2,3-Dihydroxybutanoic acid, stereoisomers

FIGURE 7.9 Stereoisomeric 2,3-dihydroxybutanoic acids. Stereoisomers I and II are enantiomers. Stereoisomers III and IV are enantiomers. All other relationships are diastereomeric (see text). 

When a molecule contains two chirality centers as does 2 3 dihydroxybutanoic acid how many stereoisomers are possible ... 

Fischer projections of the stereoisomeric 2 3 dihydroxybutanoic acids compounds I and II are erythro stereoisomers and III and IV are threo... 

Orgaific chemists use an informal nomenclature system based on Fischer projections to distinguish between diastereomers. When the carbon chain is vertical and like substituents are on the same side of the Fischer projection, the molecule is described as the erythro diastereomer. When like substituents are on opposite sides of the Fischer projection, the molecule is described as the threo diastereomer. Thus, as seen in the Fischer projections of the stereoisomeric 2,3-dihydroxybutanoic acids, compounds I and II are erythro stereoisomers and III and IV are threo. 

In Section 7.5, the term relative configuration was used to describe the stereochemical relationship between a single chirality center in one molecule to a chirality center in a different molecule. Relative configuration is also used to describe the way multiple chirality centers within the same molecule are related. The two erythro stereoisomers of 2,3-dihydroxybutanoic acid possess the same relative configuration. The relationship of one chirality center to the other is the same in both, but different from that in the threo stereoisomer. 

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