Resonance Effect or Mesomeric Effect

Mesomeric or Resonance Effect

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:31 PM IST

The mesomeric effect, also known as the resonance effect, is a key concept in organic chemistry that describes the delocalization of electrons within molecules. This effect is crucial in understanding the stability, reactivity, and electronic distribution in organic compounds.The mesomeric effect is a feature of substituents or functional groups in a molecule. The effect is symbolized by the letter ‘M’ and is used to describe the electron-withdrawing or releasing properties of substituents depending on the relevant resonance structure.

Mesomeric or Resonance Effect

Mesomeric or Resonance Effect

The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effects designated as R or M effects.

Positive Resonance Effect (+R effect): In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as:
Negative Resonance Effect (- R effect): This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example, in nitrobenzene this electron displacement can be depicted as:

The atoms or substituent groups, which represent +R or –R electron displacement effects are as follows :

+R effect: −X,−OH,−OR,−OCOR,−H₂,−NHR,−NR₂,−NHCOR
– R effect: −COOH,−CHO,>C=O,−CN,−NO₂,−SO₃H

The presence of alternate single and double bonds in an open-chain or cyclic system is termed a conjugated system. These systems often show abnormal behavior. The examples are 1,3- butadiene, aniline, and nitrobenzene etc. In such systems, the π-electrons are delocalized and the system develops polarity.

In hyperconjugation, more is the number of $\alpha$-carbons, more is the number of hyperconjugated structures and thus more is the stability. Thus, stability follows the given order:
3^o carbocation > 2^o carbocation > 1^o carbocation

Mesomeric (Resonance) Effect: The mesomeric effect refers to the delocalization of π-electrons or lone pairs of electrons through conjugated systems within a molecule. It is depicted by drawing resonance structures, which are different possible structures that represent the same molecule.
Resonance Structures: These are multiple Lewis structures that represent the delocalization of electrons within a molecule. The actual molecule is a hybrid of these structures and has a lower energy than any individual resonance form.
Positive Mesomeric Effect (+M): Groups or atoms that donate electrons through resonance, increasing electron density on other parts of the molecule. Common +M groups include −OH,−OR,−NH2, and -NHR
Negative Mesomeric Effect (-M): Groups or atoms that withdraw electrons through resonance, decreasing electron density on other parts of the molecule. Common -M groups include −NO2,−CN,−CO, and −SO2

Solved Examples Based on Mesomeric or Resonance Effect

Q.1 The order of stability of the following carbocations is:

(1) II>I>I
(2) II>∣I>I
(3) II>II>I
(4) I>II>1II

Solution:

As we have learned

The order of stability of carbocation is -

So, III > I > II

Therefore, option (1) is correct.

Q.2 In the anion HCOO− the two carbon-oxygen bonds are found to be of equal length. What is the reason for it?

(1) Electronic orbitals of carbon atom are hybridised.

(2) The C=C bond is weaker than the C−C bond

(3) The anion HCOO− has two resonating structures.

(4) The anion is obtained by removal of a proton from the acid molecule

Solution:

As we have learned

The two carbon-oxygen bonds are found to be of equal length because of the resonance, both of the carbon-oxygen bonds are of the same length as both bonds attains $\pi$ - bond character due to resonance.

It has two resonating structures.

Therefore, the correct option is (3).

Conclusion

The mesomeric (resonance) effect describes the delocalization of π-electrons or lone pairs within a molecule, contributing to its stability and reactivity. There are two types: positive mesomeric effect (+M), where groups donate electrons (e.g., −OH3,−NH2 ), and negative mesomeric effect (-M), where groups withdraw electrons(e.g., −NO2,−CN) This effect stabilizes molecules, influences reactivity, and affects properties like acidity and basicity.

Frequently Asked Questions (FAQs)

1. What is the mesomeric effect?

The mesomeric effect, also known as the resonance effect, is the redistribution of electron density within a molecule due to the interaction between pi bonds and lone pairs of electrons. It involves the delocalization of electrons across multiple atoms, resulting in increased stability of the molecule.

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

The mesomeric effect involves the delocalization of pi electrons or lone pairs, while the inductive effect is the polarization of sigma bonds. Mesomeric effects can be transmitted through conjugated systems, whereas inductive effects decrease rapidly with distance from the source.

