Mesomeric effect is defined as the polarity produced in a molecule or a conjugated system by the movement of π electrons towards or away from a substituent group. In other words, an atom or group of atoms that donates/withdraw electrons through resonance is called mesomeric effect. Thus, electron releasing or electron-withdrawing effects occur to the substituents by delocalization of π− electrons. Conjugation is a necessary criterion for mesomeric effect. It is represented by M, and such effect can be analyzed in various canonical forms.
Types of Mesomeric effect
- +M effect (positive mesomeric effect)
- -M effect )negative mesomeric effect)
+M effect (positive mesomeric effect)
When the π electrons move away from the particular group towards the rest of the molecule increases the electron density of the molecule/conjugated system. Such an effect is called positive mesomeric effect ( +M effect). It is shown by electron releasing substituent. Some of the +M substituents are −OH,−SH, -OR, -OCOR, −NH2, -NR2, –NHCOR, Ph, –CH3, –F, –Cl, -Br, –I, etc.
-M effect (negative mesomeric effect)
When the π electrons are moved from the rest of the molecule to a particular group, the electron density of the molecule/conjugated system decreases. Such an effect is called negative mesomeric effect ( -M effect). It is shown by electron-withdrawing substituent. Some of the -M substituents are NO2, –CN, –SO3H, –COOH, –COOR, –CONH2, –COO−, –CHO, –COR, and so on.
Mesomeric effect and resonance effect
Mesomeric effect is defined as the polarity produced in a molecule or a conjugated system by the movement of π electrons towards or away from a substituent group. It is represented by M.
A resonance effect is the withdrawal or release of electrons associated with a particular substituent through the delocalization of pi-electrons, which can be demonstrated by various canonical structures.
The group which shows +R effect (electron releasing) is equivalent to +M effect and the group shows –R effect (electron attracting) is equivalent to –M effect.
Electromeric effect and Mesomeric effect
Electromeric effect is a temporary effect in which dipole moment is created instantly in a molecule due to the complete transfer of a pi-electron pair by the influence of attacking reagent; electrophile or a nucleophile. Electromeric effect is of two types; +E effect and the -E effect. +E effect. +E effect occurs when π-electrons of the bond are transferred to the attacking reagent, while -E effect occurs when π-electrons of the bond are transferred to the atom to which the attacking reagent is not attached.
Mesomeric effect is defined as the polarity produced in a molecule or a conjugated system by the movement of π electrons towards or away from a substituent group. Mesomeric effect is of two types; +M effect and the -M effect. +M effect occurs when the π electrons move away from the particular group towards the rest of the molecule, and increases the electron density of the molecule/conjugated system. -M effect occurs when the π electrons are moved from the rest of the molecule to a particular group, the electron density of the molecule/conjugated system decreases.
Mesomeric effect vs Resonance effect
| Mesomeric effect | Resonance effect |
| Mesomeric effect is the polarity produced in a molecule or a conjugated system by the movement of π electrons towards or away from a substituent group. | A resonance effect is the withdrawal or release of electrons associated with a particular substituent through the delocalization of pi-electrons. |
| Positive mesomeric effect (+M) and negative mesomeric effects (-M) are the two types of mesomeric effect. | Positive resonance effect (+M) and negative resonance effects (-M) are the two types of resonance effect. |
| It is due to the substituents, either electron-withdrawing or electron-donating in a conjugated system. | It occurs due to the presence of lone pairs adjacent to the double bond. |
Mesomeric effect vs Inductive effect
| Mesomeric effect | Inductive effect |
| Mesomeric effect is the polarity produced in a molecule or a conjugated system by the movement of π electrons towards or away from a substituent group. | The polarization of one bond caused by the polarization of an adjacent bond is known as an inductive effect. |
| The mesomeric effect is a permanent effect that depends on the substituents or the functional groups in a chemical compound. | The electronegativity difference between the two atoms in the bond directly affects the inductive effect. |
| When the substituent is an electron-withdrawing group, the mesomeric effect is negative -M. | The electron-withdrawing nature of groups or atoms is called a negative inductive effect. It is indicated by -I. |
| When the substituent can be considered as an electron releasing group based on the resonance structures, the effect is positive +M. | The electron-releasing nature of groups or atoms is called a positive inductive effect. It is indicated by +I. |
FAQS
Is mesomeric effect and resonance effect the same?
No, there’s a difference between resonance and mesomeric effect. The main difference is that mesomeric effect occurs due to the substituents, either electron-withdrawing or electron-donating in a conjugated system, while resonance effect occurs due to the presence of lone pairs adjacent to the double bond.
