Organic Chemistry

38 Diastereomers with stereocenters

In simple terms, two stereoisomers are diastereoisomers of each other ifonly onechiral
center differs between the two stereoisomers. That is to say, if both molecules contain two
or more chiral centers, but if only one of the chiral centers in each molecule is different than
the other, then the two molecules are diastereoisomers of one another.

If a molecule contains a single asymmetric carbon atom orstereocenter, it will have two
mirror image forms. If a molecule contains two asymmetric carbons, there are four possible
configurations, and it would be mathematically and physically impossible for all four to be
mirror images of each other. The more chiral centers in a molecule, the more possibilities
there are for different conformers, and therefore the more possible diastereomers exist.

As an example, tartaric acid contains two asymmetric centers, but two of the configura-
tions of the tartaric acid molecule are equivalent to one another -- and together they are
called meso compounds. This configuration is not optically active, while the remaining two
configurations areD- andL- mirror images. For this reason, the meso form of tartaric acid
is a diastereomer of the other forms.

Figure 105 Figure 106 Figure 107

(natural) tartaric acid

Ldextrotartaric acid-(+)-tartaric acid

Dlevotartaric acid-(-)-tartaric acid mesotartaric acid

(1:1)

DL”racemic acid”-tartaric acid

The meso form of tartaric acid (right) is a diastereomer of the other forms.

Two common prefixes used to distinguish diastereomers arethreoanderythro. When
drawn in the Fischer projection theerythroisomer has two identical substituents on the
same side and thethreoisomer has them on opposite sites.

38.1 Carbohydrates

The families of 5 and 6 carbon carbohydrates contain many diastereomers because of the
large numbers of asymmetric centers in these molecules. Since each carbon in the primary
chain of an aldose (one type of carbohydrate) and all but one of the carbons in in the primary
chain of a ketose (another type of carbohydrate) have both a hydrogen and a hydroxyl group
attached, most of the carbons in any given sugar are actually chiral. Since the number of
possible conformers for a chiral molecule is 2 raised to thenpower (2n), wherenis the
number of chiral centers, this makes for a great deal of variability in carbohydrates and a
large number of diastereomers.