Additional Chirality Elements
Determination of Absolute Configuration of Molecules with a Chirality Plane
In order to determine the absolute configuration of a molecule with a chirality plane, the atom of highest priority (according to the CIP rules) that is directly bound to the chirality plane, but not a member of it, must first be found. This atom is then known as the pilot atom (P). In the ansa compound 13-bromo-1,10-dioxaparacyclophane, the pilot atom is the oxygen-bound methylene carbon atom that is located right next to the bromine atom in the aromatic ring. In 13-bromo-1,10-dioxaparacyclophane, the chirality plane is spread out by the aromatic ring, the bromine atom, and the oxygen atoms. Aside from the pilot atom, the next three consecutive atoms of the chirality plane are labelled a, b, and c. If there is a branching, the atom of highest CIP priority is selected in each case. In 13-bromo-1,10-dioxaparacyclophane therefore, the aromatic carbon atom that carries the bromine is selected and labelled c. Last, but not least, the absolute configuration is called (pS) if the atom sequence P - a - b - c is arranged counterclockwise, while it is classified as (pR) if the atom sequence is arranged clockwise.
In the case of (E)-cyclooctene, a pilot atom cannot be chosen according to the CIP rules, as the two carbon atoms in question have the same CIP priority. Nevertheless, it does not matter which one of them functions as the pilot atom because, in each case, the absolute configuration is the same.