- The most frequent cause of chirality in organic molecules is the occurence of a tetrahedral coordinated carbon atom with four different substituents. Such a carbon is called asymmetric carbon, chirality center or stereogenic center.
2-Butanol, a chiral molecule
2-Butanol is a small molecule with a chirality center. It is the simplest alcohol containing an asymmetric carbon. For a better illustration, the four different substitutents are shown in different colours.
Molecules I and II have a mirror-image relationship but they are not superimposable. Therefore, they are enantiomers: 2-butanol is chiral. Try to orientate the models of 2-butanol with your mouse so that they are superimposable.
Carbon atom C-2 of 2-butanol carries four different substituents H, CH3, OH, and CH2CH3. Exchange of any two of these substituents would yield the opposite enantiomer. However, such an exchange does not occur spontaneously because it requires a bond breakage. Therefore, enantiomers are different, stable, coexisting compounds.
2-Propanol, an achiral molecule
C-2 of 2-propanol does not carry four different substituents. It is not a chirality center, because the methyl groups are not distinguishable. Each 2-propanol molecule is superimposable to its mirror image. Therefore, no enantiomeric 2-propanol molecules exist, molecules III and IV are identical. Try to orientate the models of 2-propanol with your mouse so that they are superimposable.