Molecules with More Than One Chiral Center. Diastereomers

5-5 MOLECULES WITH MORE THAN ONE CHIRAL CENTER. DIASTEREOMERS 

We have seen examples of molecules with one chiral center that exist in two mirror-image configurations, which we call enantiomers. What happens when there is more than one chiral center How many stereoisomers should we expect Consider the stereoisomers of the important amino acid, threonine, (2-amino-3-hydroxybutanoic acid). For this substance, if we write all of the possible configurations of its two chiral carbons, we have four different projection formulas, 19-22, corresponding to four different stereoisomers  

Because each chiral center added to a chain doubles the number of possible configurations, we expect eight different stereoisomers with three chiral carbons, sixteen with four, and so on. The simple rule then is 2n possible different stereoisomers for n chiral centers. As we shall see later, this rule has to be modified in some special cases. 

It should be clear (and, if it isn t, ball-and-stick models will be invaluable) that 19a and 20a are mirror images of one another and that 21 a and 22a similarly are mirror images.5 What about other combinations such as 19a and 21 a or 20a and 22a If you look at the pairs closely you will find that they are not mirror images and are not identical. Such substances, related to each other in this way and which can be converted one into the other only by changing the configurations at one or more chiral centers, are called diastereomers. 

All of the threonine stereoisomers 19-22 are chiral substances that is, they are not identical with their mirror images. However, it is important to recognize that not all diastereomers are chiral. To illustrate this point, we return to the tartaric acids mentioned previously in connection with Pasteur s discoveries (Section 5-1C). 

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Molecules with More Than One Chiral Center. Diastereomers... 

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