chemical reaction (molecular) chemical reaction (molecular) reaction Chemical reactions may be elementary reactions or stepwise reactions. (It should be noted that this definition includes experimentally observable interconversions of conformers.) Detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events'). [IUPAC] chemical reaction A chemical reaction is a process that results in the interconversion of chemical species. [IUPAC] 2020-12-11 Changed labels. [Allotrope] process