. Summary
What is EDG?
EDG stands for electron donating groups. We call them “electron releasing groups (ERG)” as well. These are substituents in organic compounds that can donate some of its electron density into a conjugated pi system. This is done via resonance effect or inductive effect. This makes the pi electron system more nucleophilic.
For example, EDG, when attached to a benzene ring, the benzene ring can undergo electrophilic substitution reactions. This is because the EDG increases the electron density of the benzene ring. However, benzene usually undergoes this type of electrophilic substitution reaction. Hence the EDG can increase the reaction rate. Therefore, we call these substituents as activating groups for aromatic rings. Some examples of EDG include phenoxide, primary, secondary and tertiary amines, ether, phenols, etc.
What is EWG?
EWG stands for electron withdrawing groups. It has the opposite effect to that of EDG on an aromatic ring. Therefore, it removes electron density from a pi-electron system. This makes the pi electron system more electrophilic. Hence when these groups attach to benzene rings, they will reduce the reaction rate of electrophilic substitution reactions.
Figure 01: Nitrobenzene has a nitro group as EWG
Moreover, EWG can deactivate aromatic rings. This is done via resonance withdrawing effect or inductive withdrawing effect. For benzene, these groups can make the ortho and para positions less nucleophilic. Hence, the benzene ring tends to undergo electrophilic addition reactions at meta positions. Some examples of EWG includes trihalides, sulfonates, ammonium, aldehydes, ketones, esters, etc.
What is the Difference Between EDG and EWG?
EDG stands for electron donating groups while EWG stands for electron withdrawing groups. Both these are “electrophilic aromatic directing groups”. As a key difference between EDG and EWG, we can say that the EDG can increase the electron density of a conjugated pi system whereas the EWG decreases the electron density of a conjugated pi system. Basically, EDG can donate electrons while EWG can receive electrons. Moreover, EDG can increase the nucleophilicity of aromatic rings, which is the opposite function of EWG; it decreases the nucleophilicity of aromatic rings. Both these substituents show significant effects on the electrophilic substitution reactions of conjugated pi systems such as benzene ring; EDG can increase the reaction rate of electrophilic substitution reactions of aromatic rings whereas EWG can decrease the reaction rate of electrophilic substitution reactions of aromatic rings.
The below infographic lists out more details on the difference between EDG and EWG.
Summary – EDG vs EWG
Both EDG and EWG are electrophilic aromatic directing groups. They show opposite functions when attached to aromatic rings. Therefore, we can denote the key difference between EDG and EWG as; EDG can increase the electron density of a conjugated pi system whereas EWG decreases the electron density of a conjugated pi system.
Reference:
1. Hunt, Ian R. “Ch12: Substituent Effects.” Ch 13 – NMR Basics. Available here
2. “Electrophilic Aromatic Directing Groups.” Wikipedia, Wikimedia Foundation, 11 July 2018. Available here
Image Courtesy:
1.”Nitrobenzene resonance”By Ed (Edgar181) – Own work, (Public Domain) via Commons Wikimedia