Resonance: Definition, Examples, Rules, R-effect

Resonance: Sharing and caring make a good bonding with your family and friends. In chemistry, the bond is formed by the sharing of electrons or the transfer of electrons. Sharing of single electron pair forms sigma \(\left( {\rm{\sigma }} \right)\) bond whereas sharing two or three pair of an electron forms a pi \(\left( {\rm{\pi }} \right)\) bond.

Are these electrons involved in the bonding stability? Are these pi bonds fixed or delocalised?  Does it have any effect on the chemical property? In this article, we have provided all the information about Resonance, the rules associated with resonance, the effects, etc. Continue reading to learn more!

Resonance: Overview

The phenomenon in which a molecule is represented by several electronic structures which do not differ much in their energy contents and are obtained by the oscillation of \({\rm{\pi }}\)-electrons is called resonance. The structures are called resonance structures.

What is Resonance?

If a compound having a certain molecular formula can be represented by different structural formulae that differ only in the arrangement of the electron pairs and not of the atoms, such structures are called resonating or contributing or canonical structures. The compound as a whole cannot be represented by any of these structures but as a hybrid called resonance hybrid with characteristics of all the contributing structures.

This phenomenon is known as resonance or mesomerism. The various resonating structures for a particular molecule are represented by the notation ↔ (sign for resonance).

Resonance Structure

Benzene can be represented as a resonance hybrid of the following two canonical (Kekule) structures, i.e., I and II.

Any of these two canonical structures cannot explain all the properties of benzene. According to these structures, a molecule of benzene should have \(3\) carbon-carbon single bonds of \({\rm{1}}{\rm{.54}}\mathop {\rm{A}}\limits^{\rm{o}} \) length and three carbon-carbon double bonds of \({\rm{1}}{\rm{.34}}\mathop {\rm{A}}\limits^{\rm{o}} \) length.

But actually, it has been found that all the six carbon-carbon bonds in benzene are equal, i.e., \({\rm{1}}{\rm{.39}}\mathop {\rm{A}}\limits^{\rm{o}} \) This implies that the actual structure of benzene is neither represented by I nor by II but is a resonance hybrid of these two structures. The actual molecule or resonance hybrid of benzene is usually represented by formula III. The circle inside the ring denotes completely delocalised six \({\rm{\pi }}\)-electrons.

Examples of Resonance

Resonance hybrid of carbon dioxide \(\left( {{\rm{C}}{{\rm{O}}_{\rm{2}}}} \right)\)

Resonance hybrid of carbonate ion \(\left( {{\rm{CO}}_{\rm{3}}^{{\rm{2 – }}}} \right)\)

Resonance hybrid of Nitrate ion \(\left( {{\rm{NO}}_{\rm{3}}^{\rm{ – }}} \right)\)

Resonance hybrid of Urea \(\left( {{\rm{N}}{{\rm{H}}_{\rm{2}}}{\rm{CON}}{{\rm{H}}_{\rm{2}}}} \right)\)

Rules for Writing Resonance Structures

Below we have provided the rules for writing resonance structures for your reference:

1. The various resonance structures should differ only in the position of electrons and not in the position of atoms or nuclei.
2. All the resonance structures should have the same number of unpaired electrons.
3. In canonical structures, unlike charges, should reside on closely situated atoms. 
4. The positive charge should as far as possible reside on the less electronegative atom, whereas a negative charge should reside on a more electronegative atom.
5. All the resonance structures should have nearly the same energy.
6. A canonical structure should possess the maximum possible number of bonds. The greater the number of bonds in a canonical form, the more is its contribution to the real molecule.
7. Charged structures have less contribution towards the hybrid in comparison to uncharged structures. 
8. The contributing structures with more covalent bonds contribute more towards the hybrid than the structures with less covalent bonds.
9. The more the number of resonance structures, the more stable is the resonance hybrid.

Resonance Energy

The difference in the internal energies of the most stable canonical structures (canonical structure with least energy) for a particular compound and its resonance hybrid is known as resonance energy.

The resonance energy for benzene is nearly \({\rm{150}}{\rm{.0kj/mol,}}\) which means that the internal energy of the hybrid is nearly \({\rm{150}}{\rm{.0kj/mol}}\) less than any of the contributing structures proposed by Kekule.

