Autoxidation of hydrazones is a generally occurring reaction, leading mostly to the formation of-azohydroperoxides. All structural kinds of hydrazones, having at least one hydrogen atom on nitrogen,are prone to autoxidation; however, there are marked differences in the rate of the reaction. Hydrazonesof aliphatic ketones are 1-2 orders of magnitude more reactive than analogous derivatives of aromaticketones. Even less reactive are the hydrazones of chalcones, which function also as efficient inhibitorsof autoxidation of other hydrazones. These differences can be attributed to the reduction of the rate ofthe addition of oxygen to a hydrazonyl radical, which is a reversible reaction. In the case of con
jugatedketones, it becomes endothermic, making this elementary step slow down and the chain terminationreactions become important. Substituents influence the stability of hydrazonyl radicals and, consequently,the bond dissociation energies of the N-H bonds. In acetophenone phenylhydrazones, the substituentsplaced on the ring of hydrazine moiety exhibit a higher effect (Hammett = -2.8) than those on theketone moiety ( = -0.82), which denotes higher importance of the structure with spin density concentratedon nitrogen in delocalized hydrazonyl radical. Electronic effects of the substituents also affect the transitionstate for the abstraction of hydrogen atom by electrophilic peroxy radicals; NBO analysis display a negativecharge transfer of about 0.4 eu from hydrazone to a peroxy radical in the transition state.
