Conjugation vs Resonance
Conjugation and resonance are two important phenomena in understanding the behavior of molecules.
What is Conjugation?
In a molecule when there is alternating single and multiple bonds present, we say that the system is conjugated. For example, benzene molecule is a conjugated system. In a multiple bond, there is one sigma bond and one or two pi ponds. Pi bonds are made up by overlapping p orbitals. The electrons in the p orbitals are located perpendicular to the plane of the molecule. So when there are pi bonds in alternating bonds, all the electrons are delocalized throughout the conjugated system. In other words, we call it an electron cloud. Since electrons are delocalized, they belong to all the atoms in the conjugated system, but not for only one atom. This lowers the overall energy of the system and increase stability. Not only, the pi bonds, but also lone electron pairs, radicals or carbenium ions can take part in creating a conjugated system. In these instances, there are either non bonded p orbitals with two electrons, one electron or no electrons present. There are linear and cyclic conjugated systems. Some are restricted to one molecule only. When there are larger polymer structures, there can be very large conjugated systems. Presence of conjugation allows the molecules to act as chromophores. Chromophores can absorb light; therefore, the compound will be colored.
What is Resonance?
When writing Lewis structures, we only show valence electrons. By having the atoms share or transfer electrons, we try to give each atom the noble gas electronic configuration. However, in this attempt, we may impose an artificial location on the electrons. As a result, more than one equivalent Lewis structures can be written for many molecules and ions. The structures written by changing the position of the electrons are known as resonance structures. These are structures that only exist in theory. The resonance structures state two facts about the structure.
• None of the resonance structures will be the correct representation of the actual molecule. And none will completely resemble the chemical and physical properties of the actual molecule.
• The actual molecule or the ion will be best represented by a hybrid of all the resonance structures.
The resonance structures are shown with the arrow ↔. Following are the resonance structures of carbonate ion (CO32-).
X-ray studies have shown that the actual molecule is in between these resonances. According to the studies, all the carbon-oxygen bonds are in equal length in carbonate ion. However, according to the above structures, we can see one double bond and two single bonds. Therefore, if these resonance structures occur separately, ideally there should be different bond lengths in the ion. The same bond lengths indicate that none of these structures actually present in nature, rather a hybrid of this exists.
What is the difference between Conjugation and Resonance?
• Resonance and conjugation are interrelated. If there is conjugation in a molecule, we can draw resonance structures to it by alternating the pi bonds. Since the pi electrons are delocalized in the whole conjugated system, all the resonance structures are valid for such molecule.
• Resonance allows a conjugated system to delocalize electrons.