Fischer projection formulas point

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

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. 

In the Fischer Convention, the type of structural formula used is the Fischer projection. Thus we should convert the structural representation of the glyceraldehydes given in the problem into the Fischer projection. First, a cross is drawn. The location of the chiral (assymmetric) carbon is represented by the intersecting point of the two lines of the cross. The horizontal lines extending to the left and right of this point represent bonds extending forward, above the plane of the paper. The vertical lines represent bonds extending back, below the plane of the paper. 

We can now convert the eclipsed formula, above, into a Fischer projection. Imagine looking at it from a direction such that the carbon-carbon bond is vertical, and the groups horizontal to it point toward us for example ... 

Converting from a Newman projection to (or from) a Fischer projection, a perspective formula, or a skeletal structure is relatively straightforward, if you remember that a Fischer projection represents an eclipsed conformer. (Recall that in a Fischer projection, the horizontal lines represent bonds that point out of the plane of the paper toward the viewer, and the vertical lines represent bonds that point back from the plane of the paper away from the viewer.)... 

Groups on the right in the Fischer projection point downward in the Haworth formula. 

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