The diastereomers , also called diastereoisomers, are defined as stereoisomers that are not mirror images of each other. They are a particular type of isomers, which means that they are different compounds that share the same molecular formula.
There are different types of isomers, among which are stereoisomers. These are those in which all the atoms are joined in the same order and with the same type of bonds, but have different orientations in space (stereo means space). Within the stereoisomers, we find the diastereomers, characterized by not being mirror images of each other.
In other words, diastereomers are stereoisomers that are not enantiomers of each other.
There are different types of diastereomers including the Cis-Trans geometric isomers, the EZ, as well as some compounds with more than one chiral center.
How are diastereomers recognized?
To recognize diastereomers, we first determine whether they are Cis-Trans isomers or whether they are EZ isomers. If this happens, then they are diastereomers. Otherwise the following rules are followed:
- They must be stereoisomers, that is, they must only differ in the spatial orientation of their atoms.
- They must have two or more chiral centers.
- They must have the same configuration in at least one of their chiral centers.
- They must have the opposite configuration in at least one of their chiral centers.
Any pair of isomers that meets these characteristics will be a pair of diastereomers.
Properties of diastereomers
Their physical and chemical properties are different
Unlike enantiomers, which share the same boiling point , melting point, etc., diastereomers are quite different compounds and can have very different physical and chemical properties.
They almost always present differences in their polarity, in their vapor pressure, in their boiling and melting points, and even in their solubility in different solvents.
Differences in their physical and chemical properties make it possible to separate mixtures of diastereomers easily by means of fractional crystallization, fractional distillation, conventional chromatography, etc.
They may or may not be chiral
There can be multiple diastereomers of the same compound, and some of them can be chiral while others not, depending on whether the molecule has some kind of symmetry or not.
Having chiral centers does not ensure that the molecule is chiral. Some compounds with chiral centers can be symmetrical, so they will be overlapping with their mirror image and therefore not chiral. In these cases, they are called a meso compound .
Two diastereomers of tartaric acid are shown in the following figure. Diastereomer A is not chiral, as it is a symmetric molecule (the top half is a reflection of the bottom half). However, diastereomer B is chiral.
In the figure above, isomer A is an example of a meso compound.
They can be Cis-Trans and EZ isomers
The Cis and Trans isomers, as well as the EZ isomers of an alkene or a substituted ring, are stereoisomers that are not mirror images of each other, so they are also diastereomers.
Those diastereomers that possess chiral centers but are not meso compounds will be chiral compounds and therefore will be optically active. This means that they are able to rotate the plane of polarized light.
Nomenclature of diastereomers
The nomenclature of diastereomers that possess chiral centers (those that are not Cis-Trans or EZ), is based on the systematic IUPAC name of the compound preceded by the absolute configuration of each chiral center in the order in which they appear in the main chain.
There are several methods to identify one or another diastereomer, but the most widely used is the Cahn-Ingold-Prelog (CIP) system. This consists of the following steps:
- A level of hierarchy is assigned to the four groups attached to each chiral center. The priority of the groups is assigned according to the atomic number of the atom directly linked to the chiral center. If there are two equal atoms, we proceed to add the atomic numbers of the atoms that are linked to the first.
- The direction in which the three priority groups travel is determined when the lowest priority is pointing backwards.
- If the direction of travel is clockwise, it is assigned the R setting, otherwise it is assigned the S setting.
Examples of common diastereomers
Examples of diastereomeric carbohydrates
Diastereomers are very common in nature, especially in compounds of biological importance. The following examples are the different diastereomers of three major groups of sugars:
Ribose is one of the most important carbohydrates in nature, since it is part of both DNA and RNA that make up our genetic material. This carbohydrate has three chiral centers that give rise to eight different isomers. Here are the 4 most important diastereomers of this sugar:
As its name implies, fructose is the sugar found in fruits, and it has three chiral centers that give rise to the following diastereomers:
Glucose is the most common carbohydrate in existence and is one of the most abundant biological molecules on earth. There are 16 isomers in total, of which only 8 are present in most living things. The other 8 are only present in some cells. Below are the 8 most common diastereomers:
We can notice that when comparing any pair of diastereomers, they differ in the configuration of at least one chiral carbon, and share the configuration of at least one chiral carbon.
Example of Cis-Trans diastereomers
Example of EZ diastereomers
In this example, the numbers shown correspond to the priority of the substituents following the rules of the CIP system.