enantiomer
/ en-AN-tee-oh-mer /
Think again of your left and right hands. They are perfect mirror images, identical in every measurable part — same number of fingers, same lengths, same joints — yet they are not the same object: a right glove will never fit the left hand. Two molecules that stand in exactly this relationship are called enantiomers.
Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. At every chirality center, one enantiomer has the opposite configuration from the other (an R center becomes S, and vice versa, at all of them). Because they are mirror images, enantiomers share almost every ordinary physical property: identical melting point, boiling point, density, and solubility in ordinary solvents. They differ in just two situations — both of which require a chiral partner: they rotate plane-polarized light in equal but opposite directions, and they react at different rates with other chiral molecules (such as enzymes).
That single difference is enormous in biology. Because receptors, enzymes, and binding sites are chiral, they distinguish the two enantiomers of a molecule the way a hand distinguishes two gloves. One enantiomer of a flavour compound can taste sweet while its mirror image is bitter; one enantiomer of a drug can be a cure and its mirror image inert or even toxic. Designing or separating the correct enantiomer is therefore a central goal of pharmaceutical chemistry.
(R)- and (S)-limonene are enantiomers: the R form smells of oranges, the S form of lemons. They boil at the same temperature and have the same density, but your nose — lined with chiral receptors — tells them apart at once.
Mirror-image molecules with identical physical constants but different biological fates.
Enantiomers are a strictly pairwise relationship: a molecule has exactly one enantiomer (its mirror image). Any stereoisomer that is not that mirror image is a diastereomer, not an enantiomer — a distinction students often blur when a molecule has several stereocenters.