Mesomeric Effect

In 1938, scientist Ingold developed the concepts of mesomeric effect, mesomerism and mesomer. Interestingly, mesomerism is synonymous with resonance which was introduced by scientist Pauling.

Up until 1950, the word mesomerism was widely used in French and German language. However, in the English language, the term “resonance” has become very popular and is widely used today. On the whole, they refer to the same concept.

What Is the Mesomeric Effect?

The polarity developed between atoms of a conjugated system by the electron transfer or pi–bond electron transfer is known as the mesomeric effect. In simple terms, we can describe that the mesomeric effect occurs when π electrons move away from or towards a substituent group in a conjugated orbital system.

The mesomeric effect can be subdivided into two types:

• +M effect
• -M effect

+M effect (Positive mesomeric effect)

When the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugated system, such a phenomenon is known as the (+M) effect or positive mesomeric effect.

Example 1:

Example 2: 

• For the +M effect, the group should have either a lone pair of electrons or should have a negative charge.
• The +M effect gives a negative charge to the conjugate system, or it can be said that the electron density increases in the conjugate system due to this. These conjugate systems show more reactivity towards electrophiles and less reactivity towards a nucleophile.

Group showing +M effect

–NH, –NH₂,–NHR, –NR₂, – O, – OH, –OR, – F, – Cl, –O–COR, – NHCOR, –SH, – SR etc.

Also Read: Electrophile and Nucleophile

-M Effect (Negative Mesomeric Effect)

When the pi-bond electrons are transferred from the conjugate system to a particular group, the electron density of the conjugate system is decreased, then this phenomenon is known as the negative mesomeric (–M) effect.

Example 1: 

Example 2: 

• For the –M effect, the group should have either a positive charge or should have a vacant orbital.
• The –M effect makes the compound more reactive towards a nucleophile as it decreases the electron density in the conjugate system, and at the same time, it is less reactive towards electrophile due to the same reasons.

The group which shows the –M effect includes

–NO₂, –CN, –COX, –SO₃H, – CHO, –CONH₂, –COR, –COOH, –COOR etc.

Significance of Mesomeric Effect

• It describes the distribution of the charge in the compound and helps to decide the point at which electrophiles or nucleophiles attack.
• Useful in describing physical characteristics, such as dipole moment and bond length.

Mesomeric Effect and Resonance Effect

If two or more different structures can be drawn for a molecule or ion that have the same arrangement of atomic nuclei but differ in the distribution of electrons, this effect can be termed the resonance effect.

The different structures are called contributing or resonating structures. Not all the properties of the molecule or ion are shown by the single resonating structure, but the actual structure is a resonance hybrid of all the resonating structures.

Understanding Resonance

There is charge transfer or electron migration from one part of the compound to the other part. During this charge transfer, energy is released from the conjugate system, due to which the stability of the compound is enhanced.

This results in the formation of different structures. These structures are known as resonating structures. They have the same arrangement of atoms, but only the charge/electron distribution is different.

Although this effect is completely hypothetical, it helps to understand and explain various chemical mechanisms and reactions.

Some Important Points to Remember

The following important points must be remembered:

• The resonance structures are the hypothetical structures of the conjugated compounds which are used to explain the movement of electrons.
• The actual structure of the conjugated compound is a resultant hybrid of all the resonating structures. This phenomenon is known as delocalisation, mesomerism or resonance.

The following important conditions must be followed by resonance structures.

• The atomic structure of every resonating structure must be the same. This has to be kept in mind that resonance and tautomerism are different from each other.

As it can be observed, the position of the atoms is the same; only the electrons have been transferred from one atom to another atom.

In structure (I), hydrogen is attached to carbon number 1, but in structure (II), hydrogen is attached to oxygen number 3. Therefore, structures (I) and (II) are tautomers and not resonating structures.

• All the unpaired electrons and lone pairs must be equal in numbers in all the resonating structures. But their arrangement and distribution can vary, which results in a new structure.

Electron pairs = 16 electron pairs = 16

Unpaired electron =1 unpaired electron =1

All the resonating structures must have almost the same energy. If the energy of the structure is almost the same, then they contribute equally towards the resonance hybrid.

