Difference Between Resonance and Mesomeric Effect

Main Difference – Resonance vs Mesomeric Effect

Resonance and mesomeric effects in molecules determine the exact chemical structure of the molecule. Resonance is the effect that describes the polarity of a molecule that is induced by interaction between lone electron pairs and bond electron pairs. Mesomeric effect is the effect of substituents or functional groups on chemical compounds. The main difference between resonance and mesomeric effect is that resonance occurs due to the interaction between lone electron pairs and bond electron pairs whereas mesomeric effect occurs due to the presence of substituent groups or functional groups.

Key Areas Covered

1. What is Resonance
      – Definition, Description with Examples
2. What is Mesomeric Effect
      – Definition, Description with Examples
3. What is the Difference Between Resonance and Mesomeric Effect
      – Comparison of Key Differences

Key Terms: Bond Electron Pair, Functional Group, Lone Electron Pair, Mesomeric Effect, Negative Mesomeric Effect, Negative Resonance Effect, Polarity, Positive Mesomeric Effect, Positive Resonance Effect, Resonance Effect

What is Resonance

Resonance is the concept that describes the interaction between lone electron pairs and bond electron pairs of a molecule that eventually determine the chemical structure of that molecule. This effect can be observed in molecules having double bonds. The resonance of molecules causes the polarity of molecules.

The interaction between lone electron pairs on atoms and the pi electron bond pairs of adjacent chemical bonds results in resonances. A molecule can have several resonance forms depending on the number of lone electron pairs and pi bonds. But the actual structure of the molecule is a hybrid of all possible resonance structures.

Figure 1: Resonance Structures of NO₃

The above image shows resonance structures of nitrate ion. Here, the lone electron pairs on the oxygen atoms interact with the pi bond electrons. This results in the delocalization of electrons. The actual structure of the molecule is a hybrid structure of all these resonance structures.

Resonance effect of molecules can occur in two types: positive resonance effect and negative resonance effect. The positive resonance effect describes the delocalization of electrons in molecules having positive charges. This occurs for the stabilization of positive charges. The negative resonance effect describes the delocalization of electrons in molecules having negative charges. This occurs for the stabilization of negative charges.

The hybrid structure obtained from the resonance structures of the molecules has a lower energy than that of all the resonance structures. Therefore, the hybrid structure is the actual structure of the molecule.

What is Mesomeric Effect

Mesomeric effect is the stabilization of a molecule with the use of different functional groups or substituents. Some substituents are electron donor groups whereas some are electron withdrawing groups. This occurs because of the difference between electronegative values of atoms in these substituent groups. Ex: higher the electronegativity, higher the electron donating ability.  

Some examples of electron donating groups are –O, -NH₂, -F, -Br, etc. The effect of electron donating or release of these substituents is known as the negative mesomeric effect or M-. Some examples for electron withdrawing groups are –NO₂, -CN, -C=O, etc. The effect of electron withdrawing of these substituents is known as the positive mesomeric effect or M+.

Figure 2: Stabilization of Nitrobenzene through Positive Mesomerism

In conjugated systems (molecules having alternating double bonds), the mesomeric effect can be moved along the system. It is the delocalization of the pi bond electron pairs. This occurs for the stabilization of the molecule.  

Difference Between Resonance and Mesomeric Effect

Definition

Resonance: Resonance is the concept that describes the interaction between lone electron pairs and bond electron pairs of a molecule that eventually determines the chemical structure of that molecule.

Mesomeric Effect: Mesomeric effect is the stabilization of a molecule with the use of different functional groups or substituents.

Causative Agent

Resonance: Resonance occurs due to the presence of lone pairs adjacent to double bonds.

Mesomeric Effect: Mesomeric effect occurs due to the presence of substituents/ functional groups or conjugated systems.

Different Types

Resonance: Resonance can be found as positive resonance effect and negative resonance effect.

Mesomeric Effect: Mesomeric effect can be found as positive mesomeric effect and negative mesomeric effect.

Conclusion

Resonance and mesomeric effect are two concepts that are used to describe the stabilization of molecules via delocalization of electrons throughout the molecule. The main difference between resonance and mesomeric effect is that resonance occurs due to the interaction between lone electron pairs and bond electron pairs whereas mesomeric effect occurs due to the presence of substituent groups or functional groups.

References:

1. “Mesomeric effect.” Wikipedia, Wikimedia Foundation, 16 Sept. 2017, Available here.
2. “Resonance Effect or Mesomeric Effect – Definition & Types of Resonance effect.” JEE Class 11-12, Byjus Classes, 17 Feb. 2017, Available here.

Image Courtesy:

1. “Nitrate ion resonance structures” (CC BY-SA 3.0) via Commons Wikimedia
2. “Nitrobenzene resonance” By Ed (Edgar181) – Own work (Public Domain) via Commons Wikimedia