The resonance structures are the two or more lewis structures that can be written for the compound. However, these are not the actual structures of molecules. This is because they are hypothetical. They can exist only on paper although these structures are helpful. This is because they help us describe molecules, ions, and radicals. For which single lewis structure is not enough.
What are resonance structures?
“When two or more lewis structures of any molecule or polyatomic ions differ due to the distribution of electrons are known as resonance structure.”
The true structure is known as a resonance hybrid. In comparison, the others are contributing structures. However, the resonance hybrid has the same structure all the time. A double-headed arrow is used between any pair of contributing structures. Although the double-headed arrow does not mean, the oscillations of molecules from one resonance structure to another. Rather, it means that real structure is a mixture of contributing structures.
Rules for writing resonance structure
There are the following rules for writing the resonance structure:
All the contributing structures must be real lewis structure
All the contributing structures must follow the rule of valence. For example, the maximum number of bonds that an atom may have is one for hydrogen, three for oxygen, four for nitrogen, and four for carbon.
- When oxygen atoms form one covalent bond. At that time, it must have a negative charge.
- When it is neutral it must have two bonds.
- While oxygen has a positive charge, it has three bonds.
The structure ‘1’ and ‘2’ are resonance contributing structures. While structure ‘3’ is not a contributing structure. This is because carbon can not form five covalent bonds.
Similarly, in this example structure, ‘3’ is not a resonance contributing structure. This is because nitrogen cannot form five bonds.
The position of the atom must be the same in all resonance structure
When we write resonance structure only electrons are moved. The position of atomic nuclei must remain the same.
In the above example, the structure ‘3’ is not a resonance structure. This is because the position of hydrogen has been changed.
All resonating structures must have the same number of unpaired electrons
In the above example, structure ‘3’ is not a canonical form. This is because it has an unpaired electron. While the structure of 1,3-butadiene does not have any unpaired electrons.
All atoms involved in the resonance must lie in the same plane
The planarity is to allow the maximum overlap of p orbitals for the delocalization of electrons. For example, cyclooctatetraene has alternate double and single bonds. However, it does not have resonating structure,
This is because of the lack of planarity in the molecule. Hence it has a tube-shaped structure.
Structure with a distorted bond angle will not contribute to the resonance hybrid
The structure ‘2’ below is not considered a contributing structure of acrylaldehyde. This is because it has a distorted bond angle.
Contribution of Resonance structure
All structures do not contribute equally to the resonance hybrid. The contribution of each structure depends on its stability. However, it’s not easy to govern the stability of the structure. Therefore, there are some rules in this regard. These are given below:
Structures with more covalent bonds are more stable than those with fewer
In the example below the structure, ‘1’ is the most stable. This is because it contains more covalent bonds than others. Therefore, it contributes more.
Resonance Structure with a charge are less stable than those without a charge separation
The stability of the contributing structure decreases with the separation of charges. This is because charge separation requires energy.
In the above example, the structure ‘1’ of the vinyl chloride makes a large contribution. This is because it is without charge separation.
The Greater the charge separation, the less the stability
In the example below structure ‘1’ has less charge separation than structure ‘2’. Therefore, the stability of structure ‘1’ is greater.
Resonance structures with like charges on the same atoms are highly unstable
When the two adjacent atoms have the same charges, those canonical forms have less contribution. For example, the structure ‘3’ below has a negligible contribution. This is because two adjacent nitrogen atoms have positive charges.
A resonance structure with a negative charge on a more electronegative atom is more stable
The structure in which a negative charge is present on a less electronegative atom is less stable.
In structure ‘1’ a negative charge is present on the oxygen atom. Therefore, it is more stable. This is because oxygen is a more electronegative atom than carbon.
Resonance structure in which bond angles resembles resonance hybrid are more stable
The canonical form in which bond angles and bond length resembles resonance hybrid are more stable. As compared to those with distorted bond angle and bond length.
In the above example, the structure ‘1’ and ‘2’ has a major contribution. This is because they have equal bond energy. Rather, structures ‘3’, ‘4’ and ‘5’ are less important. This is because in these structures one bond is longer than the other.
Examples of resonance structure
The resonance structure of some molecules is given below:
The real structure of benzene is a resonance hybrid of the two Kekule and three dewar structures. The Kekule structure contributes more than the dewar structure. Therefore, principally benzene is a resonance hybrid of two Kekule structures.
In pyrrole, the lone pair of nitrogen interacts with the pi orbitals of the carbon atoms. The curved arrows show the delocalization of electrons.
Ozone is represented by the two resonating structures.
The carbonate ion cannot be fully described by a single lewis structure. It can be represented by the three resonating structures. Which are given below:
The two resonance structures are possible for the acetate ion. Which are shown below:
How do we know when to draw a resonance structure?
When one lewis structure is not possible to fully describe the bonding. Which is present between neighboring atoms. At that time we draw resonance structure.
What are resonance structures examples?
The two structures which differ in the position of multiple bonds and pi electrons are known as resonance structures. For example, benzene has two resonance structures. which are given below:
How many resonance structures are there?
Anu molecule can have two or more resonance structures.
What makes A resonance structure stable?
A resonance structure with a negative charge on the more electronegative atom is more stable.
How do write resonance structures?
We can write the resonance structure by moving the pi electrons.
- A textbook of Organic Chemistry by Ghulam Rasool Chahudary