Radical-induced oxidation of pyridazine-3,6-diol (maleic hydrazide). A model for chemiluminescent cyclic hydrazides

Abstract

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 pK_a 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.