The topic of this lecture will be diastereomers. Diastereomers are stereoisomers that are not mirror images. This is a very broad definition.
You'll notice, because of the broadness of this definition, it includes a type of isomer we've already talked about, geometric isomers. Geometric isomers, or cis-trans isomers, exist because of the lack of rotation around a double bond or ring. The definition also includes molecules that have more than one chiral center, but are not enantiomers.
mirror image isomers. Let's look at some examples. These two molecules are geometric or cistrans isomers. Because they are stereoisomers that are not mirror image isomers, they are also diastereomers. These two molecules are also geometric or cistrans isomers, and because they are non-mirror image stereoisomers, they are also diastereomers.
However, the definition of diastereomers also includes molecules we haven't talked about yet. such as these two molecules. These two molecules are related.
They are stereoisomers of one another, but they are not mirror image stereoisomers. Therefore, they are diastereomers. Remember that enantiomers are mirror image stereoisomers.
This molecule is the mirror image isomer of the structure on the top left. Therefore, these two are enantiomers. The relationship of each of the enantiomers to the third molecule is diastereomers.
Again, because they are stereoisomers that are not mirror images. This is an important distinction. Remember that enantiomers have identical physical properties, except for the direction in which they rotate plane polarized light.
This is not true for diastereomers. Diastereomers have different physical properties. This is important because it means that we can separate two diastereomers using methods that we use for separating non-isomeric molecules, such as we've used in lab.
For example, we can use recrystallization, distillation, and chromatography. Separating enantiomers is much more difficult and will be a topic that we discuss later. Now let's briefly review the types of isomers we have looked at so far.
Remember that isomers are structures that have the same molecular formula but are different compounds. There are two main types of isomers. There are constitutional, or structural, isomers, which differ in the order of attachment of atoms in the molecule, and there are stereoisomers, which have the same order of attachment of atoms, but have a different organization of those atoms in three-dimensional space.
There are two main types of stereoisomers. enantiomers, which are mirror image isomers, and diastereomers, which are non-mirror image isomers. Diastereomers can be broken up further into two types. These include geometric or cis-trans isomers, which are stereoisomers that are dependent on having a ring or a double bond and the lack of rotation around that ring or double bond, and then other diastereomers, which are diastereomers that have more than one chiral center.
Remember to be as specific as possible in your definition when you're trying to determine the relationships between two molecules.