Mesomeric Effect

In Organic Chemistry, Mesomeric effect involves the donation or withdrawal of pi electrons (π-electrons) through resonance structures. This effect basically refers to the movement or shifting of electrons within a molecule due to the influence of certain groups or atoms attached to the molecular framework. Let’s understand this effect in detail.

1.0What is Mesomeric effect

The mesomeric effect, also known as the resonance effect, is important in understanding the behavior of various substituents or functional groups within a chemical compound. This effect arises when there is an interaction involving pi bonds or between a pi bond and a lone pair of electrons on an adjacent atom. Such interactions lead to a redistribution of electron density across the molecule, creating various resonance structures.

These resonance structures are not actual structures but theoretical constructs that help visualize the electron distribution in the molecule. The true structure of the molecule is considered a hybrid of these resonance structures, where the electron density is delocalized.

Here is visual representation of electron movements in different compounds attached to Electron donating group (Hydroxy-OH) and Electron withdrawing group (Aldehyde-CHO)-

The direction of electron movement—whether toward or away from a substituent group—depends on the nature of the substituent. Electron-donating groups (EDGs) push electrons towards the pi system, enhancing electron density and stabilizing the system through a positive mesomeric effect (+M). Conversely, electron-withdrawing groups (EWGs) pull electron density away from the pi system, typically resulting in a negative mesomeric effect (-M). Thus it can be categorized into two primary types based on the nature of electron movement within the molecule:

1. Positive Mesomeric Effect (+M)

• This occurs when a substituent group donates electrons to the conjugated system. These groups typically have lone pairs of electrons that can be delocalized into the system. 
• For instance, groups like -OH (hydroxyl), -OR (alkoxy), -NH₂ (amino), and -OCH₃ (methoxy) are common donors that exhibit the +M effect. The electron donation through resonance increases the electron density in the pi system, which can stabilize charged intermediates such as carbocations and make the molecule more nucleophilic.

2. Negative Mesomeric Effect (-M):

• In this case, the substituent group withdraws electrons from the conjugated system through resonance. Groups that typically show the -M effect include -NO₂ (nitro), -CN (cyano), -SO₃H (sulfonyl), and carbonyl groups like -COH, -COR, -COOH. These groups possess multiple bonds or electronegative atoms capable of accepting electron density, which results in a decrease in the electron density in the pi system. This effect can make the molecule more electrophilic and influence the stability and reactivity, often making the molecule more susceptible to nucleophilic attacks.

2.0Mesomeric Effect v/s Inductive Effect

The inductive and mesomeric effects are two fundamental electronic effects in organic chemistry that influence molecular stability, reactivity, and chemical properties. However, they differ significantly in their mechanisms and dependency on molecular structure.

3.0Functional groups that exhibit Mesomeric and Inductive Effect

In organic chemistry, the mesomeric effect is essentially another term for resonance effect when discussing substituent groups on aromatic rings or other conjugated systems. Both terms describe the ability of electrons to be delocalized within a molecule, which can stabilize the molecule or an intermediate formed during a reaction. Thus, any group capable of showing the mesomeric effect inherently shows the resonance effect as well. These effects are typically observed with groups that can donate or withdraw electrons through pi systems (π systems).

Here are some common groups that exhibit both mesomeric and resonance effects, categorized based on whether they are electron-donating or electron-withdrawing:

4.0Applications of Mesomeric Effect

1. Compare C = C bond length

Comparing the bond lengths of C=C in different environments involves considering the electronic effects exerted by substituents attached to the double bond. Let's compare the C=C bond lengths in:

Ethene (CH₂=CH₂) and Vinylamine (CH₂=CH₂–NH₂)

• Ethene (CH₂=CH₂): In ethene, the C=C bond is a typical double bond with no additional substituents influencing its electron density through resonance or inductive effects. The bond is composed of one sigma (σ) bond formed by the overlap of sp² hybrid orbitals and one pi (π) bond formed by the side-by-side overlap of unhybridized p orbitals. This arrangement results in a bond length typically around 1.34 Å.
• Vinylamine (CH₂=CH₂–NH₂): In this molecule, the amino group (-NH₂) attached to the ethene structure can exert a significant electronic effect. The -NH₂ group is an electron-donating group via both the inductive effect and the mesomeric effect (+M). Through resonance, the lone pair electrons on the nitrogen can be delocalized into the π system of the double bond, increasing the electron density of the double bond.

So the bond length order will be (b) > (a)

2. Compare C -X bond length (where x = Halogen)

Let’s understand with an example-

Ex. Compare C-Cl bond length in given compounds-

      a.CH₂=CH-Cl     b. CH₃-CH₂-CH₂-Cl      c. Chlorobenzene

• CH₂=CH-Cl (Vinyl Chloride):  In vinyl chloride, the C-Cl bond is attached to a carbon atom that is part of a double bond (C=C). This carbon is sp² hybridized, which leads to a slightly shorter bond length compared to sp³ hybridized carbons because sp² orbitals are more s-character dominant, making them closer to the nucleus and thus shorter. Additionally, the double bond's electron-donating effect through resonance may slightly increase the electron density around the C-Cl bond, influencing its character.
• CH₃-CH₂-CH₂-Cl (n-Propyl chloride): Here, the C-Cl bond is on an sp³ hybridized carbon in a simple aliphatic chain without any double bonds nearby. This typical alkyl chloride has a C-Cl bond that is purely influenced by the inductive effect of the alkyl chain, which is relatively weak. The bond length here is characteristic of a standard sp³ hybridized C-Cl bond, typically around 1.78 Å.
• Chlorobenzene: In chlorobenzene, the chlorine atom is bonded to a carbon atom in an aromatic benzene ring. The carbon here is sp² hybridized like in vinyl chloride, but the context is different due to the aromatic ring's stability and resonance structures. In aromatic compounds, the C-Cl bond might exhibit partial double bond character due to resonance, as chlorine can participate in the conjugation with the benzene ring, withdrawing electron density through the -I and -M effects. This can lead to a slight shortening of the C-Cl bond compared to a purely aliphatic environment.

So the order of C-Cl bond length will be- . (b) > (a) > (c)

3. Stability of Ions and Molecules

• Carbocations: Carbocations are positively charged carbon atoms that are stabilized significantly by adjacent groups capable of donating electron density through the mesomeric effect. For example, a carbocation adjacent to a double bond or an oxygen atom will be more stable due to the electron donation by these groups.
• Carbanions and Free Radicals: Similarly, groups that can delocalize the negative charge or unpaired electron through resonance stabilize carbanions and free radicals.

4. Acidity and Basicity

• Acids: The mesomeric effect influences the acidity of compounds. For example, the acidity of phenols is enhanced by electron-withdrawing groups (such as nitro groups) on the aromatic ring that stabilize the negative charge on the oxygen atom after deprotonation.
• Bases: Conversely, electron-donating groups on aromatic amines increase the basicity by stabilizing the positive charge on the nitrogen atom when it accepts a proton.

5. Aromatic Substitution Reactions

• Directing Effects: In aromatic compounds, substituents that can exhibit mesomeric effects influence the positions at which electrophilic aromatic substitution reactions occur. Electron-donating groups direct substitution to the ortho and para positions, while electron-withdrawing groups typically direct substitution to the meta position.
• Reactivity: Electron-donating groups increase the overall electron density of the aromatic ring, making it more reactive towards electrophilic reagents. In contrast, electron-withdrawing groups decrease the ring’s reactivity.