Kinetics anmd Mechanism of Ring Cleavage of 1-Benzyl-4-phenyl-1,2,4-triazolium Chloride

Article abstract of DOI:10.1002/prac.19923340306

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.