Diastereomers are stereoisomers that are not mirror images of one another and are non-superimposable on one another. Stereoisomers with two or more stereocenters can be diastereomers. It is sometimes difficult to determine whether or not two molecules are diastereomers. For introductory purposes, simple molecules will be used as examples. More complex examples will be given later.
For example, consider the following molecules.
These molecules are not mirror images
of one another. Additionally, these molecules are non-superimposable
because if one of these molecules is flipped 180 degrees (so that the alcohols and methyls are aligned, as shown below), the stereochemistry is different at one carbon (the alcohols) and the same at another carbon (the methyls). Therefore, these molecules are diastereomers