Different types of Stereoisomers

Let’s begin with isomers. Isomers are different compounds with same molecular formula.  There are three major classes of isomers, constitutional, conformational isomers and stereoisomers. Constitutional isomers are generated when bond connectivity is different between atoms. Their functional groups could be same or different.

But Stereoisomers differ only in three dimensional arrangements of atoms. Therefore, they always have same functional groups.

The concept of mirror image is the key to the understanding of stereoisomerism. All objects including all molecules have mirror images. A mirror image is the reflection of the molecule in a mirror. Objects can be divided into two classes based on their mirror images.

Case 1) One example of stereoisomers- isomers with double bonds.

In the above example two molecules cis-2-butene and trans-2-butene are not mirror image of each other. They are called geometrical isomers or diastereisomers.

Case 2): If a molecule and its mirror images are superimposable then the molecule doesn’t have any optical isomerism, it will exhibit other kind of isomerism like conformational isomers. A dinner plate with no design would be an example of superimposable mirror images.

In the above example, mirror images are superimposable. They are identical and form a pair of achiral compounds also called “meso” compound.

Case 3) Nonsuperimposable mirror images are images where not all the parts of the molecules coincide when the molecules and its mirror images are placed on each other. Enantiomers are formed when a molecule and its mirror image are non superimposable on each other. Since we are talking about mirror image and superimposable, let’s look at the concept in detail.

Example#2: A pair of enantiomers of lactic acid

Remember

Enantiomers: stereoisomers that are non superposable mirror images.

Diastereomers: stereoisomers that are not mirror images.

Example: Tartaric Acid

The following configurations make a pair of enantiomers. A mixture of equal amounts of two enantiomers  is called racemic mixture.

There are a pair of diastereisomers. They are also called mesomeric structures.

Here are all four stereoisomers of tartaric acid.

Drawing Mirror images:

Following molecule has one stereoisomer marked by an asterisk. Draw three-dimensional representations for the enantiomers of each molecule:

CH3CH(Cl)CH2CH3
*

Strategy:

First, draw the carbon stereocenter showing the tetrahedral orientation of its four bonds. One way to do this is to draw two bonds in the plane of the paper, a third bond toward you  in front of the plane and the fourth bond away from you behind the plane. Next, place the four groups bonded to stereoisomer on these positions. This completes the stereo drawing of one enantiomer. To draw the other enantiomer, interchange any two of the groups on the original stereo drawing.

PRACTICE PROBLEM:

Which of the following molecules has a non superimposable mirror image?

Fischer projection: Chirality centers in organic compounds are often drawn with a different convention called Fischer projection. In Fisher projection, instead of drawing of a tetrahedron with two bonds in a plane, and one behind it, the tetrahedral structure is tipped so that both horizontal bonds come forward and vertical bonds go behind.  A Fischer projection is a two-dimensional structural notation for showing the spatial arrangement of groups about chiral centers in a molecule.

Hermann Emil Fischer

The  molecule below when projected on 2-dimensional Fischer projection it looks like below:

Here is a pair of enantiomers using Fischer projection.

Compound with two or more Chirality center:

When we have multiple carbon atoms with chiral center then it is converted in the following way.

Here are the Fischer projection formulas of all four forms of tartaric acids.

PRACTICE PROBLEMS:

Draw the Fischer Projection formula for the following compounds: