Q.What is electromeric effect? Give example.

The electromeric effect is a temporary electronic effect seen only in molecules containing a multiple bond (C=C, C=O, etc.), where the pi-electron pair shifts completely to one atom the instant a reagent attacks, and reverses back once the reagent is removed.

Comparing the electronic effects:

• Hyperconjugation (option a): permanent delocalisation of electron density from a C-H (or C-C) sigma bond into an adjacent empty/p orbital or pi system; not related to complete pi-electron transfer on demand.
• -I effect / +I effect (options b, c): the inductive effect — a permanent, weak polarisation of a sigma bond due to electronegativity differences, transmitted through the chain of sigma bonds; it does not involve pi electrons being completely transferred, and it doesn't require an attacking reagent. …

The electromeric effect is the temporary, complete transfer/displacement of a shared pair of pi-electrons of a multiple bond (double or triple) towards one of the atoms, triggered by the approach of an attacking reagent, lasting only while the reagent is present.

When a reagent approaches a molecule that contains a multiple bond (C=C, C=O, etc.), the pi-electron pair of that multiple bond can shift entirely to one of the two atoms forming the bond, in order to satisfy the demand of the incoming reagent (this generates a temporary dipole, priming the molecule for reaction). This is an intramolecular, temporary electron displacement — it is a purely electronic effect (unlike the inductive effect, which is permanent and operates through sigma bonds) and it disappears the moment the attacking reagent is remo …