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The Fischer Projection

Rules for Handling Fischer Projections

Fig.1
Fischer projection of 2-butanol.

The Fischer projection of 2-butanol (Abb. 1) shows one of several possible Fischer projections of the same enantiomer. Only specific manipulations of Fischer projections are allowed, otherwise not a projection of the same but of the opposite enantiomer would be obtained. The allowed manipulations are expressed as follows:

Rule 1
A Fischer projection may be rotated 180° in the illustration plane. 90° and 270° rotations are not allowed.
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Explanation:

A 180° rotation of the Fischer projection means that substituents that initially pointed towards or away from the viewer continue to point in these same directions. In addition, substituents that initially lie directly opposite from each other are interchanged. The overall result is basically the same as that of a 180° rotation of the whole molecule on an axis that is perpendicular to the illustration plane and passes through the asymmetric carbon (run the animation). Therefore, the new Fischer projection represents the same enantiomer.

In contrast, a 90° or 270° rotation of the Fischer projection is actually not equivalent to a 90° or 270° rotation of the whole molecule on the axis mentioned above. As a result of a 90° or 270° rotation of the Fischer projection, the substituents that initially pointed towards or away from the viewer now point in the opposite directions. In exchange, a 90° or 270° rotation of the whole molecule on the formerly mentioned axis does not alter the orientation of the substituents in relation to the illustration plane (run the animation of 90° and 270° rotation of the whole molecule).

Hint
A 90° or 270° rotation results in the Fischer projection of the opposite enantiomer with opposite absolute configuration.
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