Chiral molecules Fischer projections, drawing

Chemists use two-dimensional representations called Fischer projections to show the configuration of molecules with multiple chiral centers, especially carbohydrates. To write a Fischer projection, draw a three-dimensional representation of the molecule oriented so that the vertical bonds from the chiral center are directed away from you and the horizontal bonds from the chiral center are directed toward you. Then write the molecule as a two-dimensional figure with the chiral center indicated by the point at which the bonds cross. 

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... 

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. 

Fischer projection (Section 7.8) A two-dimensional drawing of a chiral molecule in which the chirality center is represented as a cross with the atom at its coiter. Although the four bonds to the chirality center are shown in the plane of the page, the horizontal bonds project above the plane of the page and the vertical bonds project behind the page. 

It is sometimes useful to be able to draw a schematic diagram of the stereochemistry around a chiral carbon, especially when a molecule contains more than one chiral centre. The German chemist Emil Fischer solved this problem and his method of representing chiral centres is now called a Fischer projection. 

The Fischer Projection is a two-dimensional drawing of a molecule that shows a chiral carbon at the intersection of two lines. Horizontal lines represent bonds projecting out of the page and vertical lines represent bonds that project into the page. The most oxidized carbon is always represented at the "top" of the structure. 

The method used to draw stereoisomers of this type is to first draw one isomer in the Fischer projection. Then we draw its mirror image and see if they are super-imposable, if they are not, they are stereoisomers, specifically enantiomers (see structures I and II of Fig. 2). Then we switch the first left hand group with the first right hand group and compare it with the original structure to see if they are superimposable. If they are not superimposable, they are stereoisomers (compare structure I with structure III in Fig. 2). We continue this process with all the chiral centers until we reach the other end of the molecule. 

The R and S system is useful in discussing the Fischer projection, which is one of the most important two-dimensional representations of chiral molecules. This type of drawing was initially developed to display the stereochemical relationships among carbohydrates with several chiral centers, but it is also useful in many other applications. In a Fischer projection, molecules are represented by crossing vertical and horizontal lines, with each intersection representing a carbon atom. Horizontal lines represent bonds that would project forward in space if we drew the molecule using perspective notation... 

In Summary A Fischer projection is a convenient way of drawing chiral molecules. We can rotate such projections in the plane by 180° (retains absolute configuration) but not by 90° (changes absolute configuration). Switching substituents reverses absolute configuration,... 

Either by hand or by computer, it s reasonably easy to draw and manipulate real stereochemical representations of molecules with two chiral centers, sometimes even with three. However, many biological molecules have many more chiral centers than this, and Fischer devised a way of representing these in a flat form, in which it is easy to see stereochemical relationships and, particularly, to identify meso-compounds. Fischer projections consist of a cross motif, 7.89, and in this, groups b and d are pointing toward us, out of the paper, while groups a and c point away from us, behind the paper. 

Natural products of the general formula PhCH(OH)CH(NHMe)CH3 are isolated from Ephedra species and are called ephedrine and pseudoephedrine. They have various physiological effects, including uses as decongestants and appetite suppressants. Draw all four of the possible stereoisomers as Fischer projections, and assign the stereochemistry at each chiral center. Why do ephedrine and pseudoephedrine have different physiological effects Do the natural and unnatural isomers have different effects What is the relationship of these molecules to methamphetamine ... 

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