Fischer projection, enantiomer

FIGURE 25 1 Three dimensional representations and Fischer projections of the enantiomers of glycer aldehyde... 

Relative to each other both hydroxyl groups are on the same side m Fischer pro jections of the erythrose enantiomers The remaining two stereoisomers have hydroxyl groups on opposite sides m their Fischer projections They are diastereomers of d and L erythrose and are called d and l threose The d and l prefixes again specify the con figuration of the highest numbered chirality center d Threose and l threose are enan tiomers of each other... 

Of the eight stereoisomeric aldopentoses Figure 25 2 shows the Fischer projections of the d enantiomers (d ribose d arabinose d xylose and D lyxose) Likewise Figure 25 2 gives the Fischer projections of the eight D aldohexoses... 

Let s return to bromochlorofluoromethane as a simple example of a chiral molecule. The two enantiomers of BrCIFCH are shown as ball-and-stick models, as wedge-and-dash drawings, and as Fischer projections in Figure 7.6. Fischer projections are always generated the same way the molecule is oriented so that the vertical bonds at the chirality center are directed away from you and the horizontal bonds point toward you. A projection of the bonds onto the page is a cross. The chirality center lies at the center of the cross but is not explicitly shown. 

FIGURE 7.6 Ball-and-spoke models (/eft), wedge-and-dash drawings (center), and Fischer projections (right) of the R and S enantiomers of bromochlorofluoromethane. 

Fischer projections and d-l notation are commonly used to describe carbohydrate stereochemistry. The standards are the enantiomers of glycer-aldehyde. 

Because a given chiral molecule can be drawn in many different ways, it s often necessary to compare two projections to see if they represent the same or different enantiomers. To test for identity, Fischer projections can be moved around on the paper, but only two kinds of motions are allowed moving a Fischer projection in any other way inverts its meaning. 

Problem 25.3, Which of the following Fischer projections of giyceraldehyde represent the same enantiomer ... 

However, the interchange of any two groups results in the conversion of an enantiomer into its mirror image (this applies to models as well as to the Fischer projections). 

Now we can understand why we cannot draw a Fischer projection sideways. If we did, we would be inverting the stereocenter. To draw the enantiomer of a Fischer projection, do not turn the drawing sideways. Instead, you should use the second method we saw for drawing enantiomers (place the mirror on the side of the compound and draw the reflection). Recall that this was the method that we used for drawings where wedges and dashes were implied but not shown. Fischer projections are another example of drawings that fit this criterion ... 

Now, we need to draw the enantiomer. For Fischer projections, we use the method where we place a mirror on the side, and then we draw the reflection ... 

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...

The erythro stereoisomers are characterized by Fischer projections in which analogous substituents, in this case OH and NH2, are on the same side when the carbon chain is vertical. There are two erythro stereoisomers that are enantiomers of each other ... 

Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ke-tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as d- and L-erythrulose. 

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