Radical-induced oxidation of pyridazine-3,6-diol (maleic hydrazide). A model for chemiluminescent cyclic hydrazides
Oxidative radicals such as N˙–3, CO˙–3, (SCN)˙–2 and Br˙–2 oxidize the basic form of maleic hydrazide (MH–) to the corresponding diazasemiquinone radical. The acid (MH2) is resistant towards all oxidants tested. The diazasemiquinone radical has a pKa of 6.3 ± 0.1. Neither MH˙ nor M˙– react with molecular oxygen. Dismutation of M˙– results in the formation of the diazaquinone (M), which is extremely reactive towards nucleophiles. In particular it forms maleic acid with HO–2. The radical M˙– combines with O˙–2 to yield maleic acid as well. Regeneration of maleic hydrazide in the reaction of M˙– with O˙–2 below pH 7.5 indicates pH-dependent reactivity of the intermediate hydroperoxide.