Chiral molecules designating enantiomers

A racemic mixture contains equal amounts of the (+) enantiomer and the (-) enantiomer and has the designation (+/-). Racemic mixtures are not optically active, because the rotation of the dextrorotatory enantiomer cancels out the rotation of the levorotatory enantiomer. Synthesis or isolation of a single enantiomer in the laboratory is a challenging task, and most syntheses of chiral molecules result in a racemic mixture containing both enantiomers. The Food and Drug Administration (FDA) policy statement drafted in 1992 and updated in 20053 requires pharmaceutical companies to characterize the properties of single enantiomers, and this adds difficulty, time, and expense to development of new medicines. 

Most of the molecules we find in nature are chiral—a complicated molecule is much more likely not to have a plane of symmetry than to have one. Nearly all of these chiral molecules in living systems are found not as racemic mixtures, but as single enantiomers. This fact has profound implications, for example, in the chemistry of drug design, and we will come back to it later. 

C Well, there can be unanticipated effects due to the chemical behaving in ways that are not in the original design. Do you remember Thalidomide The molecule is chiral and one enantiomer is a great remedy for nausea while the other causes birth defects. 

Two stereoisomers that are nonsuperimposable mirror images of one another are called a pair of enantiomers. Molecules that can exist in enantiomeric forms are called chiral molecules. The term simply means that as a result of different three-dimensional arrangements of atoms, the molecule can exist in two mirror-image forms. For any pair of nonsuperimposable mirror-image forms (enantiomers), one is always designated D- and the other L-. 

Trans" is implied by the (S,S) designation. The other two stereoisomers are the R,R) compound, its enantiomer (obviously also irans), and the cis isomer, a diastereomer, which is a meso R,S) compound (Section 5-6). This is the only possible chiral molecule with the formula C,H o and one ring. 

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