Stereochemical vocabulary for structures that are chiral but not asymmetric: History, analysis, and proposal for a...

Asymmetric objects are necessarily chiral, but a structure may be chiral and not asymmetric if it possesses one or more proper rotation axes. Chiral but not asymmetric molecules are important in chemistry and its applications, but no suitable term exists for the designation of such structures, and their terminology in the literature is confused and chaotic. Dissymmetric has been redefined by some authors as "chiral but not asymmetric," in conflict both with Pasteur's definition of the term as "not superposable on its mirror image" (without other restrictions, i.e., chiral) and the understanding of the term in stereochemistry. Moreover, dissymmetric and asymmetric are frequently confused because of their similar forms. Furthermore, dissymmetric is widely used in many other definitions in chemistry, physics, and other disciplines. Thus, dissymmetric is unsuitable in the new definition of "chiral but not asymmetric," and a new term is needed. The adjective "symmanumorphous" is therefore proposed for "chiral but not asymmetric". "Sym" (from symmetry) indicates the presence of some symmetry in the structure, and "manu" (from "manus," Latin for hand, e.g., manual, manuscript) refers to its handedness. "Morphous," from the Greek "morphē," that is, form, is widely used, for example, anthropomorphous, enantiomorphous, etc. Symmanumorphous is convenient and euphonious and at 15 characters (same as enantiomorphous) is not unduly long. The nouns "a symmanumorph" (a structure that is chiral but not asymmetric) and "symmanumorphism" (the phenomenon of chirality without asymmetry) are also proposed. The new terminology is adaptable in other languages and would contribute to creating order out of linguistic chaos.