Mass spectral and pyrolytic behavior of the two main products of phenylbutazone degradation: simulation of unusual mass spectral fragmentation.

The two major routes of degradation of phenylbutazone (I) are oxidation and hydrolysis. Hydrolysis gives rise to n-butylmalonic acid mono(N,N'-diphenyl)hydrazide (II). Oxidation at the C-4 position yields 1,2-diphenyl-4-n-butyl-4-hydroxpyrazolidine-3,5-dione (III), which is readily hydrolyzed to n-butyltartronic acid mono-(N,N'-diphenyl)hydrazide (IV). Whereas the mass spectra of I, III, and the methyl esters of carboxylic acids II and IV all demonstrate major peaks corresponding to their respective molecular ions, the mass spectrum of II is essentially identical with that of I, suggesting facile dehydration. In the mass spectrum of IV, the peak of highest mass is found at m/e 205; no peak could be perceived corresponding to the molecular ion or to loss of water, carbon dioxide, or both of these elements from the molecular ion. Under normal pyrolytic conditions, II is decarboxylated to N-caproylhydrazobenzene (V) and IV is readily dehydrated to yield III. The mass spectral fragmentation of IV was successfully simulated in the laboratory to give an excellent yield of aniline and alpha-keto-N-caproylaniline (VI) (mol. wt. 205). The probable course of this unusual transformation was elucidated from studies of the accelerated decomposition of IV and derivatives considered as possible intermediates in the degradation process.