Resonance Structures

Do you know how to represent compounds through Lewis dot method? Can you represent benzene that way? Oh, you can NOT! Don’t worry! That is where the concept of resonance structures comes into play. But, what is it? In this chapter, we will read more about resonance structure and how we find that. But, before we proceed to that, let us first look at what resonance effect is all about.

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What is Resonance Effect?

We cannot predict the properties of many organic compounds with the help of single Lewis dot structure. For example, let’s consider the case of benzene. Going by the Lewis dot method, we would end up predicting Benzene to have three C-C bonds and three C=C bonds. But, the actual property deviates from this prediction.

Thus, we define resonance structures for defining properties of these compounds. The resonance structures (canonical structures) are actually hypothetical. This is because they do not represent any real molecule individually. They contribute to the actual structure in proportionately according to their stability.

The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures.The resonance energy increases with the number of important contributing structures. The number of unpaired electrons is the same in the resonance structures and so also are the positions of nuclei.

Browse more Topics under Chemical Bonding And Molecular Structure

• Bond Parameters
• Covalent Compounds
• Fundamentals of Chemical Bonding
• Hybridisation
• Hydrogen Bonding
• Ionic or Electrovalent Compounds
• Molecular Orbital Theory
• Polarity of Bonds
• Valence Bond Theory
• VSEPR Theory

Stability

The stability of resonance increases with:

• Number of covalent bonds
• Number of atoms with an octet of electrons (except hydrogen which has a duplex)
• Separation of opposite charges,
• Dispersal of charge
• A negative charge if any on a more electronegative atom, a positive charge if any on the more electropositive atom, increases the stability of the atom.

More on Resonance Effect

Resonance is the phenomenon which causes a polarity to be produced in the molecule. This could happen either by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom. The delocalisation of π-electrons is what causes this effect. We can classify the resonance effect into two main categories, as described below.

1) Positive Resonance Effect (+R effect)

In the positive resonance effect, we notice that the transfer of electrons takes place away from an atom or substituent group attached to the conjugated system (presence of alternate single and double bonds in an open-chain or cyclic system) due to resonance. Some of the substituent groups which attribute to positive resonance effect are – COOH, –CHO, >C=O, – CN, –NO₂, etc.

2) Negative Resonance Effect (-R effect)

In this effect, we see that the transfer of electrons is towards the atom or substituent group attached to the conjugated system (presence of alternate single and double bonds in an open-chain or cyclic system) due to resonance. Examples of the substituent groups that attribute to negative resonance effect include – COOH, –CHO, >C=O, – CN, –NO₂, etc.

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Let us now look at resonance structures in more specific details. We will also look at some of the examples of the same.

Resonance Structures

From few experiments, it was observed that the bond parameters of some molecules were not same as what was calculated on the basis of different bond theories. Thus, to explain these differences, the theory of resonance came into the picture which suggested that whenever a single Lewis structure is insufficient to describe a molecule correctly, then multiple Lewis structures can be superimposed over each other to describe the molecule leading to hybrid structures with similar energy, position of nuclei, bonding and non-bonding pairs of electrons.

Definition

Resonance structures are the multiple Lewis structures of similar energy, the position of nuclei, bonding and the non-bonding pair of electrons that can accurately describe a molecule. They are taken as canonical structures of the hybrid molecules formed by the superimposition of multiple Lewis structures. The hybrid molecule alone can accurately describe the molecule.

An Indicative example – Resonance Structure of SO₃

The above figure shows the different canonical structures of SO­₃. They all are similar in energy, the position of nuclei, bonding and non-bonding pairs. The three O-S bond have the same bond length. This was actually not equivalent to the length of the double bond between O and S. So, it requires resonance structures to describe it correctly.

Solved Example for You

Q: Write a note on the characteristics of resonance.

Ans:

• Every structure is associated with a certain quantity of energy, which determines the stability of a molecule or ion.
• A resonance hybrid is one particular structure that is an intermediary structure between the contributing structures. The total quantity of potential energy, however, is lesser than the intermediate. This way, the molecule is a hybrid molecule.
• Resonance averages the bond characteristics as a whole.
• The canonical forms don’t exist in reality actually. They are only discussed to make the study of molecules easier for us. The superimposition of canonical forms leads to the formation of hybrid molecules. This procedure accurately describes the molecule.
• The concept of resonating structures or canonical structures came into being so as to account for the different bond parameters found in the molecule than suggested by their Lewis structures.