Depicting Carbohydrate Stereochemistry Fischer Projections

Thomson lOW Click Organic interactive to learn to draw and interpret Fischer projections of simple monosaccharides. 

Because carbohydrates usually have numerous chirality centers, it was recognized long ago that a quick method for representing carbohydrate stereochemistry is needed. In 1891, Umil Tischer suggested a method based on the projection of a tetrahedral carbon atom onto a flat surface. These Fischer projections were soon adopted and are now a standard means of representing stereochemistry at chirality centers, particuiarly in carbohydrate chemistry. 

A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. 

Because a given chiral molecule can be drawn in many different ways, it s often necessary to compare two projections to see if they represent the same or different enantiomers. To test for identity, Fischer projections can be moved around on the paper, but only two kinds of motions are allowed moving a Fischer projection in any other way inverts its meaning. 

A 90 rotation breaks the Fischer convention by exchanging the groups that go into the pJane and those that come out. In the following Fischer projections of (i )-glyceraldchyde, the —H and —OH groups come out of the plane before rotation but go into the plane after a 90° rotation. As a result, the rotated projection represents (5)-glyceraldehyde. 

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The structures above show some of the fundamental features of carbohydrates. Initially, we have drawn these compounds in the form of Fischer projections, a depiction developed for these compounds to indicate conveniently the stereochemistry at each chiral centre (see Section 3.4.10). The Fischer projection is drawn as a vertical carbon chain with the group of highest oxidation state, i.e. the carbonyl group, closest to the top, and numbering takes place from the topmost carbon. 

Since all carbohydrates have chiral carbon atoms, it was recognized long ago that a standard method of representation is needed to describe carbohydrate stereochemistry. The method most commonly used employs Fischer projections for depicting chirality centers on a flat page. 

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