The key difference between fluxionality and tautomerism is that fluxionality refers to the interchange of some or all the atoms in a molecule whereas tautomerism refers to the interchange of protons between molecules.
Both terms fluxionality and tautomerism refer to the interchanging of atom(s) between different positions such as axial and equatorial positions. If these different positions lie on the same molecule, then it is fluxionality. But if the atom that is interchanged is a hydrogen atom (a proton) and of the positions are in two different molecules, then it is called tautomerism.
CONTENTS
1. Overview and Key Difference
2. What is Fluxionality
3. What is Tautomerism
4. Side by Side Comparison – Fluxionality vs Tautomerism in Tabular Form
5. Summary
What is Fluxionality?
Fluxionality refers to the ability of a molecule to undergo dynamics in a way that some or all of the atoms in the molecule are interchanged between symmetry-equivalent positions. Almost all the molecules we know are fluxional to some extent. A good example is the bond rotations that take place in organic compounds.
Usually, we consider a molecule is fluxional if its spectroscopic signature exhibits line-broadening due to chemical exchange. However, sometimes we cannot detect this property of fluxionality via spectroscopy due to the slow rate of interchanging. In such contexts, we can use the method of isotopic labelling for this detection.
Figure 01: Chemical Structure of Phosphorous Pentafluoride
A typical molecule that has fluxionality is phosphorous pentafluoride. When considering its fluoride-NMR spectrum, it has a 31P-coupled doublet. This indicates that the molecule has fluorine atoms in equatorial and axial positions and they interchange rapidly during the process of NMR spectroscopy.
What is Tautomerism?
Tautomerism is a concept in chemistry that describes the effect of having several compounds that are capable of interconversion via relocating a proton. This effect is most common in organic compounds such as amino acids and nucleic acids. The process of this interconversion is known as tautomerization, which is a type of chemical reaction. Here, the relocation of protons means the exchange of a hydrogen atom between two other forms of atoms. The hydrogen atom forms a covalent bond with the new atom that receives the hydrogen atom. The tautomers exist in equilibrium with each other. They always exist in a mixture of two forms of the compound since they attempt to prepare a separate tautomeric form.
Figure 02: Tautomerism in Phenol
During tautomerization, the carbon skeleton of a molecule does not change. Only the position of protons and electrons are changed. Tautomerization is an intramolecular chemical process of conversion of one form of tautomer into a different form. A common example is a keto-enol Tautomerism. It is an acid or base-catalyzed reaction. Typically, the keto form of an organic compound is more stable, but in some states, the enol form is more stable than the keto form.
What is the Difference Between Fluxionality and Tautomerism?
Both the terms fluxionality and tautomerism refer to the interchanging of atom(s) between different positions. The key difference between fluxionality and tautomerism is that fluxionality refers to the interchange of some or all the atoms in a molecule, whereas the term tautomerism refers to the interchange of protons between molecules. Moreover, fluxionality occurs in the same molecule while tautomerism occurs between two molecules.
The following table summarizes the difference between fluxionality and tautomerism.
Summary – Fluxionality vs Tautomerism
Both fluxionality and tautomerism refer to the interchanging of atom(s) between different positions. The key difference between fluxionality and tautomerism is that fluxionality refers to the interchange of some or all the atoms in a molecule, whereas the term tautomerism refers to the interchange of protons between molecules.
Reference:
1. “Fluxional Molecule.” Wikipedia, Wikimedia Foundation, 18 Apr. 2020, Available here.
Image Courtesy:
1. “Phosphorus-pentafluoride-2D-dimensions” (Public Domain) via Commons Wikimedia
2. “Phenol tautomers” By Ed (Edgar181) – Own work (Public Domain) via Commons Wikimedia