Main Difference – Diastereomers vs Enantiomers
In chemistry, both diastereomers and enantiomers are forms of stereoisomers. Stereoisomers are a type of isomers. Isomers can be of two types: structural isomers and stereoisomers. Structural isomers share the same molecular formula, but there is a difference in their bond connection or order. In contrast, stereoisomers have the same molecular formula and the bond constitution but, the three-dimensional orientations of their atoms in space differ. Stereoisomers can be further divided into diastereomers and enantiomers. The main difference between diastereomers and enantiomers is that enantiomers are mirror images of each other whereas diastereomers are not mirror images of each other.
What are Enantiomers
These are stereoisomers which are mirror images of each other. That is, when the two molecules are taken separately, they are non-superimposable. This is due to the presence of a stereo centre. For a Carbon atom to become a stereo centre, it has to have four different groups attached to it. Therefore, due to the characteristics of its connectivity, these molecules are able to form mirror images of each other. These Carbon atoms are also called ‘chiral Carbons’ and a molecule to be possessing these chiral Carbons is known as its ‘chirality’. Furthermore, these chiral centres are said to have optical activity. (This is explained below).
Provided that the environment is symmetric, enantiomers have identical chemical and physical properties, but they react differently to plane polarized light. These are light waves where the vibrations occur in a single direction/plane. Un-polarized light needs to be converted to polarized light through a process called ‘polarization’. Each enantiomer rotates plane-polarized light in a different direction (one to left and other to right). However, the amount of rotation is the same as long as the concentration of each enantiomer remains the same. Therefore, enantiomers can be distinguished from each other depending on the way it reacts with plane polarized light. A mixture that contains each of the enantiomer molecules in similar concentration is called a ‘racemic mixture’. These mixtures are not optically active, as the effect from each enantiomer cancels each other’s effect. Usually, in biological systems, one enantiomer is deemed active and the other inactive. It is the active form which is recognized by receptor cells and taken up for reactions.
What are Diastereomers
As mentioned above, Diastereomers are also a type of stereoisomers. However, they are not mirror images of each other. And unlike enantiomers, they have two stereocentres. In the case of enantiomers, the each stereo centre needs to be in the opposite configuration (denoted by the R and S configuration), in order to form the mirror images. However, when it comes to Diastereomers, one stereo centre behaves like in the case for the enantiomers, and the other contains the same configuration. Therefore, even though they possess stereo centres, they do not form mirror images of each other.
‘Meso compounds’ are a special set of Diastereomers that contains a mirror plane within the molecule. This is a result of the molecule being perfectly symmetric. Diastereomers have different physical properties and differ in their reactivity.
Difference Between Diastereomers and Enantiomers
Enantiomers are stereoisomers that are mirror images of each other.
Diastereomers are stereoisomers that are not mirror images of each other.
Number of Stereocentres
Enantiomers can have one or more stereocenters.
Diastereomers usually have two stereocentres.
Enantiomers have identical physical properties, except for the rotation of plane polarized light.
Diastereomers have different physical properties.
Enantiomers have identical chemical properties.
Diastereomers differ in their reactivity.
R and S configuration
Enantiomers have opposite R and S configurations on each of their stereo centres.
Diastereomers have opposite R and S configuration at one stereo centre and the same configuration at the other stereo centre.
“Thalidomide-enantiomers” by Klaus Hoffmeier – Own work. (Public Domain) via Wikimedia Commons