Two or more compounds with the same number and kinds of atoms are called isomers. They have the same molecular formula. Structural isomers are isomers that differ in the orders in which carbon or other atoms are attached to one another. Stereoisomers are isomers with the same structural formula, which means they have the same atoms attached to each other. In a pair of stereoisomers the bonding patterns are the same. They differ only in their configuration in space. There are two types of stereoisomers, optical isomers and geometric isomers. Optical isomers rotate plain polarized light. They are optically active. Remember that only chiral molecules are optically active. Any carbon atom with four different substituents bonded to it, is called a chiral center, and the molecule to which it belongs is called a chiral molecule. Every chiral molecule has an enantiomer and enantiomer is a mirror image of the chiral molecule. Remember, however, that enantiomers are not identical because they are not super imposable on each other. Chiral molecules do not have enantiomers.

Let us review what we just learned:

1. A chiral molecule is a molecule with the chiral center and the chiral center is a carbon with four different things attached to it.
   
2. If a molecule is chiral, it is not super imposable on its mirror image, which means the molecule and its mirror image molecule are not the same.
   
3. The chiral molecule and its mirror image are called enantiomers.
   
4. An a chiral molecule is super imposable on its mirror image molecule, which means the molecule and its mirror image are, in fact, identical.

Chiral molecules do not give rise to enantiomers. Remember that every chiral molecule and its enantiomer are optically active. Polarized light can be rotated to the right or to the left. If a particular chiral molecule rotates polarized light to the right, then its enantiomer rotates the plain to the left. Similarly, if a given chiral molecule rotates the plain of polarized light to the left, it has an L-configuration and its enantiomer, which rotates the plain of polarized light to the right, has a D-configuration.

In order to figure out the configuration of enantiomers look at one enantiomer and examine the chiral carbon and its four different substituents. Look at the flat atom on each substituent and find the one that has the lowest atomic weight of the four, then when you find it, rotate it to the back. Look at the remaining three atoms and assign each one a priority number based on its atomic weight.

Assign priority one to the substituent whose first atom has the highest atomic weight. Assign priority two to the substituent whose first atom has the second highest atomic weight. Assign priority three to the substituent whose first atom has the lowest atomic weight. With your finger point to the substituents in numerical order one, two, three. If your finger is moving clockwise you are looking at the R enantiomer. If your finger is moving counterclockwise you are looking at the S enantiomer. Remember that absolute configuration does not tell you anything about the direction in which a particular enantiomer rotates the plain of polarized light.

A chiral molecule may have more than just one chiral center. When a pair of chiral molecules is not mirror images of each other, they are no longer enantiomers. They are called diastereomers. Diastereomers have different physical and chemical properties and can be separated by methods such as crystallization, chromatography and distillation. When equal amounts of enantiomers and diastereomers are mixed together, the resultant mixture is called a racemic mixture and has no optical activity. The opposite rotational effects of the enantiomers cancel each other out. Therefore, the net effect is zero optical rotation.

Geometrical isomers differ in their orientation about a double bond or ring. Geometric isomers are not optically active. An example of this is cis and trans isomers. Hydrocarbons are molecules that are made of carbon and hydrogen only.