3. What are the two types of mesomeric effects?

The two types of mesomeric effects are: 1) Positive mesomeric effect (+M), where electron density is donated to the system, and 2) Negative mesomeric effect (-M), where electron density is withdrawn from the system.

4. Can you explain the concept of resonance using the mesomeric effect?

Resonance is a consequence of the mesomeric effect. It occurs when a molecule can be represented by multiple Lewis structures that differ only in the position of electrons. These structures, called resonance forms, contribute to the overall stability of the molecule by delocalizing electrons across the system.

5. How does the mesomeric effect influence the stability of a molecule?

The mesomeric effect generally increases the stability of a molecule by delocalizing electrons. This delocalization distributes electron density more evenly, reducing charge separation and lowering the overall energy of the molecule.

6. What is a conjugated system and how does it relate to the mesomeric effect?

A conjugated system is a series of alternating single and multiple bonds. The mesomeric effect is particularly important in conjugated systems because it allows for the delocalization of electrons across the entire system, leading to increased stability and unique chemical properties.

7. How does the mesomeric effect impact the acidity or basicity of a compound?

The mesomeric effect can significantly influence acidity or basicity by affecting the stability of the conjugate base or acid. For example, a negative mesomeric effect can stabilize a conjugate base, making the original compound more acidic.

8. What is the difference between +M and -M groups?

+M groups (e.g., -OH, -NH2) donate electrons to the system through resonance, increasing electron density. -M groups (e.g., -NO2, -CN) withdraw electrons from the system through resonance, decreasing electron density.

9. How does the mesomeric effect influence the reactivity of aromatic compounds?

The mesomeric effect can activate or deactivate aromatic rings towards electrophilic substitution reactions. +M groups activate the ring by increasing electron density, while -M groups deactivate the ring by decreasing electron density.

10. Can the mesomeric effect occur in saturated compounds?

No, the mesomeric effect requires the presence of pi bonds or lone pairs of electrons. Saturated compounds only have sigma bonds, so they cannot exhibit the mesomeric effect.

11. How does the mesomeric effect influence bond lengths in a molecule?

The mesomeric effect can cause bond lengths to become intermediate between single and double bonds due to electron delocalization. This results in a more uniform distribution of electron density across the conjugated system.

12. What is hyperconjugation and how is it related to the mesomeric effect?

Hyperconjugation is a special case of the mesomeric effect involving the interaction between sigma bonds and adjacent pi bonds or empty p-orbitals. While similar to the mesomeric effect, hyperconjugation typically involves weaker electron delocalization.

13. How does the mesomeric effect impact the dipole moment of a molecule?

The mesomeric effect can either enhance or reduce the dipole moment of a molecule by redistributing electron density. This can lead to unexpected dipole moments in some conjugated systems.

14. Can the mesomeric effect overcome the inductive effect?

Yes, in many cases, the mesomeric effect can overcome the inductive effect, especially in conjugated systems. This is because the mesomeric effect typically has a stronger influence on electron distribution than the inductive effect.

15. How does the mesomeric effect influence the orientation of electrophilic substitution in benzene derivatives?

The mesomeric effect determines whether a substituent is ortho-para directing or meta-directing in electrophilic aromatic substitution. +M groups are typically ortho-para directing, while -M groups are usually meta-directing.

16. What role does the mesomeric effect play in the stability of carbocations?

The mesomeric effect can stabilize carbocations by delocalizing the positive charge through resonance. This distribution of charge reduces the overall energy of the carbocation, making it more stable.

17. How does the mesomeric effect influence the strength of carbon-carbon bonds in alkenes?

The mesomeric effect can strengthen or weaken carbon-carbon bonds in alkenes depending on the substituents. Electron-donating groups (+M) can increase electron density in the pi bond, strengthening it, while electron-withdrawing groups (-M) can have the opposite effect.

18. Can you explain the concept of cross-conjugation in relation to the mesomeric effect?

Cross-conjugation occurs when two pi systems are connected through a single sp2 hybridized carbon atom. The mesomeric effect in cross-conjugated systems is typically weaker than in linearly conjugated systems due to less efficient electron delocalization.

19. How does the mesomeric effect influence the color of organic compounds?

The mesomeric effect can impact the color of organic compounds by affecting the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Extended conjugation often leads to a smaller HOMO-LUMO gap, resulting in absorption of visible light and thus color.

20. What is the relationship between aromaticity and the mesomeric effect?

Aromaticity is a consequence of extensive electron delocalization, which is closely related to the mesomeric effect. The stability of aromatic compounds is largely due to the cyclic delocalization of electrons, which can be explained using resonance structures.

21. How does the mesomeric effect influence the reactivity of carbonyl compounds?

The mesomeric effect in carbonyl compounds affects their reactivity by influencing the polarity of the C=O bond. Electron-withdrawing groups enhance the electrophilicity of the carbonyl carbon, while electron-donating groups reduce it.

22. Can you explain the concept of push-pull systems in relation to the mesomeric effect?

Push-pull systems contain both electron-donating (+M) and electron-withdrawing (-M) groups connected through a conjugated system. The mesomeric effect in these systems leads to enhanced polarization and unique reactivity.

23. How does the mesomeric effect influence the basicity of amines?

The mesomeric effect can significantly impact the basicity of amines. Electron-donating groups (+M) increase the electron density on the nitrogen, enhancing its basicity, while electron-withdrawing groups (-M) decrease basicity by reducing electron density.

24. What is the role of the mesomeric effect in the keto-enol tautomerism?

The mesomeric effect plays a crucial role in keto-enol tautomerism by stabilizing the enol form through electron delocalization. This stabilization affects the equilibrium between the keto and enol forms.

25. How does the mesomeric effect influence the strength of hydrogen bonds?

The mesomeric effect can strengthen or weaken hydrogen bonds by affecting the electron density on the atom participating in the hydrogen bond. Electron-donating groups can enhance hydrogen bond strength, while electron-withdrawing groups can weaken it.

26. Can you explain the concept of antiaromaticity in relation to the mesomeric effect?

Antiaromaticity is related to the mesomeric effect in that it involves electron delocalization in cyclic systems. However, antiaromatic compounds have destabilizing electron delocalization, unlike the stabilizing effect seen in aromatic compounds.

27. How does the mesomeric effect influence the reactivity of conjugated dienes?

The mesomeric effect in conjugated dienes leads to electron delocalization across the entire system. This affects their reactivity, particularly in electrophilic addition reactions, where 1,2- and 1,4-addition products can form due to resonance stabilization.

28. What is the impact of the mesomeric effect on the acidity of phenols?

The mesomeric effect significantly influences the acidity of phenols. The negative charge on the phenoxide ion is stabilized by resonance, increasing the acidity of phenols compared to aliphatic alcohols.

29. How does the mesomeric effect influence the strength of carbon-oxygen bonds in ethers?

The mesomeric effect typically has little direct influence on the strength of carbon-oxygen bonds in ethers, as these bonds are sigma bonds. However, in conjugated systems containing ethers, the mesomeric effect can indirectly affect bond strengths through electron delocalization.

30. Can you explain the concept of homoconjugation in relation to the mesomeric effect?

Homoconjugation occurs when pi systems are separated by a single sp3 hybridized carbon. While not as effective as direct conjugation, homoconjugation can still allow for some electron delocalization through hyperconjugation, which is related to the mesomeric effect.

31. How does the mesomeric effect influence the stability of radicals?

The mesomeric effect can stabilize radicals by delocalizing the unpaired electron through resonance. This distribution of electron density lowers the energy of the radical, making it more stable.

32. What is the role of the mesomeric effect in the reactivity of alpha,beta-unsaturated carbonyl compounds?

In alpha,beta-unsaturated carbonyl compounds, the mesomeric effect leads to electron delocalization across the entire conjugated system. This affects their reactivity, particularly in nucleophilic addition reactions, where 1,2- and 1,4-addition can occur.

33. How does the mesomeric effect influence the strength of carbon-nitrogen bonds in amides?

The mesomeric effect strengthens the carbon-nitrogen bond in amides by delocalizing the nitrogen's lone pair into the carbonyl group. This partial double bond character makes the C-N bond in amides stronger and shorter than a typical single bond.

34. Can you explain the concept of captodative effect in relation to the mesomeric effect?

The captodative effect occurs when both electron-donating and electron-withdrawing groups are attached to the same carbon atom. This combination can lead to enhanced stabilization of radicals through a synergistic mesomeric effect.

