Please explain +E and -E electromeric effects with examples.
The electromeric effect is the movement of electrons from one atom to another as a reagent attacks a π bond.
If the attacking species is an electrophile, the π electrons are transferred towards the positively charged atom. This is the +E effect.
An example is the protonation of ethene. When the H⁺ comes near the double bond, the bond is polarized towards the proton.
If the attacking reagent is a nucleophile, the electrons are transferred away from the attacking reagent and into the π system. This is the –E Effect.
The term “electromeric effect” is somewhat outdated. We are more likely now to use words like “Use curved arrow notation to show the movement of electrons during the reaction”.