• The Octet rule must be followed by all the atoms of the resonating structure.
• All atoms that are in a conjugated system must lie in the same plane.
• The greater the number of covalent bonds, the more the stability of the resonance hybrid.
• Resonance structure involving charges is a minor contributor in resonance hybrid.
• The resonating structure containing a negative charge on the more electronegative atom and the positive charge on the less electronegative charge is more stable.
• If the structure has like charges (either positive or negative) on adjacent atoms, then that resonating structure is very unstable.
• The structure in which there is delocalisation of positive charge has more stability.
• The resonance structure whose all atoms have complete octet are more stable compared to those structures which have some atoms that have not completed their octet.

Conditions for Resonance

1. If there is a conjugation of two pi-bonds (means two pi-bonds alternately), then electrons of one pi-bond would be transferred to the other bond.

(According to I–effect)

2. If there is a conjugation of one negative charge or a lone pair electron and one pi-bond (pi- sigma- lone pair/ electron), then the electrons or a lone pair electrons are transferred towards the pi-bond.

Example:

3. If there is a conjugation of the pi bond and one positive charge (pi- sigma- +ve charge), then electrons of the pi-bond are transferred towards the positive charge.

Example:

4. If there is a conjugation of free radical and one pi bond (pi- sigma- free radical).

Example:

5. If there is a conjugation of one lone pair or negative charge and one positive charge (lone pair-sigma- +ve charge), then the lone pair electrons or negative charge are transferred towards the positive charge.

Example:

Characteristics of Resonance

i. In the resonance effect, only electrons are delocalised, not atoms.

ii. The number of lone pair electrons or the number of unpaired in all resonating structures must be equal.

iii. All the resonating structures must possess the same energy.

iv. This is a permanent effect.

v. All the resonating or canonical structures must conform to Lewis structures.

Resonance Energy

The difference between the calculated energies (heat of hydrogenation) and the experimental energy, which contributes to the stabilisation of a conjugated compound, is known as the resonance or delocalisation energy. If there is more resonance energy, the better is the resonance stabilisation.

Applications of Mesomeric Effect

Carbocation Stability

Carbocation’s stability is enhanced by resonance. All the aromatic compounds are always more stable as compared to non-aromatic compounds due to the effect of resonance.

Also Read: Carbocation

Example: Compare stability order of:

(i)

Resonance effect                     +I effect                                      – I effect

(ii)

1° carbocation   2° carbocation      3° carbocation

(iii)

Resonance increases, and stability increases.

Stability of Carbanion

(a) Carbanion’s stability is increased by resonance.

Example: Compare stability order of:

(i)

Resonance effect

The correct order of stability is I > II > III

(ii)

(I)                                               (II)

Stable by resonance                 more resonance

Stability order II > I.

(iii)

Resonance effect                   no conjugation, no resonance                resonance effect and + I effect

Stability order I > III > II

Stability of Free Radicals

(a) Resonance increases the stability of free radicals.

Example: Compare stability order of:

(i)

Less resonance                                   no resonance                                more resonance

Stability order III > I > II

(ii)

Resonance                                  more resonance                                   localised

Stability order II > I > III

Acidic and Basic Strength

(a) Acidic strength:

• Acidic strength is directly proportional to the -M effect.
• Acidic strength is directly proportional to the -I effect.
• Acidic strength is indirectly proportional to the +M effect.
• Acidic strength is indirectly proportional to the +I effect.

So, this can be remembered as:

Acidic strength ∝ -M effect ∝ -I effect ∝ 1 / +M ∝ 1/+1

(b) Basic strength order:

• Basic strength is directly proportional to the +M effect.
• Basic strength is directly proportional to the +I effect.
• Basic strength is indirectly proportional to the -M effect.
• Basic strength is indirectly proportional to the -I effect.

So, this can be remembered as:

Basic strength ∝ +M effect ∝ +I effect ∝ 1 / -M ∝ 1 / -1

Examples: Give basic strength order:

(i)

Resonance effect                                   no resonance effect                             resonance effect

So, the maximum basic and +I effect

So, the basic order — II > III > I

(ii)

Stable by                                                   localised

Resonance                                               l.p. on more EN

So, the basic order — III > II > I

Electron Displacement Effects

The organic reactions are not possible until and unless some charge or polarity is developed on the reactants, and they get attached to each other. This happens only when there is a displacement of electrons, due to which polarity develops within the reactant molecules. Such effects involving the displacement of electrons in the substrate (reactant) molecules are referred to as “electron displacement effects”.

There are 4 basic electron displacement effects.

1. Inductive effect
2. Electromeric effect
3. Mesomeric effect
4. Hyperconjugation effect

Out of these four effects, the mesomeric effect is the most widely applicable and significant effect. It is also a permanent effect.