22.2 Handedness of Carbohydrates
• Because carbohydrates contain carbon atoms bonded to four
different groups, they are chiral (that is, not superimposible
on their mirror images).
• Glyceraldehyde, the three-carbon monosaccharide that is the
simplest naturally occurring carbohydrate, has four different
groups bonded to the number 2 carbon atom making it a
chiral molecule.
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Chiral compounds like glyceraldehyde lack a plane of
symmetry and exist as a pair of non -superimposable mirror
images, enantiomers, a “right-handed” D form and a “lefthanded” L form.
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Solutions of chiral chemical compounds change the plane in
which the light is polarized. Each enantiomer of a pair rotates
the plane of the light by the same amount, but the directions
of rotation are opposite. If one enantiomer rotates the plane of
the light to the left, the other rotates it to the right.
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22.3 The D and L Families of Sugars:
Drawing Sugar Molecules
• A standard method of representation called a Fischer
projection has been adopted for drawing stereoisomers on a
flat page so that we can tell one from another.
• Bonds that point up and out of the page are shown as
horizontal lines, and bonds that point behind the page are
shown as vertical lines.
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• In a Fischer projection, the aldehyde or ketone carbonyl group
of a monosaccharide is always placed at the top.
• The result is that -H and -OH groups projecting above the page
are on the left and right of the chiral carbons, and groups
projecting behind the page are above and below the chiral
carbons.
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Tetrahedral C atoms
can be drawn with
wedges and dashes
or as Fischer
projections.
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Look at the structural formulas of the D (-OH
to the right) and L (-OH to the left) forms of
glyceraldehyde.
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Each pair of enantiomers has a different name.
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