Enantiomers vs. Diastereomers

An isomer is a molecule with the same molecular formula as another molecule, but with a different chemical structure. Isomers contain the same number of atoms of each element but have different arrangements of their atoms. Isomers do not necessarily share similar properties unless they also have the same functional groups. There are two main forms of isomers: constitutional (structural) and stereoisomer (spatial). The constitutional isomers have the same gross formula, but a different binding sequence. And stereoisomers have the same structure but a different arrangement (three-dimensional orientations) of atoms or atomic groups. They have the different configuration of the molecule. The constitutional and stereoisomers are in fact the opposite. A special subtype of chemistry studies this type of isomer of the so-called stereochemistry.

Stereoisomers can be further divided into several subcategories, but we will describe only enantiomers and diastereoisomers in this post.


The two molecules which related as object and mirror image is called enantiomers. Each isomer of the image – mirror image pair is called an enantiomer. We can also say that these two molecules are enantiomers to each other. A molecule that is not superimposable on its mirror image is said to be chiral. A chiral center is a carbon atom bonded to the four different substituents. In contrast with chiral molecules, compounds having structures that are superimposable on their mirror images are achiral. This means that the achiral center has at least two identical substituents. Here are some examples:

In the first example, we can see that four different atoms are attached to the carbon atom. Those are bromine, chlorine, fluorine, and hydrogen. The chiral center is marked by a star (*), which is the usual way of marking it. When we would put the mirror next to the molecule on the left, we would get the molecule on the right. Then, if we would rotate this molecule by 180 degrees, we wouldn’t get the same molecule as on the left. Hydrogen and fluorine will be in the same position but chlorine and bromine on the opposite. And because of that, we get non-superimposable structures. While in the other example, we have two chlorine atoms attached to the carbon. When we rotate the mirror image, we get the same molecule as on the left! This means that this molecule has superimposable structure.

These two enantiomers can be distinguished by their names by looking at their R,S designations. These designations are related to the chiral atoms. Each chiral atom is designated by R or S which depends on whether priorities of the substituents go in the clockwise (R) direction or the counterclockwise
(S). The priority of the substituent is determined by atomic numbers. Since it starts with the largest number, which means that the hydrogen will always be the number four (or in the last place). Number four is always in the back! This means that hydrogen (if it’s fourth) is on a DASH!

And it looks like this:

The name of this compound is S-bromochloroethane.

But if look at our first example, hydrogen is not on a dash! It’