Diastereomers:

When a molecule has two or more asymmetric centers, there are various possible structures that are possible and we require to makes use of the terms like stereoisomers, diastereomers and enantiomers to discuss them. To demonstrate the relative meaning of these terms, we shall look at the probable structures of the amino acid threonine. This molecule comprises two asymmetric centers. The result of it is, four different structures are possible taking place from the two different possible con?guration at each center. These are illustrated with the asymmetric centers at positions 2 and 3 entitled as R or S. The four dissimilar structures are considered to as stereoisomers. The (2S, 3R) stereoisomer is a non super-imposable mirror image of the (2R, 3S) stereoisomer and thus these structures are enantiomers having similar chemical and physical properties. The (2S, 3S) stereoisomer is the non super-imposable mirror image of the (2R, 3R) stereoisomer and thus these structures are as well enantiomers having similar chemical and physical properties. Each set of enantiomers is termed as a diastereomer. Diastereomers are not mirror images of each other and do totally dissimilar compounds that comprise different physical and chemical properties. To conclude, threonine has two asymmetric centers that mean that there are two probable diastereomers consisting of two enantiomers each, creating a total of four stereoisomers. Since the number of asymmetric centers rises, the number of possible stereoisomers and diastereomers increases. For a molecule comprising n asymmetric centers, the number of possible stereoisomers is 2ⁿ and the number of diastereomers is 2^n-1.