Fischer projection

The Fischer projection, devised by Hermann Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates and used by chemists, particularly in organic chemistry and biochemistry. The use of Fischer projections in non-carbohydrates is discouraged, as such drawings are ambiguous when confused with other types of drawing.

All nonterminal bonds are depicted as horizontal or vertical lines. The carbon chain is depicted vertically, with carbon atoms represented by the center of crossing lines. The orientation of the carbon chain is so that the C1 carbon is at the top. In an aldose, the carbon of the aldehyde group is C1; in a ketose the carbon of the ketone group has the lowest possible number (usually C2).

A Fischer projection is used to differentiate between L- and D- molecules. On a Fischer projection, the (next-to-last) carbon of D sugars are depicted with hydrogen on the left and hydroxyl on the right. L sugars will be shown with the hydrogen on the right and the hydroxyl on the left.

In a Fischer projection, all horizontal bonds project toward the viewer, while vertical bonds project away from the viewer. Therefore, a Fischer projection cannot be rotated by 90° or 270° in the plane of the page or the screen, as the orientation of bonds relative to one another can change, converting a molecule to its enantiomer. However, any rotation of 180° doesn't change the molecule's representation. Swapping two pairs of groups attached to the central carbon atom still represents the same molecule as was represented by the original Fischer projection.

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