Kinetics and mechanism of ring cleavage of 1-Benzyl-4-phenyl-1,2,4-triazolium Chloride

1-Benzyl-4-phenyl-1,2,4-triazolium chloride (1) is cleaved by aqueous sodium hydroxide to form 4-benzyl-1-formyl-1-phenyl-amidrazone ( 5 ), the structure of which is proved by mass spectroscopy. The kinetics of the cleavage reaction are studied by UV spectroscopy in methanol/water 1:1 (v/v) in the presence of Na₂CO₃/HCl buffer at pH values from 11.64 to 12.61. The reaction obeys a first-order kinetic law. The rate constants do not depend linearly on the hydroxide concentration. This is in coincidence with fast formation of the pseudobase from 1 , followed by fast deprotonation to the pseudobase anion, and slow decay to the amidrazone. The pseudobase anion is proved spectroscopically, and its spectroscopically determined pK_QOH = 12.7 coincides with that from the kinetic experiments. The equilibrium constant of pseudobase formation pK_A = 10.99 is two orders of magnitude higher than that from the azocoupling of 1 1] which shows that in the azocoupling not the pseudobase but its methyl ether (pK_QOMe = 8.7) is active. 4-Benzyl-1-formyl-1-phenyl-amidrazone ( 5 ) re-cyclizes in the presence of HCl in MeOH/H₂O to form 1 . The kinetics of this reaction are also studied.