What is the difference between electron withdrawing groups and electron donating groups?

An electron withdrawing group, or EWG, draws electrons away form a reaction centre. The presence of an electron-withdrawing substitute has a stabilizing effect when the center is an electron rich carbanion (or an alkoxide anion).

These are examples of electron-drawing groups

* halogens (F, Cl);

* nitriles CN;

* Carbonyls RCOR’

* nitro groups NO2.

An electron releasing gas or ERG (also known as electron donating or EDG’s), releases electrons into a reaction centre and stabilizes electron-deficient carbocations.

These are examples of electron-releasing groups

* Alkyl groups

* Alcohol groups

* amino groups.

The total substituent effect is the sum of both the polar and combined steric effects.

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