Electromeric Effect

Electromeric effect

Edited By Shivani Poonia | Updated on Jul 02, 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

Electromeric Effect
Solved Examples Based on Electromeric Effect
Conclusion

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).

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.

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)

1. What is the electromeric effect?

The electromeric effect is a temporary and reversible electronic displacement that occurs in multiple-bonded molecules when they are subjected to an external reagent or electric field. It involves the complete transfer of one or more π electrons, resulting in the formation of a new σ bond.

2. How does the electromeric effect differ from the inductive effect?

The electromeric effect differs from the inductive effect in several ways: 1) It occurs only in compounds with multiple bonds, while the inductive effect can occur in single-bonded molecules. 2) The electromeric effect is temporary and reversible, whereas the inductive effect is permanent. 3) The electromeric effect involves complete electron transfer, while the inductive effect involves partial electron displacement.

3. What are the two types of electromeric effects?

The two types of electromeric effects are: 1) Positive electromeric effect (+E effect), where electrons move away from the substituent group, and 2) Negative electromeric effect (-E effect), where electrons move towards the substituent group.

4. Can you explain the positive electromeric effect (+E effect)?

The positive electromeric effect (+E effect) occurs when electrons move away from a substituent group towards the rest of the molecule. This typically happens when an electron-donating group is present, and it results in the formation of a positive charge on the substituent.

5. What is the negative electromeric effect (-E effect)?

The negative electromeric effect (-E effect) occurs when electrons move towards a substituent group from the rest of the molecule. This typically happens when an electron-withdrawing group is present, and it results in the formation of a negative charge on the substituent.

6. Can you give an example of a molecule exhibiting the positive electromeric effect?

A common example of a molecule exhibiting the positive electromeric effect is aniline (C6H5NH2). The lone pair of electrons on the nitrogen atom can be donated to the benzene ring, resulting in a positive charge on the nitrogen and increased electron density in the ring.

7. What is an example of a molecule showing the negative electromeric effect?

Nitrobenzene (C6H5NO2) is an example of a molecule showing the negative electromeric effect. The strongly electron-withdrawing nitro group pulls electrons away from the benzene ring, resulting in a partial negative charge on the oxygen atoms of the nitro group.

8. Why is the electromeric effect considered a temporary effect?

The electromeric effect is considered temporary because it only occurs when a molecule is under the influence of an external reagent or electric field. Once this external influence is removed, the electron distribution returns to its original state, making the effect reversible.

9. Can the electromeric effect occur in saturated compounds?

No, the electromeric effect cannot occur in saturated compounds. It requires the presence of multiple bonds (π bonds) to facilitate the movement of electrons. Saturated compounds only have single bonds (σ bonds), which do not allow for this type of electron movement.

10. How does the electromeric effect differ in alkenes and alkynes?

The electromeric effect can occur in both alkenes and alkynes, but it is generally stronger in alkynes. This is because alkynes have two π bonds, providing more electrons for potential movement compared to alkenes, which have only one π bond.

11. Can you explain how the electromeric effect influences the reactivity of dienophiles in Diels-Alder reactions?

In Diels-Alder reactions, the electromeric effect can influence the reactivity of dienophiles by altering their electron density. Dienophiles with electron-withdrawing groups exhibit a negative electromeric effect, making them more reactive by lowering the energy of their LUMO (Lowest Unoccupied Molecular Orbital), which facilitates better overlap with the HOMO (Highest Occupied Molecular Orbital) of the diene.

12. Can you explain how the electromeric effect influences the strength of π-π interactions?

The electromeric effect can influence the strength of π-π interactions by altering the electron density in π systems. Substituents that exhibit a strong positive electromeric effect can increase the electron density in an aromatic system, potentially enhancing its ability to participate in π-π stacking. Conversely, a negative electromeric effect might reduce this electron density, potentially weakening π-π interactions.

