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  Fischer Projection Formulas Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Fischer Projection Formulas

On a two-dimensional surface the problem of drawing 3-D configurations, like a piece of paper, has been a long-standing concern of chemists. The hatched and wedge line notations we have been employing are effective, but when applied to compounds having many chiral centers, it can be troublesome. As part of his Nobel Prize-winning research on carbohydrates, the German chemist Emil Fischer devised a simple notation i.e. still extensively used. The four bonds to a chiral carbon make a cross with the carbon atom at the intersection of the vertical and horizontal lines in a Fischer projection drawing. Toward the viewer (forward of the stereogenic carbon) the two horizontal bonds are directed. The two vertical bonds are directed behind the central carbon that is away from the viewer. Because this is not the general method in which we have viewed this type of structures, how a stereogenic carbon positioned in the common two-bonds-in-a-plane orientation ( x-C-y define the reference plane ) is rotated into the Fischer projection orientation (the far right formula) the following pictures displayed. When writing Fischer projection formulas it is significant to remember these conventions. Because the horizontal bonds toward the viewer and the vertical bonds extend away from the viewer, within the plane a Fischer structure may only be turned by 180º, So maintaining this relationship. The structure must not be flipped over or rotated by 90º.

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In the above picture, the resulting formula describes (R)-(-)-lactic acid, if x = CO2H, y = CH3, a = H & b = OH. The mirror-image formula, where x = CO2H, y = CH3, a = OH & b = H, would, certainly, represent (S)-(+)-lactic acid.

The stereoisomers of 2-methylamino-1-phenylpropanol are drawn in the following manner by using the Fischer projection notation. Note: it is customary to set the longest carbon chain as the vertical bond assembly.

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In his carbohydrate studies, the value of this notation to Fischer is evident in the picture. There are eight stereoisomers of 2, 3, 4, 5-tetrahydroxypentanal, a set of compounds considered as the aldopentoses. Because in this constitution there are three chiral centers, we should suppose a maximum of 23 stereoisomers. These eight stereoisomers contain four sets of enantiomers. The four stereoisomers that result will be diastereomers if the configuration at C-4 is kept constant (R in the examples shown here). Fischer formulas for these isomers, that Fischer entitled as the "D"-family, are displayed in the diagram. Each of these compounds has an enantiomer that is a member of the "L"-family so, as supposed; there are eight stereoisomers in all. Find out whether a chiral carbon is R or S may appear not easy while using Fischer projections, but it is in fact quite simple. If the lowest priority group (generally hydrogen) is on a vertical bond, configuration is given directly from the relative positions of the three higher-ranked substituents. If the group which have the lowest priority is on a horizontal bond, the positions of remaining groups give the wrong answer (you are in looking at the configuration from the wrong side), so you just reverse it.

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The aldopentose structures displayed above in the direction are all diastereomers. A more selective phrase, epimer, is used to designate diastereomers that different in configuration at only one chiral center. So, arabinose and ribose are epimers at C-2, and lyxose and arabinose are epimers at C-3. Though, xylose and arabinose are not epimers, since their configurations different at both C-2 and C-3.

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