Instantaneous rate constants in physical organic chemistry: application to acyl transfer reactions of p‐nitrophenyl acetate to hydroxide ion

Abstract

AbstractThe instantaneous rate constant (IRC)–time profiles for single‐step mechanisms are straight lines with zero slope. The IRC–time profiles for reactions taking place in more than a single step depend upon whether decay of reactant or evolution of product concentration is monitored. When reactant decay is monitored, the IRC–time plot extrapolated to zero time is equal to the rate constant for the initial step (kf) in the reaction while the IRC–time profile for product evolution extrapolates to zero at t = 0. The IRC–time profiles for monitoring reactant decay and product evolution converge to a plateau value which can be equated to the rate constant under steady‐state conditions (ks.s). Two independent procedures for calculating IRC–time profiles were developed and tested for both simple and complex mechanisms. The first method is initiated with a least squares correlation over the first 11 points of a 2000‐point ln(1−ER)−time profile (ER designates extent of reaction). The IRC at the midpoint (IRC(6)) of the time interval is assigned the value of −S (where S is the slope of the correlation). The second correlation is over points 2–12 and provides a value of IRC(7). This procedure is continued until IRC(6) to IRC(1995) have been generated. The second method involves a 5th order polynomial smooth of the (1−ER)/time profile before calculating IRC at the midpoints between successive points. The limitations of the two IRC procedures are discussed. The IRC procedures are applied to experimental absorbance–time profiles for the acyl transfer reactions of p‐nitrophenyl acetate to hydroxide ion in water and in aqueous acetonitrile. In water, no significant deviations from pseudo first‐order kinetics were observed. A dramatic change in IRC/time profiles was observed on changing solvents to aqueous acetonitrile. Under the latter conditions the IRC analyses were observed to be consistent with a 2‐step mechanism. Copyright © 2006 John Wiley & Sons, Ltd.