How to Specify a Configuration

Two conventions are in use to describe the configuration of a stereogenic centre. The older system uses the symbols D and l, and is still employed for two important classes of compound, namely amino acids and sugars otherwise it has been superseded. The d/l system will be considered first. The use of capital letters here serves to distinguish this convention from dll (page 22), which relates only to the sense in which the plane of polarized light is rotated. 

The configuration of (-)-6 is as shown and is accordingly termed D-glyceric acid. This result shows that there is no correlation between configuration and the sign of rotation. 

Amino acids of type 7 of course may be assigned configurations in the R/S convention (Section 2.2.2.) after having read this section, attempt question 2,6 in the Problem section (page 35) to ascertain whether the four amino acids 28-31, which are all of L configuration, are all described by R or S. 

There are further rules for the use of Fischer projections (a) where relevant the vertical direction is that of the carbon chain (b) in the vast majority of cases the carbon in the highest oxidation state is located at the top of the diagram. 

Verify these rules with a molecular model of 3. 

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