A Hierarchical Classification Scheme for Chemical Reactions

Several known approaches to classification of structural changes in the course of chemical reactions are briefly discussed. The hierarchical five-level classification scheme is suggested for organic interconversions in which the bond redistributions (summarizing all required steps of the reaction process) are exactly known. The main hierarchy levels, i.e., reaction category, reaction class, interconversion type, topological type, and reaction type and also corresponding sublevels are illustrated by numerous examples. The first two levels together with reaction subcategories and subclasses relate to the nature of educt and product species (neutral or dipolar molecules, ions, radicals, etc.) and to contours and graphs formed by bonds which change their multiplicity during the reaction. The interconversion, topological, and reaction types are uniquely characterized by classification, symbolic, and skeletal equations, respectively; the subtypes are also associated with specific kinds of equations. The central hierarchy attribute, i.e., classification equation, is shown to closely correspond to what organic chemists typically use in order to systematize simple processes with cyclic and linear electron transfers. Finally, the application of the suggested classification scheme to construction of hierarchically organized codes for organic reactions is finally demonstrated in the paper.