The complete shifting of π-electron pair of a multiple bond to one of the bonded atoms in presence of the attacking reagents is called electromeric effect. Positive electromeric effect (+E)
Negative electromeric effect [−E]
In positive +E effecr, π-electrons of the multiple bonds are transfered to that atom to which the reagent gets attached. In negative −E effect, π-electrons of the multiple bonds are transferred to that atom to which the attacking reagent does not get attached.