Kinetic Studies on the Catalystless Tautomerization of the Tautomer of p-Nitrosophenol and p- Benzoquinoneoxime in Binary Media of Methanol and Water

Abstract

The nitroso form of the entitled tautomer increases with water content in the medium.The rates of nitroso-oxime tautomerization of the tautomer of p-nitrosophenol and p-benzoquinoneoxime in the absence of catalyst in binary media of 40, 50, 60 and 80 vol% water and methanol at 1525°C were measured by means of UV spectrophotometry, using a calibration curve for the reaction prepared by wave analysis.The reaction followed reversible first-order kinetics : Rate=k1 (a-c-x) -k-1 (c+x).Here, k1 and k-1 are first-order rate constants for the nitrosozation and the oximization;a, c and x are the concentration of the whole tautomer, nitroso form at the initial time, and consumed oxime or formed nitroso form at time t, respectively.The entropy term is more predominant than the enthalpy term both for the forward and reverse reactions.The slopes of log k1 and log k-1 against log aH2O were 3 and 2 for the forward and reverse reactions, respectively.From these findings, a mechanism is presented, involving the intermediate hydrogen bonding perfect annular complex by three molecules of water between hydroxylamino hydrogen and ketonyl oxygen for nitrosozation and the imperfect annular complex by two water molecules with phenolic hydrogen and/or nitroso oxygen for oximization.