13. How does the electromeric effect influence reactivity in organic compounds?

The electromeric effect influences reactivity by temporarily altering the electron distribution in a molecule. This can affect the molecule's ability to act as an electrophile or nucleophile, change its stability, and influence its reaction rates and pathways.

14. How does the presence of multiple bonds affect the electromeric effect?

Multiple bonds (double or triple bonds) are essential for the electromeric effect to occur. These bonds provide the π electrons that can be easily moved or transferred within the molecule, allowing for the temporary electronic rearrangement characteristic of the electromeric effect.

15. How does the electromeric effect impact the acidity or basicity of a compound?

The electromeric effect can impact acidity or basicity by altering the electron distribution in a molecule. For example, a positive electromeric effect can increase electron density at certain sites, potentially making the molecule more basic, while a negative electromeric effect can decrease electron density, potentially increasing acidity.

16. What role does the electromeric effect play in resonance structures?

The electromeric effect plays a crucial role in resonance structures by facilitating the movement of π electrons. This electron movement allows for the formation of different resonance structures, which contribute to the overall stability and reactivity of the molecule.

17. How does the strength of the electromeric effect compare to that of the inductive effect?

The electromeric effect is generally stronger than the inductive effect because it involves the complete transfer of electrons, whereas the inductive effect only involves partial electron displacement. However, the electromeric effect is temporary, while the inductive effect is permanent.

18. How does the electromeric effect influence the stability of carbocations?

The electromeric effect can significantly influence carbocation stability. A positive electromeric effect can stabilize a carbocation by donating electrons to the positively charged carbon, while a negative electromeric effect can destabilize a carbocation by withdrawing electrons from it.

19. What is the relationship between conjugation and the electromeric effect?

Conjugation and the electromeric effect are closely related. Conjugated systems, which have alternating single and multiple bonds, provide an ideal environment for the electromeric effect to occur. The presence of conjugation allows for more extensive electron delocalization, enhancing the impact of the electromeric effect.

20. Can you explain how the electromeric effect influences the reactivity of carbonyl compounds?

In carbonyl compounds, the electromeric effect influences reactivity by allowing electron movement between the carbon-oxygen double bond. This can make the carbonyl carbon more electrophilic or the oxygen more nucleophilic, depending on the direction of electron movement, thus affecting the compound's reactivity in various reactions.

21. How does the electromeric effect impact the orientation of electrophilic aromatic substitution reactions?

The electromeric effect can impact the orientation of electrophilic aromatic substitution reactions by influencing the electron density at different positions on the aromatic ring. Substituents that exhibit a positive electromeric effect tend to direct incoming electrophiles to ortho and para positions, while those with a negative electromeric effect often lead to meta substitution.

22. What is the difference between the mesomeric effect and the electromeric effect?

While both effects involve electron movement, the mesomeric effect is a permanent effect that occurs due to conjugation in a molecule, whereas the electromeric effect is a temporary effect induced by an external reagent or electric field. The mesomeric effect involves partial electron delocalization, while the electromeric effect involves complete electron transfer.

23. How does the electronegativity of atoms influence the electromeric effect?

The electronegativity of atoms plays a crucial role in determining the direction and strength of the electromeric effect. Highly electronegative atoms or groups tend to induce a negative electromeric effect by attracting electrons, while less electronegative atoms or groups with lone pairs can induce a positive electromeric effect by donating electrons.

24. How does the electromeric effect impact the strength of hydrogen bonding?

The electromeric effect can impact the strength of hydrogen bonding by altering the electron density around atoms involved in the hydrogen bond. A positive electromeric effect can increase electron density on an electronegative atom, potentially strengthening its ability to act as a hydrogen bond acceptor. Conversely, a negative electromeric effect can decrease this electron density, potentially weakening hydrogen bonding.

25. What role does the electromeric effect play in the mechanism of nucleophilic addition reactions?

In nucleophilic addition reactions, the electromeric effect can influence the reactivity of both the nucleophile and the electrophile. For example, in carbonyl compounds, the negative electromeric effect makes the carbonyl carbon more electrophilic, facilitating nucleophilic attack. Similarly, the electromeric effect can enhance the nucleophilicity of certain groups by increasing their electron density.

