Electromeric Effect

Electromeric Effect

This is a temporary effect involving the complete transfer of a shared pair of electrons to one or other atom joined by a multiple bond, i.e., a double or triple bond. The electromeric effect is brought into play only at the requirements of the attacking reagent, and because of this, the direction of the electromeric effect is always that which facilitates reaction.

The electromeric effect is represented as follows :
Electromeric effect
The curved arrow shows the displacement of the shared electron pair, beginning at the position where the pair was originally, and ending where the pair has migrated.  Since A has lost its share in the electron pair and B has gained this share, A acquires a positive charge and B a negative charge.

The electromeric effect is represented by the symbol E, and is said to be +E when the displacement is away from the atom or group, and –E when towards the atom or group.

N.B. – The displacement of the electron pair forming a covalent bond when a u it charge is brought up is a measure of the polarisability of that bond. It is not a permanent polarization since, when the charge is removed, the electron displacement disappears. Thus, the electromeric effect is a polarisability effect and operates in the excited state.