Electromeric Effect – Introduction, Types and Examples

The Electromeric Effect is only seen in organic molecules with many bonds. When a chemical is exposed to an attacking reagent, it produces a transient effect.

What is the Electromeric Effect and how does it work?

The Electromeric effect is the creation of a dipole in a molecule of an organic compound as a result of the complete transfer of shared pi-electron pairs to one of the atoms under the influence of an attacking reagent.

Inorganic molecules with at least one multiple bonds, this effect can be seen. When an attacking reagent comes into contact with the atoms in these multiple bonds, one pi bonding pair of electrons is entirely transferred to one of the two atoms.

The electromeric effect is a transient effect that lasts as long as the attacking reagent is present and the organic compound is exposed to it. The polarised molecule returns to its natural state after the attacking reagent is withdrawn from the solution.

Direction of the Shift

The shift of the pair of electrons will be in the following direction:

  • The shift can occur in either direction if the groups linked to multiple bonds are similar.
  • The inductive effect determines the shift of the electron pair when different groups are joined on the ends of the double bond.

The Electromeric Reaction Mechanism

An electromeric reaction’s mechanism can be stated in the following way. When an electrophile E+ (a reagent) attacks a double or triple bond, the two pi electrons constituting the pi bond are transferred to one atom or the other. 

The instantaneous development of a dipole in the molecule is caused by the transfer of the shared pi electrons. The following chemical process can be used to depict the electromeric effect. 

The displacement of the electron pair is depicted by the curved arrow in this diagram. Atom A loses its portion of the electron pair, which is now held by atom B. As a result, A receives a positive charge while B receives a negative charge.

Types of Electromeric Effect

Electromeric effects can be divided into two categories:

  • The positive electromeric effect, also known as the +E effect, is a phenomenon that occurs when a substance has a positive charge.
  • The -E effect, also known as the negative electromeric effect, is a phenomenon that occurs when a substance has a negative charge.

+E Effect or Positive Electromeric Effect

When electrophiles attack, the pi-electrons are moved to the atom with a positive charge. The chemical reaction described below can reflect this effect, which is denoted by the letter +E. When acids are added to alkenes, the +E effect is observed.

-E Effect or Negative Electromeric Effect

When a nucleophile attacks, the electrons are moved away from the nucleophile and into the pi system. This effect is denoted by the letter -E, and it can be represented chemically by the following reaction (See figure 3). In the reaction of cyanide ion addition to carbonyl compounds, the -E effect is observed.

Examples of the Electromeric Effect

The reaction of an alkene with Br₂ in CCl₄

Temporary polarisation occurs as the reagent bromine approaches an alkene, with the C2 atom obtaining a negative charge and the C1 atom gaining a positive charge. The electrophile Br+ attacks alkenes, producing a cyclic bromonium ion as an intermediary. Br attacks the cyclic bromonium ion, yielding vicinal dibromide as a result.

Hydrogen halide addition

Both an electrophile (proton) and a nucleophile (hydrogen halide) are present in hydrogen halides (halide). The electrophile assaults the double bond and absorbs a set of pi electrons, which it then attaches to the molecule (carbocation). The nucleophile (halide) completes the reaction and results in the formation of a new molecule.

Addition reaction with nucleophiles

When negatively charged nucleophiles approach a carbonyl molecule, the carbonyl group becomes polarised, and the nucleophile assaults the molecule’s positive core. The following is a diagram of the reaction.

The reaction of electrophilic addition

The action of electrophiles such as H+ causes polarisation of the carbon-carbon double bond in symmetrical alkenes or alkynes.

Benzenoids undergo electrophilic substitution reactions.

When benzene is attacked by an electrophile, it undergoes polarisation.

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