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⁻

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.