Positive Mesomeric Effect

According to History, As of 1938, the scientist Ingold developed the concept of mesomeric effect, as well as the concept of mesomerism and mesomer. However, A Scientist -named Pauling introduced the concept of resonance which is also known as mesomerism.

It is known that the word ‘mesomerism’ was used in a wide range in Germany and France. By the way, the word ‘resonance’ in English is very popular nowadays and widely used today. Above you can see that both the concepts are the same. We discuss mesomerism mesomer as well.

What is the Mesomeric Effect?

The effect of the mesomer is defined as the polarity produced in the molecule or system conjugated with the movement of electrons towards or away from the substituent group. In other words, an atom or atomic group that donates/pulls electrons through resonance is called a mesomeric effect. Thus, electron release or electron withdrawal effects occur in substituents with electron delocalization π. Conjugation is the criteria needed for the mesomeric effect. This is represented by M, and these effects can be analyzed in various canonical forms. Let’s discuss the positive and negative mesomeric effects in detail.

Positive Mesomeric Effect(+M effect)

When electrons π move away from certain groups towards the remaining molecules increase electron density from the molecular / conjugation system. Such effects are called positive mesomeric effects (+m effect ). In simple words you can define the Positive Mesomeric Effect as When pi electrons or electrons are transferred from a particular group to the conjugate system, thereby increasing the density of the electron from the conjugation system, such a phenomenon is known as an +m effect or positive mesomeric effect. It was shown by Electron releasing substituents. Some substituents + m, -sh, -or, -cor, -nh₂, -nhcor, pH, -b, -Br, -b, etc. Let’s understand positive mesomeric effects with the help of examples: 

Example: 01

Example: 02

• For + M effects, the Group must have a pair of single electrons or must have a negative charge.
• The + M effect provides a negative charge to the conjugate system or it can be said that the electron density increases in the conjugate system because of this. This conjugate system shows more reactivity to electrophiles and lacks reactivity to nucleophiles.

The group that shows the +m effect are as follow:

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

Negative Mesomeric Effect

When electrons π are moved from the remaining molecules to a particular group, the electron density of the molecular / conjugation system decreases. Such effects are called negative mesomeric effects (-m effect). This is indicated by the substituents of electron withdrawal. Some substances are NO₂, -Cn, -So₃h, -COOH, -Cool, -CooOh₂, -Cho, and so on. In more simple words you can say that When the pi bond electrons are transferred from the conjugated system to a particular group so that the electron density of the conjugated system is reduced, then this phenomenon is known as a negative mesomeric (-m) effect. Let’s understand it with examples: 

Example 01

Example 02: 

From the above, we can come to the conclusion that: 

• For the -m effect, the group must have a positive charge or must have an empty orbital.
• -M Effect means that the compound is more reactive towards a nucleophile since it decreases the electron density in the conjugated system and at the same time it is less reactive towards the electrophile due to the same reasons.

Importance of Positive and negative mesomeric effects

• Describes the distribution of the load in the compound, it helps to decide the point at which the electrophile or nucleophiles attack.
• Useful for describing physical characteristics such as the time of dipole, and the length of the union.

What is Resonance?

There is load transfer or electronic migration from a part of the compound elsewhere. During this load, the energy is released from the conjugated system because the stability of the compound is improved.

This results in the formation of different structures. These structures are known as resonant structures. These resonance structures have the same arrangement of atoms, but only the load/electronics distribution is different.

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

Conclusion

In the above article, we learnt about the meaning of positive and negative mesomeric effects with examples and understand the resonance in an easy language. We learn that When electrons π move away from certain groups towards the remaining molecules increases electron density from the molecular / conjugation system. Such effects are called positive mesomeric effects (+m effect ). And for the negative mesomeric effects, it applies oppositely. I hope the above article helped you a lot to understand the concept and you enjoyed the article. And if you have a query regarding these comments below, our team will reply to you as soon as possible.