Wedge-dash projection

FIGURE 12.5 The structure of methane, as represented by (a) its structural formula, (b) a ball-and-stiek model, (e) a space-filling model, and (d) a wedge-dash projection showing the geometry of the molecule. 

There is an entirely different way to draw stereocenters (instead of using regular bond-line drawings with dashes and wedges). Fischer projections are helpful for drawing molecules that have many stereocenters, one after another. These drawings look like this ... 

Figure 1.19 The structure and bond angles of ethene. The plane of the atoms is perpendicular to the paper. The dashed edge bonds project behind the plane of the paper, and the solid wedge bonds project in front of the paper.

Keeping in mind the three-dimensional properties of molecules, Newman projections can be converted to wedged-dashed structures or Fischer projections as desired. It is important to develop facility for manipulating structures and... 

Figure 1. The structure of strychnine. A strychnine molecule shown (A) in line form and (B) as a space-filling representation. In this line diagram and others in this chapter, wedged bonds project above the plane of the paper and dashed bonds project into it.

PROBLEM 22.2 Use solid-wedge, dashed-wedge notation to write three-dimensional representations for the following molecules shown in Rscher projection. Label each stereogenic carbon as either R) or (5). 

Methane is a tetrahedral molecule its four hydrogens occupy the corners of a tetra hedron with carbon at its center We often show three dimensionality m structural for mulas by using a solid wedge ) to depict a bond projecting from the paper toward you and a dashed wedge (i 111 ) for one receding away from you A simple line (—)... 

Lei s relurn fo bromochlorofluoromelhane as a simple example of a chiral mole cule The Iwo enanliomers of BrClFCH are shown as ball and slick models as wedge and dash drawings and as Fischer projections m Figure 7 6 Fischer projeclions are always generated Ihe same way Ihe molecule is oriented so lhal Ihe verlical bonds al Ihe chiralily center are directed away from you and Ihe horizonlal bonds poinl toward you A projeclion of Ihe bonds onto Ihe page is a cross The chiralily center lies al Ihe center of Ihe cross bul is nol explicilly shown... 

FIGURE 7 6 Ball and spoke models [left) wedge and dash drawings [center) and Fischer projections [right) of the R and S enan tiomers of bromochloro fluoromethane... 

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. 

Among the various ways in which the staggered and eclipsed forms are portrayed, wedge-and-dash, sawhorse, and Newman projection drawings are especially useful. These are shown for the staggered conformation of ethane in Figure 3.2 and for the eclipsed conformation in Figure 3.3. 

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. 

Before we can talk about drawing Newman projections, we need to first review one aspect of drawing bond-line stractures that we did not cover in Chapter 1. To show how groups are positioned in 3D space, we often use wedges and dashes ... 

You might be wondering how you would determine the conhguration of a stereocenter when you are given a Fischer projection. If each stereocenter is drawn as two wedges and two dashes, how do you figure out how to look at the stereocenter The answer is simple. Just choose one dash and one wedge, and draw them... 

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 ... 

FIGURE 3-3 Stereoisomerism in n-amino acids, (a) The two stereoisomers of alanine, l- and o-alanine, are nonsuperimposable mirror images of each other (enantiomers), (b, c) Two different conventions for showing the configurations in space of stereoisomers. In perspective formulas (b) the solid wedge-shaped bonds project out of the plane of the paper, the dashed bonds behind it. In projection formulas (c) the horizontal bonds are assumed to project out of the plane of the paper, the vertical bonds behind. However, projection formulas are often used casually and are not always intended to portray a specific stereochemical configuration. 

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. 

Newman projection Dash-line-wedge formula... 

---