A study of the mechanism of the oxidative cyclization of benzaldehyde semicarbazones induced by cupric perchlorate in acetonitrile

Treatment of benzaldehyde semicarbazones 1a‐iwith cupric perchlorate in acetonitrile at 40 provided selectively the corresponding 1,2,4‐triazolin‐5‐ones 2a‐i. The relative rate constants for 2a‐iformation were determined by the competitive method. The results obtained showed that electron‐donating substituents (methyl and methoxy) increase the reaction rate, while the reverse was found for electron‐withdrawing substituents (chloro and nitro group). The reactivity data are discussed on the grounds of two possible mechanisms.