Electromeric effect

Electromeric effect refers to a molecular polarizability effect occurring by an intramolecular electron displacement (sometimes called the ‘conjugative mechanism’ and, previously, the ‘tautomeric mechanism’) characterized by the substitution of one electron pair for another within the same atomic octet of electrons. However, this term is now considered and this effect is considered along with the inductive effect.

This effect is shown by those compounds containing multiple bonds. When a double bond or triple bond is exposed to an attack by a reagent, a pair of bonding electrons involved in the pi bond is transferred completely from one atom to another. This effect will remain as long as the attacking reagent is present. As soon as the reagent is removed, the polarized molecule will come back to the original state.

+E and -E groups

Electromeric effect can be classified into +E and -E effects based on the direction of transfer of the electron pair.

When the electron pair moves towards the attacking reagent, it is termed as the +E effect. The addition of acids to alkenes is an example of the +E effect. After the transfer takes place, the reagent gets attached to the atom where the electrons have been transferred to.

The -E effect can be found in reactions such as addition of cyanide ion to carbonyl compounds. In these reactions, the electron pair moves away from the attacking reagent.

>C=O

As another example, consider the carbonyl group, >C=O, present in electromeric effect aldehydes and ketones. When a negatively charged reagent approaches the molecule seeking positive site, it causes instantaneous shift of the bond electron pair of the C=O bond in carbonyl group to oxygen (which is 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. It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.

Conclusion

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.

Nowadays electromeric effect is often considered along with inductive effect as electron displacement, but can also be applied to radiation theory.