Electromeric Effect

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This is a temporary effect and takes place between two atoms joined by a multiple bond, i.e., a double or triple bond. It occurs at the requirements of the attacking reagent, and involves instantaneous transfer of a shared pair of electrons of the multiple bond to one of the linked atoms.

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

For example, consider the carbonyl group, >C=O, present in aldehydes and ketones. When a negatively charged reagent say approaches the molecule seeking positive site, it causes instantaneous shift of electron pair of carbonyl group to oxygen (more electronegative than carbon). The carbon thus becomes deprived of its share in this transferred-pair of electrons and acquires positive charge. In the meanwhile oxygen takes complete control of the electron pair and becomes negatively charged. Therefore, in the presence of attacking reagent, one bond is lost and this negatively charged attacking reagent links to the carbon having positive charge.

This phenomenon of movement of electrons from one atom to another at the demand of attacking reagent in multibonded atoms is called electromeric effect, denoted as E effect. The electromeric shift of electrons takes place only at the moment of reaction. Like the inductive effect, the electromeric effect is also classified as +E and E:

• 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,