Question

Please explain +E and -E electromeric effects with examples.

Answer 1

Electromeric effect:

• The electromeric effect is a temporary effect operating in unsaturated compounds only at the demand of a nearby reagent.
• It involves the complete transfer of $\mathrm{\pi }$ electrons of multiple bonds.
• During the attack of a carbonyl group by a nucleophile the $\mathrm{\pi }$-electron pair-bonding the Carbon and Oxygen is transferred entirely to Oxygen, that is,

• Here the positively charged Carbon in the above-polarised structure of the carbonyl group is then the center of attack by the nucleophilic reagent.
• The displacement of the electron pair to Oxygen rather than to Carbon is due to the greater electronegativity of the Oxygen atom as compared to the Carbon atom.
• The Electromeric effect is also of two types i.e. +E effect and –E effect.

+E effect:

• When the transfer of $\mathrm{\pi }$ electrons takes place towards the atom where the attacking reagent (Electrophile) gets finally attached, the effect is called +E effect.

• The example of the +E effect is

• Here the double bond breaks and the transfer of $\mathrm{\pi }$ electrons takes place towards the Hydrogen ion

–E effect

• When the transfer of $\mathrm{\pi }$ electrons occurs away from the attacking reagent ('Nucleophile), the effect is called -E effect.

• The example of -E effect is

• Here the double bond breaks and the transfer of $\mathrm{\pi }$ electrons occurs away from the Cyanide ion.