Unusual reactivity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with Fe³⁺ controlled by competing reactions

2,2-Diphenyl-1-picrylhydrazyl (DPPH) is a stable organic free radical widely used in various fields as a model free radical. There is scarce information about the stability of this compound in the chemical environments in which it is used. Side reactions between DPPH and other species can alter the precision of experiments that use DPPH, such as the evaluation of antioxidant properties amongst others. Following recent investigations highlighting reactions between DPPH and metal cations or Lewis acids, a quantitative reaction between DPPH and Fe³⁺ in acetonitrile was studied in the present work. UV-Vis spectroscopy and electrochemistry were used to monitor the reaction. The results obtained indicate a fast and multistep reaction between Fe³⁺ and DPPH that can be simplified as a simple redox reaction with the formation of Fe²⁺ and DPPH⁺. The reaction mechanism proposed follows complex steps involving two competing phenomena: a disproportionation which accelerates the reaction and an oxidation process that slows it down through DPPH regeneration.