Mesomeric Effect

Mesomeric Effect, often referred to as the resonance effect, is an important concept in organic chemistry that influences the distribution of electrons within molecules. Understanding of Mesomeric Effect is crucial for solving the behaviour of chemical compounds and their reactivity. The mesomeric effect shows how electrons move around in molecules, impacting their stability and behaviour. The mesomeric Effect is important to understand the mechanism of organic chemistry reactions.

Let’s understand what is Mesomeric Effect, types of Mesomeric Effect, Examples of Mesomeric Effect.

What is Mesomeric Effect?

Mesomeric Effect is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and a lone pair of electrons present on an adjacent atom. This change in electron arrangement results in the formation of resonance structures that hybridize into the molecule’s true form. Mesomeric effect is used to describe the electron-withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter M.

Types of Mesomeric Effect

The mesomeric effect is subdivided into two types:

• +M Effect
• -M Effect

+M Effect (Positive Mesomeric Effect)

+M effect occurs when the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugate system. The substituent, in this case, is an electron-donating group.

Example of + M Effect

For example, consider the following resonance structures of phenol:

As you can see, the oxygen atom in the hydroxy group donates electrons into the benzene ring, increasing the electron density of the ring. This is an example of the +M effect.

Examples of groups showing +M effect includes -NH, -NH2, -NHR, -NR2, -O, -OH, -OR, -F, -Cl, -O-COR, -NHCOR, -SH, -SR, etc.

The order of +M Effect is given as

−O⁻ > −NH₂ > −OR > −NHCOR > −OCOR > −Ph > −CH₃ > −F > −Cl > −Br > −I

-M Effect (Negative Mesomeric Effect)

-M Effect occurs when the pi-bond electrons are transferred from the conjugate system to a particular group, decreasing the electron density of the conjugate system. The substituent, in this case, is an electron-withdrawing group.

Example of -M Effect

For example, consider the following resonance structures of nitrobenzene:

As you can see, the nitro group withdraws electrons from the benzene ring, decreasing the electron density of the ring. It is an example of the -M effect.

Examples of groups exihibiting the -M effect includes -NO₂, -CN, -COX, -SO₃H, -CHO, -CONH₂, -COR, -COOH, and -COOR.

The order of -M effect is given as

−NO₂ > −CN > −SO₃H > −CHO > −COR > −COOCOR > −COOR > −COOH > −CONH₂ > −COO⁻

Mechanism of Mesomeric Effect

The mechanism of the mesomeric effect involves the following steps:

• The pi electrons in the conjugate system interact with the electrons in the substituent group, leading to a redistribution of electron density.
• In the +M effect, the substituent group gains electrons, increasing its electron density and becoming more negatively charged. In the -M effect, the substituent group loses electrons, decreasing its electron density and becoming more positively charged.
• The change in electron density affects the compound’s chemical properties, such as its reactivity towards nucleophiles or electrophiles.

Factors Influencing Mesomeric Effect

Key facotrs affecting the mesometic effect are:

• The strength of the mesomeric effect is influenced by the ionization potential of the compound, with lower ionization potential leading to a more potent effect.
• Additionally, the mesomeric effect is affected by the presence of other groups that can either support or decrease its impact. For example, groups with a positive inductive effect (+I) or positive mesomeric effect (+M) can enhance the mesomeric effect. In contrast, groups with a negative inductive effect (-I) or negative mesomeric effect (-M) can diminish it.

Mesomeric Effect vs Inductive Effect

Mesomeric and Inductive Effects are two important mechanisms in proceeding of organic reactions. The difference between Mesomeric Effect and Inductive Effect is tabulated below:

Mesomeric Effect	Inductive Effect
Operates in unsaturated compounds, i.e., compounds containing pi bonds.	Operates in saturated compounds, i.e., compounds containing sigma bonds.
Involves the complete transfer of electron pairs, leading to the development of unit positive and negative charges.	Involves the slight displacement of electron pairs, leading to the development of partial charges.
It is transmitted through the entire chain, provided conjugation is present.	It travels a short distance before becoming insignificant beyond the chain’s third carbon atom.

Resonance

Resonance is a concept within the Valence Bond Theory of bonding that describes the delocalization of electrons within molecules. It involves constructing multiple Lewis structures that, when combined, represent the complete electronic structure of the molecule. Resonance is beneficial for analyzing delocalized electrons where a single Lewis structure cannot fully describe the bonding.

Resonance structures should follow these rules:

• Have the same number of electrons
• Do not add or subtract any electrons.
• Follow the rules of writing Lewis Structures.
• Maintain the same hybridization.
• Keep the same skeleton of the structure.
• Have the same number of lone pairs

Characteristics of Resonance

The characteristics of resonance can be determined by examining the types of bonds present and identifying different arrangements possible within the molecule.

• If a molecule has multiple areas where there could be different bonds or orientations of electron sharing, it likely has resonance structures.
• Resonance contributors of a molecule can only differ in the way electrons are formally assigned to atoms in the Lewis structure depictions of the molecule.
• Resonance structures should follow specific rules, including having the same number of electrons, not changing the molecular shape, maintaining the same hybridization, and keeping the same number of lone pairs.
• The resonance hybrid, which is a combination of these structures, is more stable than any individual resonance structures, and some resonance structures contribute more to the stability of the molecule than others.

