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In the case of certain molecules, a single Lewis structure cannot explain all the molecule’s properties. Under the framework of VBT, resonance is an extrapolated idea that a Lewis structure can describe the bonding in a chemical species. For many chemical species consisting of atoms obeying the octet rules, possibly bearing the formal charges and connected by bonds of positive integer order, it is not sufficient for describing all properties. In this situation, several contributing structures are helped to understand the properties; in this case, resonance is considered. As we have answered the first question, we will discuss the condition for resonance in chemistry.*-
Condition for resonance in chemistry
The contributing structures should have the same position as the constituting atoms. They only differ in the position of electrons.
The contributing structure should have the nearly same energy.
The constituting structure should have a negative charge on the more electronegative atom and a positive charge on the electropositive atom.
In the structures, like charges should not be present on adjacent atoms while unlike charges should not be widely separated.
Resonating structures must have the same no. of electrons and the same overall charge.
As resonance is a broad topic, it takes time to understand the topic completely. We have to remember many interlinked concepts before applying them to the concept. Through the concepts given above, we can surely answer the question ‘What is the condition for Resonance?’ and many more questions related to the general idea about conditions for resonance in chemistry. We will discuss ‘series resonance’ and its conditions.
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Condition For Series Resonance
If there are two pi bonds or more than two in conjugation, electrons of one pi bond are transferred towards another pi bond.
If a lone pair or negative charge is in conjunction with pi bonds, electrons of negative charge or lone pair are transferred towards the pi bond.
If there is a positive charge (vacant orbital) and a pi bond is in conjugation, the pi bond is transferred towards a positive charge.
If there is a free electron and pi bond in conjugation, then the pi bond breaks down, and one electron from the pi bond and another free electron participate in bonding.
If there is a lone pair or negative charge and a positive charge is in conjugation, then the electron of the lone pair or negative charge is transferred towards the positive charge.
Also see
Guidelines for Drawing Resonating Structures
Use curved arrows to indicate the movement of electrons in the resonance structure; the new resonance structure should be a product automatically obtained by following arrows.
Calculate the formal charge in the new structure and label any non-zero formal charges.
Quantum Mechanical Aspect of Resonance
Resonance has deep importance in the mathematical formalism of Valence Bond Theory. Quantum Mechanics requires the wave function of a molecule to obey its observed symmetry. If a single contributing structure does not match this, resonance is invoked. For example, Benzene VBT begins with 2 Kekule structures that do not individually possess the six pole symmetry of the whole molecule. The theory constructs the actual wave function as a linear superposition of the wave functions representing the two structures. As both Kekule structures have equal energy, they are equal contributors to the overall structure. The superposition is an equally-weighted average 1:1 linear combination of the two in the case of Benzene. The symmetric combination gives the ground state, while the antisymmetric combination gives the first excited state. The superposition or resonance is written with undetermined coefficients, which are then variationally optimised to find the lowest possible energy for the given set of basis wavefunctions. When more contributing structures are included, the molecular wave function becomes more accurate, and more excited states can be derived from different combinations of contributing structures.
Key Points About Resonance
In resonance, only electrons are transferred, not atoms
The number of electrons or unpaired or paired electrons in all resonating structures should be the same.
It is a permanent effect.
All the resonating or canonical structures must follow the Lewis structures.
Conclusion
Resonating structures can be visualised by drawing a Lewis structure; however, it is important to note that these structures cannot be observed in nature. The molecule doesn’t go back and forth between these configurations; rather, it is an intermediate structure with overall lower energy than each of the possible configurations. For that reason, in Organic Chemistry, resonance is a very important concept to determine the stability, acidity, basicity, aromaticity, and many such properties of a molecule, which is further used to predict the nature of a reaction, or in the prediction of the desired product from a particular reaction. That is why it is almost necessary to know about the various conditions of resonance in chemistry.
