Lactic acid, Fischer projections

A simpler representation of molecules containing asymmetric carbon atoms is the Fischer projection, which is shown here for the same lactic acid configurations. A Fischer projection involves... 

Enantiomers have very similar chemical properties, but they rotate polarized light in opposite directions (optical activity, see pp. 36,58). The same applies to the enantiomers of lactic acid. The dextrorotatory L-lactic acid occurs in animal muscle and blood, while the D form produced by microorganisms is found in milk products, for example (see p.l48). The Fischer projection is often used to represent the formulas for chiral centers (cf.p. 58). 

Figure 1 Fischer projections of (/t)- and (S)-enantiomers of lactic add (X = Me), mandelic add (X = Ph), glyceric acid...

FIGURE 1.14B Fischer projection of the lactic acid molecule. 

In order of decreasing rank, the substituents attached to the stereogenic center in lactic acid are —OH, —C02H, —CH3, and —H. The Fischer projection given for (-F)-lactic acid (a) corresponds to the three-dimensional representation (b), which can be reoriented as in (c). When (c) is viewed from the side opposite the lowest ranked substituent (H), the order of decreasing precedence is anticlockwise, as shown in (d). (+)-Lactic acid has the 5 configuration. 

Draw the Fischer projections for the pair of enantiomers of lactic acid, CH3—CH(OH)—COOH. 

A 90 Totaiion, howevei, 3jreak ihe Fiwher convention by exchani the groups that go into the plane and those that come out. In the lo wing. Fischer projection of lactic acid, the -H and -OH gr< come out of the plane before rotation but go into the plane after a rotation. As a result, the rotated proJe>rtion represents iS)-lactic... 

Biological reduction of pyruvic acid, catalyzed by the enzyme lactate dehydrogenase, gives (+)-lactic acid, represented by the Fischer projection shown. What is the configuration of (+)-lactic acid according to the Cahn-Ingold-Prelog R-S notational system ... 

Consider the formula for (i )-lactic acid, to the left of the mirror in Figure 5.10. If we project that three-dimensional formula onto a plane, as illustrated in Figure 5.11, we obtain the flattened Fischer projection formula. 

PROBLEM 5.15 Draw a Fischer projection formula for (S)-lactic acid. 

Fischer projection formulas are often used to depict compounds such as glyceraldehyde, lactic acid, and tartaric acid. Draw Fischer projections for both enantiomers of (a) glyceraldehyde, (b) tartaric acid, and (c) lactic acid, and specify the (R) or (S) configuration at each chirality center. [Note that in Fischer projection formulas the terminal carbon that is most highly oxidized is placed at the top of the formula (an aldehyde or carboxylic acid group in the specific examples here).]... 

Reduction of pyruvic acid by NADH using the liver enzyme lactate dehydrogenase yields exclusively (5)-lactic acid. Write the Fischer projection of this product. Why does only a single product form ... 

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