Resonance Condition

For a molecule or ion to exhibit resonance, it must meet certain conditions:

• The arrangement of atomic nuclei must be the same in all resonance contributors. It means the atoms cannot move around; only the electrons can rearrange.
• The molecule or ion must have at least one delocalized electron. A delocalized electron is an electron that is not confined to a single bond between two atoms. It is spread out over several atoms or bonds.
• The molecule or ion must have a system of alternating single and double bonds. It is known as a conjugated system. The conjugation allows the electrons to move freely from one part of the molecule to another.
• The resonance contributors must have the same number of paired and unpaired electrons. It means that the overall charge of the molecule or ion must remain the same in all resonance contributors.
• The resonance contributors should be of similar energy. This means that the resonance contributors should have similar electron distributions. If the resonance contributors are too different in energy, the hybrid structure will not be well-defined.

Applications of Mesomeric Effect

The mesomeric effect has a wide range of applications in organic chemistry which are:

• Carbocation stability: The mesomeric effect can stabilize carbocations by delocalizing the positive charge over multiple atoms. This makes the carbocation less susceptible to attack by nucleophiles. For example, the benzyl carbocation is more stable than the tert-butyl carbocation due to the resonance stabilization the phenyl ring provides.
• Aromaticity: Aromatic compounds are highly stable due to the extensive resonance stabilization that they undergo. This stability is due to the delocalization of electrons over a conjugated system of atoms. For example, benzene is a highly aromatic compound due to the resonance stabilization of the six carbon atoms and six pi electrons.
• Acid-base reactions: The mesomeric effect can influence the acid-base properties of compounds. For example, benzoic acid is stronger than acetic acid due to the resonance stabilization of the benzoate ion
• Carbanion Stabilization: The mesomeric effect stabilizes the carbocation by decreasing the electron density of the Carboanion. It makes the Carboanion less susceptible to attack by electrophiles.
• Acidic and Basic Strength: The strength of acids is directly proportional to the -M effect, and base strength is proportional to the +M effect.
• Free Radical Stability: Resonance increases the stability of free radicals, making them more stable

Limitations of Mesomeric Effect

Limitations of Mesomeric Effects are:

• The mesomeric effect is qualitative and does not provide quantitative information about the extent of charge distribution within a compound.
• It does not account for the actual movement of electrons but instead provides a simplified representation of the electron distribution in a molecule.
• The mesomeric effect does not explain the energy changes associated with the resonance structures.
• It does not provide a complete picture of the electronic structure of a molecule, as it oversimplifies the electron distribution.

Mesomeric Effect IIT JEE Questions

Q1. Resonance structures of molecules does not have

1. Identical arrangements of atoms
2. Nearly the same energy content
3. The same number of paired electrons
4. Identical Bonding

Ans: (4) Resonance molecule does not have Identical Bonding.

Q2. Which of the following resonating structures of 1-methoxy-1, 3-butadiene is the least stable.

1. CH2-CH=CH-CH=O+-CH3
2. CH2=CH2-CH-CH=O+-CH3
3. CH2-CH+-CH=CH-O-CH3
4. CH2=CH-CH-CH+-O-CH3

Ans: (3) The octet of every atom is complete in this Structure, and the positive charge of the carbocation is stabilized by the lone pair of the adjacent oxygen atom.

Q3. The Correct acidity order of the following: I) Phenol ll) 4-chloro Phenol lll) Benzoic Acid lV) 4-Methyl Benzoic Acid

1. (lll) > (lV) > (ll) > (l)
2. (lV) > (lll) > (l)> (ll)
3. (lll) > (ll) > (l) > (lV)
4. (ll) > (lll) > (lV )> (l)

Ans: (1) As Acidity ~ 1/+M effect

Mesomeric Effect – FAQs

1. What is Mesomeric Effect?

Mesomeric effect, also known as the resonance effect, is a property of substituents or functional groups in a chemical compound that describes the electron-withdrawing or releasing properties of those groups based on their ability to delocalize electrons within the conjugated system.

2. What are the Two types of Mesomeric Effects?

Positive mesomeric effect (+M effect): This occurs when the substituent group donates electrons to the conjugated system, increasing its electron density.

Negative mesomeric effect (-M effect): This occurs when the substituent group withdraws electrons from the conjugated system, decreasing its electron density.

3. How does Mesomeric Effect differ from the Inductive Effect?

Mesomeric Effect: Operates in unsaturated compounds (pi bonds), involves complete electron pair transfer, and operates over a longer distance within a conjugated system.

Inductive Effect: Operates in saturated compounds (sigma bonds), involves partial electron displacement, and is effective over a short distance along a chain.

4. What are the Factors that Influence the Strength of Mesomeric Effect?

The strength of the mesomeric effect is influenced by:

• Ionization potential of the compound
• Presence of other groups:

5. What are the Applications of the Mesomeric Effect?

The mesomeric effect is important for understanding and predicting the behavior of organic compounds. It has applications in:

• Explaining the stability and reactivity of molecules
• Predicting the site of electrophilic or nucleophilic attack
• Understanding physical properties such as dipole moment and bond length

6. What is +M effect?

+M Effect occurs when a group donates electrons to a conjugated system, increasing electron density. Example: -OH in phenol.

7. What is -M effect?

-M Effect happens when a group withdraws electrons from a conjugated system, decreasing electron density. Example: -NO2 in nitrobenzene.

8. What is Electron Displacement Effect?

Electron Displacement Effcet describes how groups influence electron distribution in a molecule.

9. What are the 4 basic Electron Displacement Effect?

4 basic electron displacement effects are:

• +M Effect (Electron Donation)
• -M Effect (Electron Withdrawal)
• +I Effect (Electron Donation via Inductive Effect)
• -I Effect (Electron Withdrawal via Inductive Effect)