Electronic and Steric Effects
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A mesomeric effect is an electron redistribution that occurs via a pi orbital, quite often via conjugated systems.
The factors affecting the electron availability of a compound might reasonably be thought to have far-reaching consequences upon its reactivity with various compounds. For example, an area of high electron density is unlikely to be attacked by OH–, but an area of low electron density is likely to be far more susceptible to attack by the same reagent.