Electromeric Effect can be observed only in organic compounds which contain multiple bonds. It is a temporary effect that arises when the compound is subjected to an attacking reagent.
What is the Electromeric Effect?
The instantaneous formation of a dipole in the molecule of an organic compound due to the complete transfer of shared pi electron pairs to one of the atoms under the influence of an attacking reagent is referred to as the Electromeric effect.
This effect can be observed in organic compounds that contain at least one multiple bond. When the atoms participating in this multiple bond come under the influence of an attacking reagent, one pi bonding pair of electrons is completely transferred to one of the two atoms.
The electromeric effect is a temporary effect that remains as long as the attacking reagent is present and exposed to the organic compound. Once this attacking reagent is removed from the system, the molecule that was polarized goes back to its original state.
Types of Electromeric Effects
The electromeric effect can be broken down into two types, namely the +E effect and the -E effect. This classification is done based on the direction in which the electron pair is transferred.
This effect occurs when the electron pair of the pi bond is moved towards the attacking reagent. The +E effect can be observed in the addition of acid to alkenes. The attacking reagent attaches itself to the atom which obtained an electron pair in the transfer.
The +E effect is generally observed when the attacking reagent is an electrophile and the pi electrons are transferred towards the positively charged atom. An example where the +E effect occurs is the protonation of ethene which is illustrated below.
This effect occurs when the electron pair of the pi bond is moved away from the attacking reagent. The attacking reagent attaches itself to the positively charged atom in the molecule, i.e. the atom which lost the electron pair in the transfer.
The -E effect is generally observed when the attacking reagent is a nucleophile and the pi electrons are transferred to the atom which the attacking reagent will not bond with. An example where the -E effect occurs would be the addition of nucleophiles to carbonyl compounds as illustrated below.