Transition State Structure for the Water-Catalyzed Hydrolysis of p-Nitrophenyl Trifluoroacetate in Acetonitrile

Abstract

The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 degrees C. The reaction is found to be third order in water. The reaction is also third order in D(2)O and in methanol when D(2)O and methanol replace water as the solvent. The kinetic solvent isotope effect is (kH(2)O/kD(2)O) = 2.90 +/- 0.12. Proton inventories at each molarity of water studied are consistent with an eight-membered cyclic transition state model. In this model, three protons undergo bonding changes. Such an eight-membered transition state model can easily accommodate linear hydrogen bonds for the three transferred protons. These results are consistent with the experimental findings of Bell and Critchlow(1) on the reversible addition of water to 1,3-dichloroacetone in dioxane and the theoretical findings of Wolfe and co-workers(2) on the hydration of formaldehyde.

Solvent effect and proton inventory in the hydrolysis of p‐methylphenyl trichloroacetate

“…The first value is very similar to that observed by Venkatasubban et al for p-nitrophenyl trifluoroacetate, a fractionation factor of 0.697 AE 0.005 for a three-proton model. 4 Structure 3 is consistent with the observed effect and we can see that the three hydrogens H a are most likely responsible for the global isotope effect, since the hydrogens H b contribute little or nothing, since they are expected to have a fractionation factor of 1.…”

“…Despite this fact, we believe that the choice of an eightmembered transition state, rather than a six-membered transition state involving two water molecules, is appropriate given that the former has bond angles that accommodate approximately linear hydrogen bonds. 4,27 The water-assistance mechanism in ester hydrolysis has a substantial advantage in comparison with a single water attack. Molecular orbital calculations carried out by Hori et al 10 for the oxygen exchange accompanying the alkaline hydrolysis of methyl acetate showed that the activation barrier for the water-assisted mechanism is lower [3.8 kcal mol À1 (1 kcal ¼ 4.184 kJ)] than that of the hydroxide attack without water assistance (21.8 kcal mol À1 ).…”

“…Cyclic transition-state structures are frequently postulated in the literature, [4][5][6]9,10 including for proton transfer in water 12,28 and acid-12 and base-catalyzed [10][11][12] hydrolysis of esters. In pure water or in water-rich solvent mixtures, water-to-water hydrogen bonds predominate and a multiple number of protons participate in the transition state for the hydrolysis of p-methylphenyl trichloroacetate.…”

“…For example, reaction of p-nitrophenyl trifluoroacetate is thought to proceed with three protons undergoing bonding changes in an eight-membered transition state (1). 4 A great number of both experimental [4][5][6][7][8] and theoretical [8][9][10][11][12] studies have been devoted to the hydrolysis reactions of aldehydes, ketones, esters and amides. [13][14][15][16][17] It has been observed that the nucleophile attack on a carbonyl group is mediated by water molecules that previously hydrated the carbonyl group.…”

Spontaneous hydrolysis of ethyl formate: Isobaric activation parameters

“…These kinetic features suggest transition state (TS) structures in which many solvent molecules are immobilized and subjected to electrostriction associated with the solvation of developing charges and the trans- There is experimental [16,18] and theoretical [21,22] basis to consider a cyclic atomic array made of one RCO-X molecule and three water molecules, as a more complete picture of the TS complex. Esters of formic acid also undergo neutral hydrolysis, at faster rates compared with the corresponding acetates.…”