Kinetics of the pH‐independent hydrolyses of 4‐nitrophenyl chloroformate and 4‐nitrophenyl heptafluorobutyrate in water‐acetonitrile mixtures: consequences of solvent composition and ester hydrophobicity

Abstract

AbstractThe pH‐independent hydrolyses of 4‐nitrophenyl chloroformate, NPCF and 4‐nitrophenyl heptafluorobutyrate, NPFB in aqueous acetonitrile were studied spectrophotometrically from 15 to 45 °C. The binary solvent composition covers water concentrations from 2.349 to 53.207 and from 2.745 to 53.333 mol L−1 for NPCF and NPFB, respectively. For both esters, the dependence of log (kobs), the observed rate constant, on log [water] is sigmoidal. The approximate kinetic orders with respect to water were found to be 2 and 3 for NPCF and NPFB, respectively. ΔG≠ gradually decreases as a function of increasing [water], due to a complex, quasi‐mirror image compensation of ΔH≠ and ΔS≠; both parameters increase. The structures of the transition states were probed by a proton inventory study, carried out in the presence of L2O mole fractions (L = H or D) of 0.190, 0.540, 0.890 and 0.180, 0.529, 0.890, for NPCF and NPFB, respectively. Plots of observed rate constants versus the atom fraction of deuterium in the solvent curve downward. Cyclic transition state models were fitted to the kinetic data; these models contain the ester and two water molecules (NPCF) or three water molecules (NPFB). Thus, the sigmoidal dependences of log (kobs) on log [water] are not due to a change in the number of water molecules in the transition states as a function of increasing [water]. The binary solvent mixture is micro‐heterogeneous; there exists two “micro‐domains,” one consists predominantly of coordinated water molecules, the other consists mostly of acetonitrile hydrogen‐bonded to water molecules. NPCF is 232 times more soluble in water than NPFB. That is, the former ester is dissolved in the outer, more polar periphery of these micro‐domains whereas the more hydrophobic NPFB is dissolved in their inner, less polar interiors. This conclusion is corroborated by comparing the dependence on log [water] of log [kobs], and of ET, the empirical solvent polarity parameter, as measured by solvatochromic probes of increasing hydrophobicity. Copyright © 2006 John Wiley & Sons, Ltd.