What is an enantiomer?

Molecules that contain the same atoms linked in the same way, but differing in their three-dimensional arrangement, are known as stereoisomers. Commonly, stereoisomers exist due to the presence of a carbon atom within the molecule, which binds to four different atoms or molecular structures. Such a carbon atom is called a chiral centre.

Enantiomers are stereoisomers that are non-superimposable mirror images of each other and, according to the sequence of atoms attached to the chiral centre, are termed S- and R-enantiomers. They have identical physicochemical properties, except that they rotate the plane of polarised light in opposite directions. This can be compared to a right and left hand. Both have five fingers - the thumb counts as the first finger followed by the index finger etc. - yet they differ, as they are not superimposable mirror images of each other.

Citalopram is a racemic mixture, i.e., a mixture of the S-enantiomer and the R-enantiomer in equal proportions. A mixture of the possible stereoisomers of a compound in equal proportions is called a racemate (racemic mixture).