Q.Explain electromeric effect.

The electromeric effect (E) is a temporary effect, present only in unsaturated compounds ($>C=C<$ or $>C=O$) and only while a reagent is actually attacking -- it vanishes the instant the reagent is removed, unlike the permanent inductive, resonance and hyperconjugation effects. On approach of a reagent, the mobile $\pi$-electron pair shifts instantaneously and completely toward or away from that reagent, forming a new bond.

+E (positive electromeric effect): the $\pi$-electrons shift toward the attacking reagent. Example: addition of $H^+$ to an alkene -- the $\pi$ electrons move to bond with the incoming proton, forming a new C-H bond and leaving a carbocation on the other carbon. …