Resonance Effect and Resonance Structure

Resonance Effect

This topic explains the resonance effect or Mesomeric effect and its types. In organic chemistry, the behavior of electrons differs when the elements other than that of the carbon atoms and hydrogen actively take part in the formation of molecular bonds.

The electronic factors influencing the organic reactions includes the electromeric effect, the inductive effect, resonance effects, hyperconjugation, etc. All these factors relate to the organic molecules, in a diverse manner. Most of the biological molecules consist of a combination of these six elements: carbon, nitrogen, hydrogen, oxygen, sulfur, phosphorus. Yet, they do not prevent the organic compounds from taking on the diverse properties of their chemical reactivity and physical characteristics.

The organic molecules also exhibit the property of resonance or mesomerism. In organic chemistry, the factor called resonance or Mesomerism describes the delocalized electrons within the certain molecules where one single Lewis structure does not express the bonds. An ion or molecule with these delocalized electrons can be represented by contributing several structures which are called resonance structures.

Resonance Effect or Mesomeric effect In Chemistry

The withdrawal effect or releasing effect of electrons attributed to a particular substituent through the delocalization of π or pi-electrons that can be seen by drawing various canonical structures is called resonance effect or mesomeric effect. M or R symbols are used to represent resonance effect.

Resonance Structures

The Above figure shows the different resonance structures of different compounds with resonance effects.

The concept of resonance effect tells about the polarity induced in a molecule by the reaction between a lone pair of electron and a pi bond. It also occurs by the interaction of 2 pi bonds in the adjacent atoms. Resonance in simple is the molecules with multiple Lewis structures. Resonance in chemistry helps in understanding the stability of a compound along with the energy states.

Definition– The resonance effect can be defined as a chemical phenomenon which is observed in the characteristic compounds having double bonds in the organic compounds. The organic compounds contain these double bonds in the structures and usually have the overlapping of the p-orbitals on the two adjacent sides of carbon atoms.

Types of Resonance Effects

There are two types of Resonance effects namely positive resonance effect and negative resonance effect.

  1. Positive Resonance Effect- Positive resonance effect occurs when the groups release electrons to the other molecules by the process of delocalization. The groups are usually denoted by +R or +M. In this process, the molecular electron density increases. For example- -OH, -SH, -OR,-SR.
  2. Negative Resonance Effect- Negative resonance effect occurs when the groups withdraw the electrons from other molecules by the process of delocalization. The groups are usually denoted by -R or -M. In this process, the molecular electron density is said to decrease. For example- -NO2, C=O, -COOH, -C≡N.

Practise This Question

Among the following, the least stable resonance structure is: