[Chemistry Class Notes] on Electromeric Effect Pdf for Exam

Organic reactions are influenced by several electronic factors like electromeric effect, inductive effect, resonance effect, and hyperconjugation. Compounds having similar formula units and structures can react differently due to the electronic factors. 

Electrons present in a pi (π) bond can be polarized with little effort. In organic molecules with at least one multiple bond, this effect can be seen. When charged reagents like electrophiles or nucleophiles approach them, the electrons get polarized and displaced towards one of the constituents of the atom. The mechanism involved is electrostatic attraction or repulsion. The atom that acquires the pair of electrons becomes negatively charged while the other atom gets a positive charge. 

What is Electromeric Effect?

The electromeric effect is a reversible reaction where there is a complete transfer of a pi-electron pair due to the influence of an electrophile or a nucleophile. It is a transient effect that occurs when a reagent assaults a multiple-bonded molecule, producing a full shift of pi electrons to one of the bonds of two atoms. The effect disappears upon withdrawal of the attacking reagent.

The electromeric effect is also non-specific in terms of its direction which always favours the reaction. It is seen only when there is an electron attacking reagent and is also referred to as the E-effect. Curved arrow notation is also used to show the movement of electrons from one site to another. 

Polarity is created by the entire transfer of the shared pair of electrons. When the inductive and electromeric effects work in opposite directions, the inductive effect takes precedence. The entire transfer of a shared pair of electrons to one of the atoms is the inductive effect.

Direction of the Shift 

The direction of the shift of the pair of electrons will be as follows:

• If the groups linked to a multiple bond are similar, a shift can occur in either direction.

• If dissimilar groups are linked on the ends of the double bond, the shift of the electron pair is decided by the inductive effect.

 

Mechanism of the Electromeric Reaction

The mechanism of an electromeric reaction can be explained in the following manner.

When a double or a triple bond is subjected to an attack by an electrophile E+ (a reagent), the 2 pi electrons forming the pi bond are transferred to one atom or the other. The transfer of the shared pi electrons leads to the instantaneous formation of a dipole in the molecule. The electromeric effect can be represented by the following chemical reaction (See figure 1). 

The curved arrow in this figure depicts the displacement of the electron pair. The atom A loses its share in the electron pair and the atom B has acquired this share. Thus, electronically, A acquires a positive charge while B acquires a negative charge.

Types of Electromeric Effect

There are two distinct types of electromeric effect:

• The +E effect, which is also referred to as the positive electromeric effect.

• The -E effect, which is also referred to as the negative electromeric effect.

 

+E Effect or Positive Electromeric Effect

Electrophiles are electron acceptors and when they attack, the pi-electrons are shifted to the atom with a positive charge. This effect is depicted as +E and can be represented by the chemical reaction shown below (See figure 2). The +E effect is observed in the addition of acids to alkenes. 

 

-E Effect or Negative Electromeric Effect

Nucleophiles are electron donors and if they attack, the electrons are shifted away and into the pi system. This effect is represented as -E and can be represented by the following chemical reaction (See figure 3). The -E effect is observed in the reaction of the addition of cyanide ion to carbonyl compounds. 

 

Examples of the Electromeric Effect

As the reagent bromine approaches alkene, temporary polarization develops with the C2 atom gaining a negative charge and the C1 atom acquiring a positive charge. Alkenes are attacked by the electrophile Br+ to produce a cyclic bromonium ion as an intermediate. The cyclic bromonium ion is then attacked by Br− giving vicinal dibromide as the product

Hydrogen halides provide both an electrophile (proton) and a nucleophile (halide). The electrophile attacks the double bond, takes up a set of pi electrons and attaches it to the resulting molecule (carbocation). The nucleophile (halide) completes the reaction to give rise to a new molecule. Prominent examples of the same have been illustrated in figures 5 and 6. 

When negatively charged nucleophiles come in the vicinity of the carbonyl molecule, the carbonyl group gets polarised and the nucleophile attacks the positive center of the molecule. The reaction can be represented as follows (See figure 7).

Symmetrical alkenes or alkynes involve polarisation of the carbon-carbon double bond due to the action of electrophiles such as H+ (See figure 8). 

These reactions involve polarisation in benzene when attacked by an electrophile (See figure 9). 

Differences Between Electromeric Effect and Inductive Effect

There are significant differences between the electromeric effect and the inductive effect. While the electromeric effect is reversible the inductive effect is a permanent state of polarization. When both inductive and electromeric effects occur simultaneously, the electromeric effect predominates over the inductive effect. The differences are shown in the table. 

Difference Between Electromeric and Mesomeric Effects