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The Fischer Projection

Projection & nomenclature

In regard to the illustration of the three-dimensional structure of chiral molecules, one must confirm to a few rules, so that the configuration can be unequivocally interpreted from a two-dimensional drawing. For that matter, the Fischer projection, already introduced by Emil Fischer in 1891, is still in use today, particularly in the case of carbohydrates and amino acids.

Fig.1
D-glyceraldehyde

In order that the Fischer projection may be obtained, the longest continuous carbon chain must first of all be arranged in the overall eclipsed conformation. Subsequently, the chain is vertically aligned in such a way that the highest oxidized carbon atom is located at the top (COOH > CHO > CH2OH > CH3). The chain is then rotated around a vertical axis, so that the vertical bonds of the carbon chain point away from the viewer, while the horizontal bonds of the substituents point towards the viewer. All bonds are illustrated as unbroken lines. In the end, the molecule is projected (flattend) onto the plane (paper or blackboard).

If the substituent of the chiral atom is on the right side of the chain, the configuration is called D. If it is on the left side, the configuration is called L. In Fig. 1 D-glyceraldehyde is shown. It belongs to the D series of aldoses.

In the case of carbohydrates, the configuration is named according to the position of the hydroxy group (red in Fig. 2) of the chiral atom which is farthest away from the highest oxidized carbon in the chain. In the case of amino acids, the configuration is named according to the position of the α amino group (green in Fig. 2).

Fig.2

The D,L descriptor is always assigned to the whole molecule. The absolute configuration of the remaining chirality centers is then unequivocally determined by the compound name.

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