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.