Electromeric & Mesomeric Effects

Electromeric Effect ($$E$$): Temporary complete transfer of a shared pair of $$\pi$$ -electrons to one atom in a multiple bond under the influence of an attacking reagent.

$$+E$$: $$\pi$$ -electrons transfer toward the atom to which the reagent attacks.

$$-E$$ : $$\pi$$-electrons transfer away from the atom to which the reagent attacks.

Mesomeric Effect ($$M$$ ): Permanent delocalization of $$\pi$$ -electrons or lone pairs through conjugation.

$$+M$$ Order:

$$\text{--O}^- > \text{--NH}_2 > \text{--NR}_2 > \text{--OH} > \text{--OR} > \text{--NHCOCH}_3 > \text{--F} > \text{--Cl}$$

$$-M$$ Order:

$$\text{--NO}_2 > \text{--CN} > \text{--CHO} > \text{--C(=O)R} > \text{--COOH} > \text{--COOR}$$