Question

Please explain +E and -E electromeric effects with examples.

Answer 1

The Electromeric effect is the development of a dipole in a molecule of an organic compound as a result of the full transfer of shared pi electron pairs to one of the atoms under the action of an attacking reagent. In organic molecules with at least one multiple bond, this effect can be seen. When an attacking reagent comes into contact with the atoms in this multiple bond, one pi bonding pair of electrons is fully transferred to one of the two atoms.

Complete answer:
The electromeric effect is a transient effect that lasts as long as the attacking reagent is present and the organic molecule is exposed to it. The polarised molecule returns to its natural state after the attacking reagent is withdrawn from the solution. The +E effect and the -E effect are two forms of electromeric effects. The direction in which the electron pair is exchanged is used to classify them.
When the pi bond's electron pair is pushed towards the attacking reagent, this result happens. When acid is added to alkenes, the +E effect is visible. The attacking reagent binds to the atom that received an electron pair during the transfer. When the attacking reagent is an electrophile and the pi electrons are transported to the positively charged atom, the +E effect occurs.
When the pi bond's electron pair is shifted away from the attacking reagent, this result happens. The attacking reagent binds to the molecule's positively charged atom, i.e. the atom that lost an electron pair during the transfer. When the attacking reagent is a nucleophile and the pi electrons are moved to an atom that the attacking reagent will not bind with, the -E effect occurs.
The +E effect occurs when the $\Pi$ – bond's electrons are transferred to the atom to which the reagent is ultimately bound.
The -E effect occurs when the $\Pi$– bond's electrons are moved to an atom other than the one to which the reagent is eventually bound.
The impact of adding acid to an alkene is an example of the +E effect.
Adding cyanide ion to the carbonyl group is an example of the -E effect.

Note:
The inductive effect should not be confused with the electromeric effect. Due to the difference in electronegativity between the connected atoms, the inductive action causes dipoles to develop. It does not require numerous bonds, and it is a permanent effect, unlike the electromeric effect.