Use and Abuse of the DPPH• Radical

The 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radical is approaching 100 years from its discovery in 1922 by Goldschmidt and Renn. This radical is colored and remarkably stable, two properties that have made it one of the most popular radicals in a wide range of studies. First, there is the evaluation of the antioxidant abilities of phenols and other natural compounds (A–H) through a "test" that—at a closer look—is utterly inappropriate. In fact, the test-derived EC50, that is, the concentration of A–H able to scavenge 50% of the initial DPPH•, is not a kinetic parameter and hence its purported correlation with the antioxidant properties of chemicals is not justified. Kinetic measurements, such as the second-order rate constants for H-atom abstraction from A–H by DPPH•, in apolar media, are the only useful parameters to predict the antioxidant ability of A–H. Other applications of DPPH• include kinetic and mechanistic studies, kinetic solvent effects, EPR spectroscopy, polymer chemistry, and many more. In this review these applications are evaluated in detail by showing the usefulness of some and the uselessness of others. The chemistry of DPPH• is also briefly reviewed.