35. How does the mesomeric effect influence the reactivity of aromatic heterocycles?

The mesomeric effect in aromatic heterocycles affects their electron distribution and reactivity. For example, in pyridine, the nitrogen atom withdraws electrons through the mesomeric effect, making the ring less reactive towards electrophilic substitution compared to benzene.

36. What is the impact of the mesomeric effect on the strength of carbon-halogen bonds?

The mesomeric effect can influence the strength of carbon-halogen bonds, particularly in conjugated systems. Electron-donating groups can weaken the C-X bond by increasing electron density, while electron-withdrawing groups can strengthen it.

37. How does the mesomeric effect influence the UV-Vis absorption spectra of organic compounds?

The mesomeric effect can lead to extended conjugation, which typically results in a bathochromic shift (shift to longer wavelengths) in UV-Vis absorption spectra. This is due to the decreased energy gap between the HOMO and LUMO.

38. Can you explain the concept of mesomeric effect in relation to molecular orbital theory?

In molecular orbital theory, the mesomeric effect is related to the mixing of p-orbitals to form delocalized π molecular orbitals. This delocalization lowers the overall energy of the molecule and explains the stability associated with resonance.

39. How does the mesomeric effect influence the strength of intermolecular forces?

The mesomeric effect can impact intermolecular forces by altering the electron distribution within molecules. This can affect dipole-dipole interactions, hydrogen bonding, and even dispersion forces in some cases.

40. What is the role of the mesomeric effect in the stability of transition states during organic reactions?

The mesomeric effect can stabilize or destabilize transition states in organic reactions by delocalizing charge or electron density. This can significantly influence reaction rates and the overall energy profile of the reaction.

41. How does the mesomeric effect influence the reactivity of conjugated polyenes?

The mesomeric effect in conjugated polyenes leads to electron delocalization across the entire system. This affects their reactivity, particularly in electrocyclic reactions and cycloadditions, where the extended conjugation plays a crucial role.

42. Can you explain the concept of through-conjugation in relation to the mesomeric effect?

Through-conjugation occurs when two pi systems are connected through a saturated group that allows for some electron delocalization. While not as effective as direct conjugation, through-conjugation can still exhibit some mesomeric effects.

43. How does the mesomeric effect influence the strength of carbon-sulfur bonds?

The mesomeric effect can influence the strength of carbon-sulfur bonds, particularly in conjugated systems. Electron-donating groups can weaken the C-S bond by increasing electron density, while electron-withdrawing groups can strengthen it.

44. What is the impact of the mesomeric effect on the reactivity of enolates?

The mesomeric effect in enolates leads to delocalization of the negative charge, stabilizing the anion. This affects their reactivity, particularly in alkylation and aldol reactions, where the charge distribution influences the nucleophilicity of the enolate.

45. How does the mesomeric effect influence the strength of pi-pi interactions?

The mesomeric effect can impact pi-pi interactions by altering the electron distribution in aromatic systems. Electron-rich aromatics (due to +M groups) can have stronger pi-pi interactions with electron-poor aromatics (due to -M groups).

46. Can you explain the concept of extended conjugation in relation to the mesomeric effect?

Extended conjugation occurs when multiple pi bonds are connected in series, allowing for extensive electron delocalization through the mesomeric effect. This often leads to increased stability and unique spectroscopic properties.

47. How does the mesomeric effect influence the reactivity of nitriles?

The mesomeric effect in nitriles involves the delocalization of the nitrogen's lone pair into the triple bond. This affects their reactivity, particularly in nucleophilic addition reactions, where the carbon atom becomes more electrophilic.

48. What is the role of the mesomeric effect in the stability of ylides?

The mesomeric effect plays a crucial role in stabilizing ylides by delocalizing the negative charge. This delocalization reduces the overall energy of the ylide, making it more stable and influencing its reactivity in reactions like the Wittig reaction.

49. How does the mesomeric effect influence the strength of metal-ligand bonds in organometallic compounds?

The mesomeric effect can impact metal-ligand bonds in organometallic compounds by altering the electron density on the ligand. This can affect the strength and nature of the bond, influencing the overall properties and reactivity of the organometallic complex.

50. Can you explain the concept of cross-conjugated annulenes in relation to the mesomeric effect?

Cross-conjugated annulenes are cyclic compounds with alternating single and double bonds, but with some bonds that are not part of the main conjugated system. The mesomeric effect in these compounds can lead to interesting electronic properties and reactivity due to the unique pattern of electron delocalization.