What does enantiomer mean?
An enantiomer is one member of a pair of molecules that share the same atoms connected in the same order but exist as non-superimposable mirror images of each other — a relationship directly analogous to the left and right human hand. Such molecules possess what chemists call chirality, or 'handedness'. Enantiomers have identical melting points, boiling points, and densities, yet they interact differently with other chiral substances, including nearly everything in living systems: enzymes, receptors, and DNA. This makes the distinction profoundly consequential in medicine and pharmacology, where one enantiomer of a drug may be therapeutic while its mirror image is inert or even toxic — the historical case of thalidomide being the most notorious example. Chemists distinguish enantiomers with R/S notation and separate them by techniques such as chiral chromatography, and the term is essential vocabulary in organic chemistry, stereochemistry, and pharmaceutical science.
nounOne of a pair of molecules that are mirror images of each other and are not superimposable, much like a left hand and a right hand. Enantiomers have identical physical properties but can differ markedly in their biological activity.
- Chemistry: either of a pair of molecules that are non-superimposable mirror images of each other, differing only in the spatial arrangement of atoms around one or more chiral centres.
"The drug was marketed as a single enantiomer after researchers found the other caused harmful side effects."
"The two enantiomers of limonene smell strikingly different — one of oranges, the other of lemons."
"Because enzymes are themselves chiral, they often bind one enantiomer far more tightly than the other."
Regular plural. Because enantiomers occur in pairs by definition, the plural is extremely common in scientific writing, though 'each enantiomer' or 'one enantiomer' is used when referring to a single member of a pair.
"The reaction produced a mixture containing both enantiomers in equal proportions, known as a racemic mixture."
Thalidomide's tragedy hinged on enantiomers: one mirror-image form eased morning sickness while its twin caused devastating birth defects — proof that in chemistry, handedness matters.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of enantiomer
The word 'enantiomer' derives from Greek 'enantios' (ἐναντίος), meaning 'opposite' or 'contrary', combined with '-mer', from Greek 'meros', meaning 'part'. It entered English usage in the late nineteenth to early twentieth century alongside the development of stereochemistry by scientists such as Louis Pasteur, Jacobus van't Hoff, and Joseph Le Bel, who established that molecules could exist as mirror-image pairs. The same Greek root appears in related terms such as 'enantiomorph' (a mirror-image form) and 'enantiodromia' (the running of things to their opposite).
Related word forms
How enantiomer is actually used
Enantiomer is a technical term used chiefly in chemistry, pharmacology, and biochemistry. It is formal register and rarely encountered outside scientific writing. In speech, chemists commonly refer to specific forms using the R/S notation (e.g., 'the R-enantiomer') rather than vague descriptors like 'left-handed' or 'right-handed', though the handedness analogy is widely used pedagogically.
Easily confused with enantiomer
Diastereomers are stereoisomers that are NOT mirror images of each other, whereas enantiomers always come as non-superimposable mirror-image pairs.
Isomers are any molecules with the same molecular formula but different structures; enantiomers are one specific subtype distinguished by being mirror images.