Q.Explain electromeric effect.

Step 1. Definition. The electromeric effect is a temporary effect that operates only in unsaturated compounds (containing $>C=C<$ or $>C=O$) and only in the presence of an attacking reagent. Unlike the inductive, resonance and hyperconjugation effects, it is not a permanent feature of the molecule -- it appears the instant a reagent approaches and vanishes again once the reagent is withdrawn.

Step 2. Mechanism. When a suitable reagent approaches a multiple bond, the mobile $\pi$-electron pair shifts instantaneously and completely, forming a new bond either to or away from that reagent.

Step 3. +E effect. When the $\pi$-electrons shift toward the attacking reagent, it is called the positive electromeric effect (+E). 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. …

Define the electromeric effect as a temporary, reagent-triggered pi-electron shift, then give one +E example (H+ addition to an …

• Confusing the electromeric effect with the (permanent) resonance effect -- both involve pi-electron movement, but the electromeric effect switches on only while a reagent is attacking and switches off again, whereas resonance is a fixed structural feature. …