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.[1] 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 nucleophiles to carbonyl compounds. In these reactions, the electron pair moves away from the attacking reagent. When a negatively charged reagent approaches the molecule seeking partially positive carbon, it causes instantaneous shift of an electron pair of the C=O group towards the more-electronegative oxygen atom. 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.

Comparison between inductive effect and electromeric effect

Inductive Effect Electromeric Effect
The polarization of single σ covalent bond to the electronegativity difference between the bonding atoms is called inductive effect The complete shift of the π bond electron pair of a double bond or triple bond to one of the atoms joined by it in the presence of a suitable electrophilic reagent is called electromeric effect
It is a permanent effect It is a temporary effect
It doesn't need the presence of a reagent It needs the presence of an electrophilic or nucleophilic reagent

References

  1. ↑ . IUPAC GoldBook


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