A deep insight into the mechanism of the acid-catalyzed rearrangement of the Z-phenylhydrazone of 5-amino-3-benzoyl- 1,2,4- oxadiazole in a non-polar solvent

The conversion of the Z-phenylhydrazone of 5-amino-3-benzoyl-1,2,4- oxadiazole (1a) into the relevant 1,2,3-triazole (2a) has been quantitatively studied in toluene in the presence of several halogenoacetic acids (HAAs, 3a-h). Again, the occurrence of two reaction pathways has been pointed out: they require one or two moles of acid, respectively, thus repeating the situation previously observed in the presence of trichloroacetic acid. The observed rate constant ratios (kIII/kII) are only slightly affected by the nature of the acid used. To gain a deeper insight into the action of the acids used we have measured the association constants of the HAAs (3a-h) with 4-nitroaniline (4) in toluene. Also in this case, the formation of two complexes requiring one (K2) or two (K3) moles of acid has been evidenced, but now the K3/K2 ratios are significantly affected by the strength of the acid examined. The variation of the K 3/K2 ratios larger than those concerning the k III/kII ratios appears useful to enlighten the very nature of the acid-catalyzed pathways in toluene, which has been elucidated also carrying out the rearrangement in the presence of mixtures of tribromo- and trichloro-acetic acids at different concentrations.