26. How does the electromeric effect influence the UV-Vis spectroscopy of organic compounds?

The electromeric effect influences UV-Vis spectroscopy by affecting the energy levels of molecular orbitals, particularly in conjugated systems. Compounds with strong electromeric effects often show bathochromic shifts (shifts to longer wavelengths) in their absorption spectra due to the increased delocalization of electrons, which reduces the energy gap between the ground and excited states.

27. Can you explain how the electromeric effect impacts the strength of carbon-carbon bonds?

The electromeric effect can impact the strength of carbon-carbon bonds by altering the electron density distribution. In cases where the electromeric effect leads to increased electron delocalization, it can strengthen certain carbon-carbon bonds by increasing their double bond character. Conversely, if the effect leads to localized charge accumulation, it may weaken some carbon-carbon bonds.

28. How does the electromeric effect influence the rate of SN1 reactions?

The electromeric effect can significantly influence the rate of SN1 reactions by affecting the stability of the carbocation intermediate. A positive electromeric effect from neighboring groups can stabilize the carbocation, increasing the reaction rate. Conversely, a negative electromeric effect can destabilize the carbocation, potentially slowing down the reaction.

29. What is the relationship between the electromeric effect and molecular polarity?

The electromeric effect can contribute to molecular polarity by causing temporary shifts in electron density within a molecule. While it doesn't create permanent dipoles like the inductive effect, the temporary electron redistribution can influence a molecule's overall polarity, especially in the presence of an external electric field or during reactions.

30. How does the electromeric effect impact the acidity of carboxylic acids?

The electromeric effect can impact the acidity of carboxylic acids by influencing the stability of the conjugate base (carboxylate ion). A negative electromeric effect can stabilize the carboxylate ion by delocalizing the negative charge, thereby increasing the acid strength. Conversely, a positive electromeric effect can destabilize the carboxylate ion, potentially decreasing acidity.

31. Can you explain how the electromeric effect influences the basicity of amines?

The electromeric effect influences the basicity of amines by affecting the availability of the lone pair of electrons on the nitrogen atom. A positive electromeric effect can increase electron density on the nitrogen, enhancing its ability to donate electrons and thus increasing basicity. A negative electromeric effect would have the opposite impact, potentially decreasing basicity.

32. How does the electromeric effect contribute to the color of organic dyes?

The electromeric effect contributes to the color of organic dyes by influencing the extent of electron delocalization in conjugated systems. Greater delocalization, often enhanced by electromeric effects, typically results in a smaller HOMO-LUMO gap, leading to absorption of longer wavelength light. This shift in absorption can significantly affect the perceived color of the dye.

33. What is the role of the electromeric effect in the mechanism of electrophilic addition reactions?

In electrophilic addition reactions, the electromeric effect plays a crucial role by facilitating the movement of electrons within the π bond. This electron movement allows for the formation of a carbocation intermediate, which is a key step in many electrophilic addition mechanisms. The stability of this carbocation can be influenced by neighboring groups exhibiting electromeric effects.

34. How does the electromeric effect influence the strength of intermolecular forces?

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.

35. Can you explain how the electromeric effect impacts the reactivity of alkenes in addition polymerization?

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.

36. How does the electromeric effect influence the strength of carbon-heteroatom bonds?

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.

37. What is the relationship between the electromeric effect and aromaticity?

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.

38. How does the electromeric effect impact the stereochemistry of addition reactions?

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.

39. How does the electromeric effect contribute to the mechanism of elimination reactions?

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.

40. What is the relationship between the electromeric effect and hyperconjugation?

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.

41. How does the electromeric effect influence the strength of coordinate covalent bonds?

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.

42. Can you explain how the electromeric effect impacts the reactivity of conjugated dienes?

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.

43. How does the electromeric effect contribute to the concept of resonance energy?

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.