Polar effect

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.

Examples of electron releasing groups are

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

Polar mutation

A polar mutation affects expression of downstream genes or operons. It can also affect the expression of the gene in which it occurs, if it occurs in a transcribed region. These mutations tend to occur early within the sequence of genes and can be nonsense, frameshift, or insertion mutations. Polar mutations are found only in organisms containing polycistronic mRNA.

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