Electromeric Effect: Types, Examples, and Mechanism

The electromeric effect is a reversible process in which a pi-electron pair is completely transferred due to the influence of an electrophile or a nucleophile.

• It is a temporary event that happens when a reagent assaults a multiple-bonded molecule, causing a complete shift of pi electrons to one of the bonds of two atoms.
• The effect disappears when the assaulting reagent is removed.
• Organic compounds are sometimes influenced by electronic factors electromeric effect, inductive effect, resonance effect, and hypercoagulation.
• Compounds that have similar formula units and structures tend to react differently in the presence of electronic factors.
• There are two types of electromeric effect: +E effect and -E effect.

Key Terms: Electromeric effect, Inductive effect, Organic compound, Pi electrons, Dipole molecules, Reagent, Multiple bond, Covalent bond, Molecules

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What is the Electromeric Effect?

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The spontaneous formation of a dipole molecule of the organic compound because of the complete transfer of shared pi electrons pairs to other atoms in the influence of the attacking reagent present is referred to as the Electromeric effect.

• The effect can only be observed in organic compounds that have a minimum of one multiple bond.
• The effect is temporary and can be observed till the time the attacking reagent is present and exposed to an organic compound.
• Once the attacking reagent is removed from the existing setting the effect breaks and the polarized molecules fall back to their original states.

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Direction of the Shift

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

• If the group is linked to multiple bonds that are similar, the shift will 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.

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Mechanism of Electromeric Effect

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The simple mechanism of the Electromeric effect can be explained in the following manner.

• When a double or triple-bond compound is subjected to an attacking reagent (electrophile E+), the pi electrons are transferred to different atoms.
• The transfer of the shared pi electrons supports the spontaneous formation of a dipole in the molecule.
• The Electromeric effect can be represented as the arrow which adheres to the displacement of the electron pair.

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Characteristics of Electromeric Effect

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The following are the characteristics of the electromeric effect

• It is a temporary effect and involves a complete transfer of pi-electrons to one of the bonded atoms.
• It operates in organic compounds that contain at least one multiple bond.
• It works only in the presence of an attacking reagent.
• The effect is lost in the absence of an attacking reagent.

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Types of Electromeric Effects

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There are two types of Electromeric effects:

• The E+ effect is also recalled as a positive Electromeric effect.
• The E- effect, is also recalled as the negative Electromeric effect.

+E Effect or Positive Electromeric Effect

Reagents (Electrophiles) show a positive nature towards electrons and when they attack, the pi-electrons are moved to the atom with a positive charge. The +E effect is observed when acids are added to alkenes.

-E Effect or Negative Electromeric Effect

Nucleophiles are electron donors and when they tend to attack, the electrons move away and into the pi system. The -E effect is observed when cyanide ion is added to carbonyl compounds.

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Examples of the Electromeric Effect

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Following are the examples of the electromeric effect

The reaction of an alkene with Br₂ in CCl₄

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.

Addition of hydrogen halides

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.

Nucleophilic addition reaction

When negatively charged nucleophiles come into the vicinity of the carbonyl molecule, the carbonyl group gets polarised, and the nucleophile attacks the positive center of the molecule.

Electrophilic addition reaction

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

Electrophilic substitution reactions of benzenoids

These reactions involve polarisation in benzene when attacked by an electrophile.

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Differences Between Electromeric Effect and Inductive Effect

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The following are the differences between the electromeric effect and the inductive effect

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Things to Remember

• The electromeric effect is a reversible process in which an electrophile or nucleophile entirely transfers a pi-electron pair.
• The electromagnetic effect is temporary because it remains till the attacking reagent is present.
• Using curved arrows, the electromeric effect can be represented as it symbolizes the electron shift.
• Electromeric effect is of types: Positive Electromeric effect and Negative Electromeric effect.
• The Electromeric effect disappeared upon the withdrawal of the attacking reagent.
• The complete transfer of a shared pair of electrons involves an electromeric effect.
• The inductive effect is observed in molecules having polar covalent bonds, but not necessarily multiple bonds.