Stability of N-heterocyclic oxime derivatives. Part V. Kinetics of the reaction of 2-hydroxyiminomethyl-1-methylpyridinium iodide and isopropyl methylphosphonofluoridate in water at 15, 25, and 35° and at pH 5·7–7·7

Abstract

The reaction of 2-hydroxyiminomethyl-1-methylpyridinium iodide and isopropyl methylphosphonofluoridate in water was found to be a multistep reaction in which the first step is an attack on the organophosphorus compound by the oximate ion to yield a phosphylated oxime. The reaction is first-order with respect to both reactants, and the second-order rate constant at 25° is 5·85 I. mole^–1 sec.^–1, with enthalpy and entropy of activation of 5·9 kcal. mole^–1 and –35·2 cal. mole^–1 deg.^–1, respectively. The phosphylated oxime subsequently undergoes elimination catalysed by hydroxide ions and oximate ions, which yields as end product the 2-cyano-1-methylpyridinium cation. For the hydroxide-ion-catalysed reaction the second-order rate constant at 25° is 19,000 I. mole^–1 sec.^–1 with enthalpy and entropy of activation of 13·7 kcal. mole^–1 and 7·0 cal. mole^–1 deg.^–1 respectively. For the oximate-ion-catalysed reaction the corresponding values 1·0 l. mole^–1 sec.^–1, 13·4 kcal. mole^–1, and –13·5 cal. mole^–1 deg.^–1 were found. The rate constants for the decomposition of the phosphylated oxime were calculated with the aid of a computer from the observed acid production of the reactions at constant pH. Decomposition of the phosphylated oxime catalysed by water or hydronium ions was not detected.