Resonance Effect or Mesomeric Effect

In conjugated systems (having alternate Sigma and Pi bonds), the electrons can flow from one part of the system to the other due to resonance. The flow of electrons from one part of the conjugated systems to the other, creating centres of low and high electron density due to the phenomenon of resonance, is called resonance effect (\({\rm{ R-}}\) effect) or mesomeric effect (\({\rm{ M-}}\) effect). It is also called \({\rm{n – \pi }}\) or \({\rm{\pi – \pi }}\) conjugation. It is of two types:

1. \({\rm{ + R}}\) or \({\rm{ + M}}\) effect: Groups that donate electrons to the double bond or to a conjugated or aromatic system are said to have \({\rm{ + R}}\) or \({\rm{ + M}}\) effect. In other terms, when the transfer of electrons away from the atom or group in conjugation with a π-bond, the mesomeric effect is said to be \({\rm{ + R}}\) or \({\rm{ + M}}\) effect.

Some groups which show the +M effect are:
\({\rm{ – OH,\; – OR,\; – SH,\; – SR,\; – N}}{{\rm{H}}_{\rm{2}}}{\rm{,\; – NHR,\; – N}}{{\rm{R}}_{\rm{2}}}{\rm{, – Cl,\; – Br,\; – I,}}\) etc.

2. \({\rm{ – R}}\) or \({\rm{ – M}}\) effect: Groups that withdraw electrons from the double bond or from a conjugated or aromatic system towards themselves due to resonance is said to have the  \({\rm{ –
R}}\) or \({\rm{ – M}}\) effect. In other terms, when the transfer of electrons towards the atom or group in conjugation with a \({\rm{\pi }}\)-bond, the mesomeric effect is said to be the  \({\rm{ – R}}\) or \({\rm{ – M}}\) effect. \({\rm{ – CHO,\; – COOR,\; – CN,\; – N}}{{\rm{O}}_{\rm{2}}}{\rm{,\; > C = O,}}\)

Applications of Resonance

1. It has been found very useful in calculating interionic distance, the partial ionic character of bonds, and the energy of the molecules.
2. It is helpful in explaining the mechanism of several chemical reactions.
3. Resonance along with hydrogen bond is helpful in various weighs in biological systems.
4. It explains the existence of odd electron bonds.
5. With the help of resonance energy, the relative electronegativity of the atoms can be determined. Relative electronegativity \(= \,0.208\sqrt {{\mathop{\rm Re}\nolimits} sonance\,energy} \)
6. Several observed facts in organic chemistry can easily be explained on the basis of the concept of resonance.

Summary

Resonance is all about the distribution of electrons and their resonance structure in organic and inorganic compounds. Resonance is also referred to as mesomerism. It occurs owing to the delocalisation of electrons in unsaturated systems. Resonance helps us to know the exact structure of the molecule.

It is important to note that several resonating structures for a particular molecule are represented by the notation ↔ (sign for resonance). Furthermore, Resonance and hydrogen bond collectively is helpful in several weighs in biological systems. It also helps in describing the mechanisms of chemical reactions.

FAQs on Resonance

Q.1. Which systems can show resonance?
Ans: Conjugated systems i.e., compounds with alternate Sigma and Pi bonds and compounds having lone pairs of electrons show resonance. 

Q.2. What does resonance mean?
Ans: In chemistry, resonance is also called mesomerism. It is a way of describing bonding in certain molecules or ions by the combination of several contributing structures.

Q.3. Why is resonance in chemistry?
Ans: Resonance in chemistry is due to the delocalisation of electrons in unsaturated systems. It helps to know the actual structure of the molecule.

Q.4. What is meant by resonance in chemistry?
Ans: The phenomenon, in which a molecule is represented by several electronic structures which do not differ much in their energy contents and are obtained by the oscillation of π-electrons, is called resonance.

Q.5. Describe some of the applications of resonance in chemistry.
Ans: Some of the applications of resonance are as follows:
1. Resonance is very useful in calculating interionic distance, the partial ionic character of bonds, and the energy of the molecules.
2. It is helpful in explaining the mechanism of several chemical reactions.
3. Resonance along with hydrogen bond is helpful in various weighs in biological systems.

Study Materials on Embibe

Make the use of following study materials from Embibe which will definitely help you in your exams:

We hope this detailed article on Resonance proves helpful to you. If you have any queries on this article or in general about this chapter, ping us through the comment box below and we will get back to you as soon as possible.