Resonance Structures

What are Resonance Structures?

Resonance structures are the Lewis structure sets that describe the electron's delocalization in a molecule or a polyatomic ion.

In most of the cases, a single Lewis structure has always failed to explain the bonding in either a polyatomic or molecule ion because of the presence of fractional bonds and partial charges in it. In such cases, the structures of resonance can be used to describe chemical bonding.

Define Resonance Structures?

To define resonance in chemistry, the bonding in specific ions or molecules by merging several contributory forms or structures, jointly known as resonance structures or canonical structures within the valence bonding theory, into a hybrid resonance (or the hybrid structure).

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The different structures of resonance of the carbonate ion (CO32-) can be illustrated above. Also, electrons' delocalization can be described through the fractional bonds (denoted by dotted lines) and the fractional charges present in a resonance hybrid.

At times, the resonance structures are not equivalent, and it is essential to determine which one (s) describes the best actual bonding. A formal charge is used to predict which resonance structures are most favoured.

Resonance Structures of NO2– Ion

In the case of nitrite ion, the bond lengths present in both nitrogen-oxygen bonds are equal. Whereas the Lewis dot structures of the NO2– ion highlight a difference in the 2 N-O bonds' bond order. Moreover, the resonance hybrid of this polyatomic ion, which is obtained from its various resonance structures, is used to explain the equal bond lengths, same as illustrated below.

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The resonance hybrid of the NO2– ion suggests that every oxygen atom holds a partial charge of magnitude with -½. The bond length of the N-O bonds can be given as 125 pm.

Resonance Structures of NO3– Ion

Nitrogen is considered to be the central atom in the nitrate ion. It is singly bonded to 2 oxygen atoms and doubly bonded to 1 oxygen atom. The oxygen atoms which are singly bonded to the nitrogen hold a charge of -1 (to satisfy the octet configuration). The central nitrogen atom also holds a charge of +1, and the overall charge on the nitrate ion is given as -1. The 3 possible resonance structures of NO3– can be illustrated as follows.

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If a resonance hybrid of the polyatomic molecule is drawn from the set of Lewis structures that is provided above, the partial charge present on every oxygen atom will be equal to -(⅔). And, the net charge on the central atom remains as +1. This resonance hybrid can be illustrated as follows.

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The resonance hybrid of the ozone molecule holds a +1 charge associated with the oxygen at the centre and also a partial charge of -(½), which is associated with the other oxygen atoms.

Resonance Structures of the Carbonate (CO2−3) Ion

Carbonate ion is considered as a common polyatomic ion that is found in baking powder, baking soda, and limestone. The addition of acid to the carbonate ion causes the carbonic acid formation, which decomposes rapidly into carbon dioxide and water. In baking, the carbon dioxide, which is released, will cause bread to rise and makes its texture lighter.

Our first attempt in drawing carbonate ion's lewis dot structure results in the structure as represented below. Carbonate ion holds 24 electrons, where 2 of them are responsible for a charge of -2. This is probably from the electrons of sodium, calcium, or whatever salt resulted in a cation that the donated electrons to the carbonate anion. Still, the carbon atom present in the pure structure needs to share the electrons to satisfy the octet rule at which the atom is most likely involved in electron sharing.

Using the formal charges present on the atoms, we could reconfigure our electrons to participate in the double bond with the carbon molecule. At this point, the positive charge present on the carbon atom is gone, and all the valence is filled. Then, the octet rule is satisfied. The formal charge's sum is equivalent to the charge present on the carbonate ion. This is considered to be a good Lewis dot structure for carbonate.

Resonance Structure of Nitrobenzene

The electron density present in the aromatic ring of nitrobenzene molecule is less than the benzene owing to the presence of an electron-withdrawing group, which contains a double bond adjacent to the nitrobenzene's phenyl ring as illustrated by the nitrobenzene's resonance structures.

Therefore, the phenyl ring of the nitrobenzene is less nucleophilic compared to the benzene. From the resonance structures, the para and ortho positions are positive. Hence, for the electrophilic aromatic substitution reaction, the electrophile will not be going to react at these positions, whereas, instead, at the meta position. Therefore, it appears that if a double bond falls in conjugation with the phenyl ring, which is the electrophilic aromatic substitution product will be said as the meta substituted product.

Resonance Structures of Benzene

Benzene is a much essential aromatic hydrocarbon of organic chemistry. It has the chemical formula C6H6. The benzene molecules contain a cyclic structure consisting of alternating single bonds and double bonds between the adjacent carbon atoms. Every carbon atom is also bonded to 1 hydrogen atom. The 2 possible resonance structures of the benzene molecule can be illustrated as follows.

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The benzene molecule can be stabilized using resonance, where the pi electrons are delocalized around the ring structure. This particular delocalization causes every carbon-carbon bond to hold a bond order of 1.5 by implying that they are stronger compared to the regular C-C sigma bonds. In the benzene's resonance hybrid, the delocalization of pi electrons can be described using a circle inside of the hexagonal ring.

In benzene molecule, Kekule's first suggested 2 cyclohexatriene Kekule structures that have been taken together, and they constitute the general structure as the contributing structures. The hexagon replaces 3 double bonds in the hybrid structure on the right side and also represents 6 electrons in a collection of 3 molecular orbitals with a nodal plane in the molecule plane.

Therefore, in this manner, we can also discuss the resonance structures of a few polyatomic ions and molecules.