Electromeric Effect

Explain Electromeric Effect

Organic reactions are often affected or influenced by various electronic factors. Such factors include the likes of electromeric effect, resonance effect, inductive effect, and hyperconjugation. Because of the electronic effect, compounds with the same structure, formula, and units can behave differently.

Electromeric Effect Definition

In a pi (π) bond the electrons involved are easily polarized. When charged reagents such as electrophiles and nucleophiles come near, their electrons are polarized and lead to their displacement towards one of the constituent atoms. The mechanism is called electrostatic repulsion or attraction. The atom that gains or attracts the electron pair turns negatively charged and the other atom will have a positive charge.

The electromeric effect is a reversible reaction. The electron transfer happens due to the influence of the electrophile or nucleophile. When these agents disappear, their effects are also withdrawn.

The electromeric effect is non-specific in direction of electron transfer. It always happens that benefits the reaction. The electromeric effect happens only in presence of an electron attacking reagent. This effect is referred to as the E effect.

The direction of the Shift

During the electromeric effect, the direction of the movement of the pair of electrons follows a certain pattern under specific conditions as follows:

• When there are groups that are bonding to multiple bonds that have similar characteristics, the movement of electrons can be in either direction. It will be difficult to predict the flow in such conditions. 
• When the groups bonded are different at the end of the double bond present, the electron movement will be decided according to the inductive effects. It means it depends on the dipole in the compound to enable electron movement between the poles, resulting in either sigma bonds or pi bonds. 

Mechanism of the Electromeric Reaction

The mechanism is easier to understand in a double or a triple bond. When any of these are subjected to a reagent or an electrophile E+, the electrons that form the pi bond will move towards one of the atoms. This causes an instant dipole formation inside the molecule. 

The effect will be reversed when the reagent is removed. 

For example, here atom A loses its electron pair to atom B. Thus atom A will get a positive charge and atom B will have a negative charge. 

Types of Electromeric Effect

There are two types of electromeric effects: positive and negative electromeric effects. The former is denoted as the +E effect and the latter is the -E effect. 

 +E Effect or Positive Electromeric Effect

This takes place with the electron pair that is shared moves towards the attacking reagent. Here, a bond is formed with the atom that has got these shared pairs of electrons by the attacking regent. Here, the positively charged reagent is looking for a negatively charged site. 

 The +E effect is commonly seen in the addition reaction of acids with alkenes. 

 -E Effect or Negative Electromeric Effect

Nucleophiles are electron donors. The opposite of the positive electromeric effect is seen here where the shared pair of electrons move away from the attacking reagent. An example of this -E effect is seen in the addition of cyanide ions with carbonyl compounds.