The polar effect or electronic effect in chemistry is the effect exerted by a substituent on modifying electrostatic forces operating on a nearby reaction center. The main contributors to the polar effect are the inductive effect, mesomeric effect and the through-space electronic field effect.
An electron withdrawing group or EWG draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect.
Examples of electron withdrawing groups are
An electron releasing group or ERG (may also be called electron donating groups or EDG's) releases electrons into a reaction center and as such stabilizes electron deficient carbocations. An ERG can essentially promote groups into have a higher effect. These higher effects are defined as steric effects to a degree, however other effects include observations to changes in polarity, which can thus produce an entirely different molecule. This entirely different molecule is the ERG, plus whatever substituent was left behind. This is partially true but in truth the entirely different molecule is composed of parts and can be further differentiated and stabilized as separate molecules as well, although this can prove difficult for the beginner. Different steric effects can produce different molecules as well, and these essentially allow certain molecular formations to take themselves apart, and if this is done, then the ERG can be recognized as a separate molecule. This is difficult but can prove that ERGs form themselves not due to some type of separation but due to the fact that the presence of weak electronic sources, that an ERG must form. If it does not then the electron weak reaction center will fall apart, and return to its original formation, and thus the separation must occur due to the steric effect itself, and not the fact that the alcohols donate electrons, but due to the change in electrons that allows the center to remain stable without the need for the other molecules to be present (Examples).
Examples of electron releasing groups are
The total substituent effect is the combination of the polar effect and the combined steric effects.
In electrophilic aromatic substitution and nucleophilic aromatic substitution substituents are divided into activating groups and deactivating groups where the direction of activation or deactivation is also taken into account.