Electromeric effect

The ELECTROMERIC effect is, by contrast with the inductive and resonance effects covered above, a genuinely TEMPORARY electronic effect -- one that is shown ONLY by multiple-bonded groups, and only under two conditions holding simultaneously: the molecule must be in an excited (reacting) state, and there must actually be a reagent approaching it at that moment. When a reagent does approach a multiple bond under these conditions, that bond's ENTIRE shared electron pair shifts COMPLETELY onto just one of the two multiply-bonded atoms in a single step, immediately generating a fully charge-separated structure -- illustrated using a generic carbonyl compound reacting with a proton (H⊕): as the incoming $H^{\oplus}$ approaches and a new O-H bond starts to form, the carbonyl's own C=O pi electron pair shifts entirely onto the oxygen atom, converting $>\!C{=}O$ directly into $>\!C^{\oplus}{-}O{-}H$, with the resulting positive charge now sitting on what was the carbonyl carbon. The single most important thing distinguishing the electromeric effect from the inductive and resonance effects is that it is genuinely TEMPORARY in the strictest sense: it exists only for as long as the reagent is actually present and actively interacting with the multiple bond, and it disappears completely, reverting the molecule back to its unperturbed ground state, the instant th …