It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only.
It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow. There are two distinct types of electromeric effect.
(i) Positive Eelctromeric Effect (+E effect) In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example :
(ii) Negative Electromeric Effect (–E effect) In this effect the π- electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example:
When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.