Stereoisomers, Fischer convention

As stated earlier, geometric isomers of alkenes are broadly classified as diastereomeric, since they are stereoisomers that are not enantiomers. The usual specification of geometry as cis or trans is deeply entrenched, but suffers from the same problems of ambiguity as does the Fischer convention, in that it is based on... 

Geometric isomers of alkenes are diastereomeric, since they are stereoisomers but not enantiomeric. The specification of the geometry of double bonds as cis and trans suffers from the same ambiguity as specifying configuration by the Fischer convention that is, it requires a subjective judgment about the similarity of groups. The sequence rule is the... 

We can draw these three stereoisomers as Fischer projections, reversing the configurations at both centres to get the enantiomeric stereoisomers, whilst the Fischer projection for the third isomer, the meso compound, is characterized immediately by a plane of symmetry. For (-l-)-tartaric acid, the configuration is 2R, >R), and for (—)-tartaric acid it is (2S,3S). For both chiral centres, the group of lowest priority is hydrogen, which is on a horizontal line. In fact, this is the case in almost all Fischer projections, since, by convention, the vertical... 

FIGURE 7-2 Three ways to represent the two stereoisomers of glyc-eraldehyde. The stereoisomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. By convention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader vertical bonds project behind the plane of the paper, away from the reader. Recall (see Fig. 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away. 

The main organic acids in grapes are described (Table 1.1) according to the conventional Fischer system. Besides tartaric acid, grapes also have a stereoisomer in which the absolute configuration of the two asymmetrical carbons is L, but whose optical activity in water, measured on a polarimeter, is d (or +). There is often confusion between these... 

Fischer projections make it easier to draw and compare stereoisomers. In this convention, horizontal lines represent bonds projecting away from the viewer while vertical lines project toward the viewer. 

Throughout this chapter we have written three-dimensional structures for particular stereoisomers and it is important to know how these have been determined. How, for example, do we know that (+)-alanine has the absolute configuration (1) and not (2) The answer is that until 1951 this was not known and the three-dimensional structure of stereoisomers was shown according to a convention introduced in the last century by Emil Fischer. According to this convention it was assumed that (+)-glyceraldehyde had the three-dimensional structure (67). Once this assumption had been made a self-consistent system of conventional representations could be applied to many other chiral compounds by chemical correlation with (67). In 1951 a group led by Bijvoet in Utrecht determined the absolute configuration of the... 

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