<P> Enantiomer members often have different chemical reactions with other enantiomer substances . Since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms . In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiologic effects, while the other enantiomer is less active, inactive, or sometimes even productive of adverse effects . Owing to this discovery, drugs composed of only one enantiomer ("enantiopure") can be developed to enhance the pharmacological efficacy and sometimes eliminate some side effects . An example is eszopiclone (Lunesta), which is enantiopure and therefore administered in doses that are exactly 1 / 2 of the older, racemic mixture called zopiclone . In the case of eszopiclone, the S enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive . A dose of 2 mg of zopiclone must be administered to produce the same therapeutic effect as 1 mg of eszopiclone, and that appears to be the only difference between the two drugs . </P> <P> In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures . In the absence of an effective enantiomeric environment (precursor, chiral catalyst, or kinetic resolution), separation of a racemic mixture into its enantiomeric components is impossible . </P> <P> The R / S system is an important nomenclature system for denoting distinct enantiomers . Another system is based on prefix notation for optical activity: (+) - and (−) - or d - and l - . The Latin for left and right is laevus and dexter, respectively . Left and right have always had moral connotations, and the Latin words for these are sinister and rectus (straight, proper). The English word right is a cognate of rectus . This is the origin of the L, D and S, R notations, and the employment of prefixes levo - and dextro - in common names . </P> <P> Most compounds that contain one or more asymmetric carbon atoms show enantiomerism, but this is not always true . There are a few known compounds that do have asymmetric carbon atoms, but, being non-dissymmetric with respect to the whole molecule, do not show enantiomerism . Thus, meso tartaric acid has two asymmetric carbon atoms, but samples still exhibit optical inactivity because each of the two halves of the molecule is equal and opposite to the other and thus is superposable on its geometric mirror image . </P>

Which of the following compounds is the (r) enantiomer