Reaction of 2,2′-Diphenyl-l-picrylhydrazyl with Hydroperoxides

Abstract

IT has been reported that the stable free-radical 2,2′-diphenyl-1-picrylhydrazyl (DPPH) is entirely unsatisfactory for most rate measurements of free-radical production1,2. These findings have been confirmed and extended during our investigations on the use of DPPH for the quantitative determination of hydroperoxides of unsaturated fatty acids3. We have found that the rate of the reaction of DPPH with different hydroperoxides depends on the concentration of both reactants. Furthermore, we have discovered that while the rate of the heat-induced decomposition of, for example, methyl oleate hydroperoxides is 0.01 per cent/h, at 70° C, the rate increases a thousandfold in the presence of DPPH4. It was, therefore, concluded that the function of DPPH is not that of a reliable ·OH counting device, which may reflect the primary O-O bond cleavage of long-chain, unsaturated fatty hydroperoxides. For the same reasons, it was thought that the actual chemical reaction between DPPH and hydroperoxides should be different from that previously reported5–8.