Diastereomers are the type of stereoisomers which bears two or more chirality centers, it may not be a mirror image of its isomer. Such isomers that are not the mirror images of each other and hence chiral centers are called diastereomers.
Example: 3-bromo-2-butanol forms four isomers. Structure I & II and III & IV are enantiomers whereas Structure I & III, I & IV and II & III, II & IV are diastereomers.
The number of diastereomers increases as the number of chiral centers increases, thus a compound containing two chirality canters will have four possible configurations.
X = 2n
Where X is the number of possible isomers and n is the number of chiral centers.
For example: 2,3-dichlorobutane contain two chiral centers.
So here n = 2 because it contains two chiral centers.
X = 22
X = 4
Note: One less isomer forms in meso compounds. If meso compound contain two chiral centers then there will be formation of three isomers instead of four.
For example: Meso-tartaric acid
Physical Properties of Diastereomers
Diastereomers have different physical properties. The two diastereomers of 2,3-dibromosuccinic acid have melting points that differ by nearly 100 °C.
Most of the common sugars are diastereomers of glucose. All these diastereomers have different physical properties.
For example: Glucose and Galactose are diastereomeric sugars that doffer only in the stereochemistry of one chiral carbon atom C4.
Diastereomers of Glucose
The glucose has six carbon which has four chiral centers and 16 diastereomers, eight out of which are shown by the Fischer projection follows with their common names. The remaining eight isomers of this series are known as L-forms. For e.g. L-Glucose.
Meso compounds that are achiral even though they have chiral carbon atoms. Half the molecule is the mirror image of the other half. They have internal mirror plane of symmetry and superimposable to each other.
They are optically inactive because one chiral centre will rotate the plane polarized light in one direction and other chiral centre will cancel that rotation by rotating the plane polarized light by the same magnitude but opposite direction. Thus, the resultant rotation is zero and meso compounds do not rotate the plane polarized light.
D- and L- Configuration
D- or L- assigned according to the stereochemistry of the highest numbered chiral carbon in the Fischer projection.
If the hydroxyl group is pointing to the right in the Fischer projection, the sugar is designated as D.
If the hydroxyl group is pointing to left in the Fischer projection, the sugar is designated as L.
Most naturally occurring carbohydrates are of the D-configuration.