The key difference between tautomerism and metamerism is that tautomerism refers to the dynamic equilibrium between two compounds having the same molecular formula whereas metamerism refers to structural isomerism in which different alkyl groups are attached to the same functional group.
Isomerism is the existence of chemical compounds having the same structural formula but different spatial arrangements. Hence, isomers have the same number of atoms in each element, but their arrangement is different. Isomers can be mainly categorized into two groups as structural isomers and stereoisomers. Tautomerism and metamerism are two subdivisions of structural isomerism.
CONTENTS
1. Overview and Key Difference
2. What is Tautomerism
3. What is Metamerism
4. Side by Side Comparison – Tautomerism vs Metamerism in Tabular Form
5. Summary
What is Tautomerism?
Tautomerism is a concept in chemistry which describe the effect of having several compounds that are capable of interconversion via relocating a proton. This phenomenon is most common in amino acids and nucleic acids. The process of this interconversion is ‘tautomerization’. Tautomerization is actually a chemical reaction. In this process, the relocation of protons means the exchange of a hydrogen atom between two other forms of atoms. Here, the hydrogen atom forms a covalent bond with the new atom that receives the hydrogen atom. After the formation, the tautomers exist in equilibrium with each other. These compounds always exist in a mixture of two forms of the compound since they attempt to prepare a separate tautomeric form.
When tautomerization takes place, the carbon skeleton of the molecule does not change. In fact, only the position of protons and electrons are changed. We can classify the tautomerization process as an intramolecular chemical process of conversion of one form of tautomer into a different form. For example, keto-enol tautomerism, which is an acid or base-catalyzed reaction, is a common reaction. Usually, 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 Metamerism?
Metamerism occurs when alkyl groups on the sides of the functional groups differ from each other. This means it is an unequal distribution of carbon atoms. Metamerism belongs to the same homologous series, which means that the number of carbon atoms can be increased gradually to get different isomers. Therefore, the structures differ only by the number of CH2 groups in the main carbon chain.
The alkyl groups are always attached to sides of a divalent atom like oxygen or sulfide, or alkyl groups may be attached to a divalent group such as -NH-. We may rarely find metamerism because of these limitations. Therefore, most of the compounds that we can find in metamerism are ethers and amines.
For example, diethyl ether and methyl propyl ether are metamers. Here, the functional group is ether, and the divalent atom is an oxygen atom. Diethyl ether has two ethyl groups whereas methyl propyl ether has a methyl and a propyl group on the sides of the oxygen atom.
What is the Difference Between Tautomerism and Metamerism?
The key difference between tautomerism and metamerism is that tautomerism refers to the dynamic equilibrium between two compounds having the same molecular formula whereas metamerism refers to structural isomerism in which different alkyl groups are attached to the same functional group. In tautomerism, isomers differ from each other according to the position of protons whereas, in metamerism, isomers differ from each other according to the alkyl groups attached to the main functional group.
Below infographic summarizes the difference between tautomerism and metamerism.
Summary – Tautomerism vs Metamerism
Tautomerism and metamerism are two concepts in organic chemistry. The key difference between tautomerism and metamerism is that tautomerism refers to the dynamic equilibrium between two compounds having the same molecular formula whereas metamerism refers to the structural isomerism in which different alkyl groups are attached to the same functional group.
Reference:
1. Helmenstine, Anne Marie “Isomer Definition and Examples in Chemistry.” ThoughtCo, Feb. 11, 2020, Available here.
Image Courtesy:
1. “Phenol tautomers” By Ed (Edgar181) – Own work (Public Domain) via Commons Wikimedia