Additional Chirality Elements
Heteroatoms as Chirality Centers
- Each atom that carries four different substituents in a tetrahedral arrangement is a chirality center.
Silicon and germanium are, like carbon, elements of the fourth main group and appear as chirality centers in chiral compounds. Nitrogen, as an element of the fifth main group, possesses a special characteristic. In amines, it carries three substituents and a lone electron pair. Including the lone electron pair, it is tetrahedrally surrounded by four "substituents" and is sp3-hybridized. In amines, nitrogen could therefore appear as a chirality center. However, at room temperature no enantiomers are observed. The inversion of the absolute configuration of amines requires only about twice of the energy which is required for a carbon-carbon single bond rotation. This is because, in the transition state, only three substituents are arranged coplanar around the nitrogen, while the lone electron pair occupies a p orbital that is perpendicular to the substituents plane. Therefore, the inversion of the absolute configuration of amines, which is called amine inversion, occurs rapidly at room temperature and even below. As a result, pure enantiomers cannot be isolated. Quarternary ammonium ions that have four different substituents are chiral and can be, under certain circumstances, configurationally stable enough to be resolved into their enantiomers. Elements of the third period invert more slowly. Therefore, several chiral phosphorus and sulfur compounds such as phosphines, sulfoxides, sulfinic esters, sulfonium salts, and sulfites have been resolved into their enantiomers.
- Chirality is not really a special characteristic of carbon compounds. Rather, it depends on the spatial shape of molecules.