Kinetic Isotope Effects for Acyl Transfer from <i>p</i>-Nitrophenyl Acetate to Hydroxylamine Show a pH-Dependent Change in Mechanism

Abstract

Kinetic isotope effects were measured for the acyl transfer reaction from p-nitrophenyl acetate to hydroxylamine at pH 6.0 and 12.0. The isotope effects measured and their values at pH 6.0 and 12.0, respectively, are as follows: leaving group oxygen-18 isotope effect, 1.0310 and 1.0074; the carbonyl oxygen-18 isotope effect, 1.0082 and 1.0008; the β-deuterium isotope effect, 0.9644 and 0.9516; the carbonyl carbon-13 isotope effect, 1.0287 and 1.0337; the nitro group nitrogen-15 isotope effect, 1.0009 and 1.0011. The solvent deuterium isotope effect is 1.4 at pH 7.5 and 0.85 at pH 12.0. The leaving group oxygen-18 isotope effect was measured as a function of pH between 6.0 and 12.0 and was found to have an inflection at pH 10.0 between the plateau regions at high and low pH. The results are consistent with rate limiting breakdown of an intermediate for the low pH reaction and attack of hydroxylamine anion with concerted expulsion of nitrophenolate ion at high pH.