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
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.