Projection formulae

Structures (I), (II) and (IV) are convenient projection formulae formulae such as (IV) and (VII) are more realistic representations, but even these are only approximations as the rings are not flat. 

Sight down the C 2—C 3 bond and draw Newman projection formulas for the... 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

In the Fischer convention, the ermfigurations of other molecules are described by the descriptors d and L, which are assigned comparison with the reference molecule glyceraldehyde. In ertqrloying the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a w as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a marmer that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away fiom the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

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. 

Since the main chain in this representation is in an entirely staggered conformation, whereas in the Fischer projection formulas the conformation represented is completefy eclipsed, an anti relationsh between two adjacent substituents in an extended structure corresponds to being on the same side in a Fischer projection formula (erythro) whereas a syn relationship corresponds to being on opposite sides in die Fischer projection (three). 

Without consulting chapter figures, draw Fischer projection formulas for glycine, aspartate, leucine, isoleucine, methionine, and threonine. 

The rules previously mentioned for assignment of a- and /3-configurations can be readily applied to Haworth projection formulas. For the D-sugars, the anomeric hydroxyl group is below the ring in the a-anomer and above the ring in the /3-anomer. For L-sugars, the opposite relationship holds. 

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

It is rather difficult to represent a tetrahedron which is three-dimensional with a drawing of a formula that is two-dimensional. Two types of representations are used, perspective and projection formulae. Fischer projections are widely used because of their simplicity. 

The conformations of H2NS02F and HC(S02F)3 as a result of rotation about single bonds are shown in Figure 8 by projection formulas. The S—C bond length in the latter is 1.831(5) A. 

FIGURE 9. The molecular model and torsional form of pentafluoroben-zene sulfonyl chloride29. The projection formula represents a view along the S—C bond. 

FIGURE 12. Two stable torsional forms of vinyl methyl sulfone33. The projection formulas represent a view along the S—C(vinyl) bonds-... 

In Newman projection formulas (the two figures on the right) the observer looks at the C C bond head on. The three lines emanating from the center of the circle represent the bonds coming from the front carbon, with respect to the observer. 

---