Figure 1. Left: This molecule has a stereocenter. The carbon indicated by the arrow has four different attachments: CH3, CH3CH2CH2, (CH3)2CH and Cl. Right: This molecule does not have a stereocenter. The carbon indicated by the arrow has three different attachments: CH3, CH3CH2CH2 (two different conformations but still the same group) and Cl.Most instances of chiral molecules that you will encounter at this level of organic chemistry owe their chirality to the presence of one or more stereocenters. It is not necessary for a molecule to have a stereocenter to be chiral. Make models of the enantiomers of 2,3-pentadiene (1,3-dimethylallene) shown below. Having a stereocenter does not mean a molecule will always be chiral. Explore this point with a molecular model of meso-tartaric acid, a compound critical to Pasteur's studies of optical activity in organic compounds.
In the next section we explore the importance of recognizing chirality in molecular structure. Because stereocenters are the origin of chirality of most chiral organic molecules, it is useful to be able to recognize stereocenters within a molecule.
Example 1: In each molecule shown below, identify the stereocenter(s).
Solution 1:
a. When looking for stereocenters, pay careful attention to the "four different attachments" criterion. Recall that hydrogens of "stick structures" are usually not drawn. Also note attachments that differ only in conformation do not make a stereocenter. The only carbon of 2-butanol that bears 4 different attachments is the alcohol carbon.
b. 2-Methyl-3-pentanol has only one stereocenter: the alcohol carbon. Don't neglect the hydrogen on this carbon just because it isn't shown. The other carbon bearing wedge and broken line bonds is not a stereocenter, because it has two of the same thing (methyl groups) attached. We often use the wedge and broken line notation to imply a three dimensional arrangement of atoms, but its does not always occur on a stereocenter.
Exercises: Locate all stereocenters in each structure.
