Stereogenic Centers and Axes: A Comparison of the Chiral Topologies Available to Cabcd and abC=C=Ccd

In this article we provide a comparative analysis of molecular chirality as a consequence of the presence of stereogenic centers and axes. We have found this to be a useful and informative classroom exercise that helps to put some of the most important stereochemical principles on a firmer footing. Chirality in molecules incorporating a stereogenic center can be fully explained using the regular tetrahedron as a model. Analysis of chirality in molecules incorporating a stereogenic axis, on the other hand, requires the use of a less regular, “stretched” or “extended”, tetrahedron. The key difference between the two types of molecule is the number of chiral topologies available to each. Molecules incorporating a stereogenic center have a single chiral topology in which all four substituents are necessarily different. Molecules incorporating a stereogenic axis, however, have three different chiral topologies available to them. In one of these all substituents are different but in the others only three or even only two unique substituents are required. It is these latter topologies that usually give students most difficulty.