The electromeric effect is a TEMPORARY shift of pi electrons to one atom of a multiple bond, occurring ONLY in the presence of an attacking reagent.
- +E effect: Electrons shift toward the attacking reagent (more common)
- -E effect: Electrons shift away from the attacking reagent
Key differences from mesomeric effect:
| Electromeric | Mesomeric |
|---|---|
| Temporary | Permanent |
| Requires reagent | Intrinsic property |
| Complete transfer of pi electrons | Partial delocalization |
Seen in: Electrophilic additions to C=C and C=O, nucleophilic additions to C=O.