Isomer Definition and Examples in Chemistry

Two categories of isomers are structural isomers and stereoisomers

These are the chemical structures of the isomers of dioxin.

Todd Helmenstine/

An isomer is a chemical species with the same number and types of atoms as another chemical species but with distinct properties because the atoms are arranged into different chemical structures. When atoms can assume different configurations, the phenomenon is termed isomerism. There are several categories of isomers, including structural isomers, geometric isomers, optical isomers, and stereoisomers. Isomerization can occur spontaneously or not, depending on whether the bond energy of the configurations is comparable.

Types of Isomers

The two broad categories of isomers are structural isomers (also called constitutional isomers) and stereoisomers (also called spatial isomers).

Structural Isomers: In this type of isomerism, the atoms and functional groups are joined differently. Structural isomers have different IUPAC names. An example is the position change seen in 1-fluoropropane and 2-fluoropropane.

Types of structural isomerism include chain isomerism, where hydrocarbon chains have different degrees of branching; functional group isomerism, where a functional group may split into different ones; and skeletal isomerism, where the main carbon chain varies.

Tautomers are structural isomers that can spontaneously convert between forms. An example is keto/enol tautomerism, in which a proton moves between a carbon and oxygen atom.

Stereoisomers: The bond structure between atoms and functional groups is the same in stereoisomerism, but the geometrical positioning can change.

This class of isomers includes enantiomers (or optical isomers), which are nonsuperimposable mirror images of each other, like left and right hands. Enantiomers always contain chiral centers. Enantiomers often display similar physical properties and chemical reactivities, although the molecules may be distinguished by how they polarize light. In biochemical reactions, enzymes usually react with one enantiomer in preference to the other. An example of a pair of enantiomers is (S)-(+)-lactic acid and (R)-(-)-lactic acid.

Alternatively, stereoisomers may be diastereomers, which aren't mirror images of each other. Diastereomers may contain chiral centers, but there are isomers without chiral centers and those that aren't even chiral. An example of a pair of diastereomers is D-threose and D-erythrose. Diastereomers typically have different physical properties and reactivities from each other.

Conformational Isomers (conformers): Conformation may be used to classify isomers. Conformers may be enantiomers, diastereomers, or rotamers.

There are different systems used to identify stereoisomers, including cis-trans and E/Z.

Isomer Examples

Pentane, 2-methylbutane, and 2,2-dimethylpropane are structural isomers of each other.

Importance of Isomerism

Isomers are especially important in nutrition and medicine because enzymes tend to work on one isomer over another. The substituted xanthines are a good example of an isomer found in food and drugs. Theobromine, caffeine, and theophylline are isomers, differing in the placement of methyl groups. Another example of isomerism occurs in phenethylamine drugs. Phentermine is a nonchiral compound that can be used as an appetite suppressant yet doesn't act as a stimulant. Rearranging the same atoms yields dextromethamphetamine, a stimulant stronger than amphetamine.

Nuclear Isomers

Usually the term isomer refers to different arrangements of atoms in molecules; however, there are also nuclear isomers. A nuclear isomer or metastable state is an atom that has the same atomic number and mass number as another atom of that element yet has a different excitation state within the atomic nucleus.

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Helmenstine, Anne Marie, Ph.D. "Isomer Definition and Examples in Chemistry." ThoughtCo, Aug. 26, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 26). Isomer Definition and Examples in Chemistry. Retrieved from Helmenstine, Anne Marie, Ph.D. "Isomer Definition and Examples in Chemistry." ThoughtCo. (accessed June 2, 2023).