Electromeric effect: The temporary effect which operates in compounds having multiple bonds (double or triple) under the influence of an attacking species.

As a result, one pi electron pair of the multiple bond gets completely transferred to the more electronegative atom.

 

eg. Consider a carbonyl group. When a nucleophile approaches, it causes the the shift of pi electrons to the more electronegative atom(Oxygen). Thus O becomes negatively charged and C becomes deprived of electrons and becomes positively charged.

 

It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.

 

There are 2 types of Electromeric effects:

When the transfer of electrons takes place towards the attacking reagent, it is called + E (positive electromeric) effect. For example,

When the transfer of electrons takes place away from the attacking reagent, it is called, -E (negative electromeric) effect. For example,

 

 

if an alkyl group carrying at least one hydrogen atom is attached to an unsaturated carbon atom, it releases the electrons of carbon-hydrogen single bond towards the multiple bond. For example, the hyper conjugation in propene is shown below: