What is Resonance?

Resonance is the phenomenon of describing bonds in polyatomic ions and certain molecules after merging the contributory structures. Several contributing factors or the contributing structures play an essential role in the resonance of a structure – these are also called the canonical or resonance structures. They play a crucial role in chemistry and help in understanding the structures of many molecules in a better way. The valence bond theory generally uses them to explain the chemical bonding between elements.

Definition and Examples of Resonance in Compounds

Resonance, also known as mesomerism, refers to the shuffling or rotation of the double bonds in some compounds. This mesomerism helps enhance the compound’s stability, thus reducing its reactivity. It also describes the covalent bonding in some chemical compounds like Benzene, Ozone, etc. It also refers to a way of explaining and representing delocalised electrons in some molecules, where the Lewis structure does not provide a satisfactory explanation.

It occurs in the presence of a nonbonding electron or when a pi-bond is changing positions. The pi-electron positions or the nonbonding electron positions are responsible for the change of positions of an atom. All the resonance structures are required to have the same energy. It takes place in unsaturated systems due to the delocalisation of electrons.

Let us take the example of the carboxylate anion. In this anion, the location of the double bond gets changed to provide more stability to the structure. You can also take the example of the Allylic radical. Here also, the position of the double bond gets altered to impart better stability to the entire structure.

Chemical Structures to Understand Resonance

The resonance structure of Benzene

Benzene (C₆H₆) is an essential hydrocarbon for studying organic chemistry. It is a cyclic structure made of alternating single and double bonds between carbon atoms. There can be two possible resonance structures in Benzene – one of them by delocalising the pi-electrons around the ring structure of the Benzene. As a result, the bonds get stronger than the C-C sigma bonds (in the previous structure).

Another way can be to delocalise the 3 double bonds in the hybrid structure towards the right side and then represent 6 molecules in the 3 molecular orbits. These orbitals have a nodal plane in the molecular plane.

The resonance structure of Carbonate ion

Carbonate ion is greatly useful in the chemical field and widely used for cooking. It is a polyatomic ion generally found in baking powder or baking soda. During cooking, it releases carbon dioxide and makes the food contents aerated, making them fluffy and soft. This technique is used for making cakes, bread, etc. As carbonate ions come into contact with acid, carbonic acid is produced, a crucial ingredient in aerated drinks.

2 out of 24 electrons of the Carbonate ion have the -2 charge. The carbon atoms are required to share electrons to satisfy the octet rule and thus, attain a certain level of stability. Thus, the reconfiguration of electrons takes place to make double bonds with the carbon molecule. The valence gets filled, and the positive charge on the carbon atom gets cancelled.

Importance of Resonance in Chemistry

• Resonance is an essential organic chemistry phenomenon that helps understand the various chemical structures of compounds, radicals, ions, etc.

• It imparts stability to the chemical structures of various compounds by altering the position of the double bonds present in them. It can be seen in various compounds and molecules, including Benzene, Nitrobenzene, etc.
• It helps in the explanation of the reactivity of the compounds. The more stability a compound exhibits, the less reactive it is in nature. Both these characteristics have a relation of inverse proportionality between them.

Conclusion

Resonance is an essential aspect of organic chemistry. If the electron pattern of a molecule cannot be described or explained with the help of one Lewis structure, it is called a resonance hybrid. It helps understand the reactivity of compounds formed with the help of covalent bonds. Covalent bonds are associated with the double bonds (usually) and, thus, are related to resonance and contributing structures in more than one way.