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Electromeric effect

Electromeric effect

Shivani PooniaUpdated on 02 Jul 2025, 06:29 PM IST

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 bonds 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.

This Story also Contains

  1. Electromeric Effect
  2. Solved Examples Based on Electromeric Effect
  3. Conclusion
Electromeric effect
Electromeric effect

In this article, we will cover the topic (Hyperconjugation). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry).

Electromeric Effect

It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent.

The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E.

In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example:

This phenomenon occurs when the pi bond’s electron pair is shifted toward the attacking reagent. The +E effect is visible when acid is added to alkenes. A positive electron transfer (or +E effect) occurs when an electrophile attacks a positively charged atom and the pi electrons are transferred to the positively charged atom. The protonation of ethene is an illustration of the +E effect. This phenomenon occurs when the attacking reagent’s electron pair is shifted away from the pi bond’s electron pair. The attacking reagent binds to the molecule’s positively charged atom, i.e., the atom that lost an electron pair during the transfer.

Recommended topic video on(Electromeric effect)


Solved Examples Based on Electromeric Effect

Q.1 Displacement of $\pi$ electron of a multiple bond towards the atom or away from the atom at the demand of reagent is called

(1) Electromeric effect

(2) Inductive effect

(3) Mesomeric effect

(4) Hyperconjugation

Solution:

As we have learned

The displacement of $\pi$ electrons in multiple bonds towards the atom or away from the atom at the demand of a reagent is called the electromeric effect.

Hence, the answer is the option (1).

Q.2 Select an incorrect statement about Electromeric effect-

(1) It involves polarisation of $\pi$ electrons in the presence of a reagent

(2) It is a temporary effect

(3) Electrophilic reagents are generally the cause for this effect in Alkenes

(4) The polarisation of $\pi$ electrons persist even after the reagent has been removed from the system

Solution:

As we have learned

The electromagnetic effect is a temporary effect in which the $\pi$ electrons are polarised in the presence of an attacking reagent.

The polarization is not present in the absence of the reagent

Hence, the answer is the option(4).

Q.3 The interaction between the π bond and a lone pair of electrons present on an adjacent atom is responsible for :

1) (correct)Resonance effect

2)Electromeric effect

3)Inductive effect

4)Hyperconjugation

Solution

→ The interaction between the π bond and the lone pair of electrons present on the adjacent atom is responsible for the resonance effect.

$\overbrace{\mathrm{CH}_2}=\mathrm{CH}-\stackrel{\ominus}{\mathrm{O}} \mathrm{H} \longleftrightarrow \stackrel{\ominus}{\mathrm{C}} \mathrm{H}_2-\mathrm{CH}=\stackrel{\oplus}{\mathrm{O}} \mathrm{H}$

Conclusion

Inductive, electromeric, and resonance effects are three of the most frequently observed electronic effects in an organic reaction generated by the attacking reagent. Each of these three effects results in the formation of polarity in the organic substrate. The electromeric effect can be defined as a transient effect that creates polarity in an organic molecule with pi-linked atoms. Electromeric interactions are categorized as$+E$ or $-E$ depending on the atom with which the assaulting reagent engages.

Frequently Asked Questions (FAQs)

Q: How does the electromeric effect contribute to the concept of resonance energy?
A:
The electromeric effect contributes to resonance energy by facilitating the movement of electrons that allows for the formation of different resonance structures. The ability of electrons to delocalize through electromeric effects contributes to the overall stability of the molecule, which is reflected in its resonance energy. Generally, stronger and more extensive electromeric effects can lead to greater resonance stabilization and higher resonance energy.
Q: How does the electromeric effect influence the strength of intermolecular forces?
A:
The electromeric effect can influence the strength of intermolecular forces by altering the electron distribution within molecules. This can affect dipole-dipole interactions, induced dipole interactions, and even van der Waals forces. For example, a strong electromeric effect might enhance molecular polarizability, potentially increasing the strength of induced dipole interactions.
Q: Can you explain how the electromeric effect impacts the reactivity of alkenes in addition polymerization?
A:
In addition polymerization of alkenes, the electromeric effect can impact reactivity by influencing the electron density of the double bond. Alkenes with substituents that exhibit a positive electromeric effect may be more reactive towards electrophilic initiators due to increased electron density at the double bond. Conversely, a negative electromeric effect might decrease this reactivity.
Q: How does the electromeric effect influence the strength of carbon-heteroatom bonds?
A:
The electromeric effect can influence the strength of carbon-heteroatom bonds by altering the electron distribution. A positive electromeric effect towards the heteroatom can strengthen the bond by increasing its double bond character. Conversely, a negative electromeric effect away from the heteroatom might weaken the bond by decreasing electron density between the atoms.
Q: What is the relationship between the electromeric effect and aromaticity?
A:
The electromeric effect and aromaticity are closely related, as both involve the movement and delocalization of π electrons. In aromatic systems, the cyclic delocalization of π electrons can be viewed as a special case of the electromeric effect. Substituents on aromatic rings can exhibit electromeric effects that influence the overall aromaticity and reactivity of the system.
Q: How does the electromeric effect impact the stereochemistry of addition reactions?
A:
The electromeric effect can impact the stereochemistry of addition reactions by influencing the stability and formation of reaction intermediates. For example, in electrophilic addition to alkenes, the electromeric effect can stabilize certain carbocation intermediates, potentially leading to preferential formation of specific stereoisomers.
Q: How does the electromeric effect contribute to the mechanism of elimination reactions?
A:
In elimination reactions, the electromeric effect can play a role by influencing the electron distribution in the reacting molecule. For example, in E2 reactions, substituents exhibiting a positive electromeric effect can stabilize the developing negative charge in the transition state, potentially facilitating the elimination process.
Q: What is the relationship between the electromeric effect and hyperconjugation?
A:
While both the electromeric effect and hyperconjugation involve electron delocalization, they differ in nature. The electromeric effect is a temporary, reagent-induced effect involving complete electron transfer, while hyperconjugation is a permanent effect involving partial delocalization of σ electrons with adjacent π or p orbitals. However, both effects can work together to influence molecular properties and reactivity.
Q: How does the electromeric effect influence the strength of coordinate covalent bonds?
A:
The electromeric effect can influence the strength of coordinate covalent bonds by affecting the electron-donating ability of the atom providing the lone pair. A positive electromeric effect can enhance this electron-donating ability, potentially strengthening the coordinate bond. Conversely, a negative electromeric effect might reduce the electron-donating capacity, potentially weakening the coordinate bond.
Q: Can you explain how the electromeric effect impacts the reactivity of conjugated dienes?
A:
The electromeric effect impacts the reactivity of conjugated dienes by influencing the electron distribution across the conjugated system. This can affect the relative reactivity of different positions within the diene, influencing the ratio of 1,2-addition versus 1,4-addition products in reactions like electrophilic addition. The electromeric effect can also influence the diene's ability to participate in cycloaddition reactions like the Diels-Alder reaction.
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