Isomeer

Isomeer is a term used in chemistry to describe a compound that has the same molecular formula as another compound but a different arrangement of atoms. In English, the standard word for this concept is isomer. The term isomeer appears in some languages and historical or translated texts as a direct equivalent of isomer.

The core idea of isomerism is that molecules with the same formula can exist in multiple distinct structures, leading to different properties. Isomers are typically categorized by how their atoms are arranged. Structural (or constitutional) isomers differ in the connectivity of atoms, such as when a carbon skeleton is rearranged or a functional group is relocated. Stereoisomers have the same connectivity but differ in spatial arrangement.

Stereoisomers are further divided into geometric (cis/trans) isomers and optical isomers. Geometric isomers arise around restricted rotation, notably around carbon–carbon multiple bonds or in ring systems, producing cis and trans forms with different properties. Optical isomers, or enantiomers, are non-superimposable mirror images, often due to chiral centers, and can have markedly different interactions with biological systems. Diastereomers are stereoisomers that are not mirror images of each other.

Examples help illustrate isomerism. n-Butane and isobutane are structural isomers with the same formula, C4H10. Cis-2-butene and trans-2-butene are geometric isomers with C4H8. The enantiomers of lactic acid are optical isomers with identical formulas but different three-dimensional arrangements that can lead to different biological effects.

Understanding isomerism is important for predicting physical properties, reactivity, and biological activity.