Mesomeric Effect, +M effect, -M effect

Table of Contents

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

-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.

-M effect

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 effectResonance 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 effectInductive 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.

Mesomeric effect

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