Spectator catalysis in the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by "hydroxypropyl-β-cyclodextrin"

1996 ◽  
Vol 74 (5) ◽  
pp. 745-752 ◽  
Author(s):  
Timothy A. Gadosy ◽  
Oswald S. Tee
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Competitive Inhibition ◽  
Strong Dependence ◽  
Limited Range ◽  
Initial State ◽  
Cleavage Rate ◽  
Apparent Dissociation Constant ◽  
Potential Inhibitor ◽  
Potential Inhibitors

Aliphatic alcohols that form host–guest complexes with "hydroxypropyl-β-cyclodextrin" retard the cleavage of m-nitrophenyl acetate by hydroxypropyl-β-cyclodextrin in basic aqueous solution, due to competitive inhibition. By contrast, these same species do not inhibit the reaction of p-nitrophenyl acetate and p-nitrophenyl hexanoate to the same extent and, in some cases, the addition of alcohols serves to increase the rate of reaction. The observed reaction kinetics require the presence of a process that has one molecule of the "potential inhibitor" in the transition state for ester cleavage. Rate constants, ka, for the reaction of the {ester•hydroxypropyl-β-cyclodextrin} complexes with a series of potential inhibitors show a strong dependence on the ability of the potential inhibitor to bind to the cyclodextrin. On the other hand, rate constants for the kinetically equivalent reaction of the ester with the {cyclodextrin•potential inhibitor} complex show little dependence on the alcohol structure and they vary over a very limited range. The negative logarithms of the apparent dissociation constant of the potential inhibitor from the transition state show a strong dependence on the ability of the potential inhibitor to bind to hydroxypropyl-β-cyclodextrin, indicating that the binding of the potential inhibitor in the initial state and the transition state is similar. It is concluded that the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by hydroxypropyl-β-cyclodextrin in the presence of 14 potential inhibitors can occur with the ester largely outside of the hydroxypropyl-β-cyclodextrin cavity during the transition state, allowing the cavity to be occupied by a molecule of potential inhibitor. Key words: cyclodextrin, spectator catalysis, esterolysis.

Effect of Medium on Reactivity for Alkaline Hydrolysis of p-Nitrophenyl Acetate and S-p-Nitrophenyl Thioacetate in DMSO-H2O Mixtures of Varying Compositions: Ground State and Transition State Contributions

2021 ◽  
Author(s):  
Ik-Hwan Um ◽  
Seungjae Kim
Keyword(s):  
Transition State ◽  
Alkaline Hydrolysis ◽  
Ground State ◽  
Rate Constants ◽  
Kinetic Data ◽  
Reaction Medium ◽  
Stepwise Mechanism ◽  
Rate Determining Step ◽  
Leaving Group ◽  
Hydrolysis Of

Second-order rate constants (kN) for reactions of p-nitrophenyl acetate (1) and S-p-nitrophenyl thioacetate (2) with OH‒ have been measured spectrophotometrically in DMSO-H2O mixtures of varying compositions at 25.0 ± 0.1 oC. The kN value increases from 11.6 to 32,800 M‒1s‒1 for the reactions of 1 and from 5.90 to 190,000 M‒1s‒1 for those of 2 as the reaction medium changes from H2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H2O. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state (TS), i.e., the reactions of 1 and 2 with OH‒ proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step (RDS). Addition of DMSO to H2O would destabilize OH‒ through electronic repulsion between the anion and the negative-dipole end in DMSO. However, destabilization of OH‒ in the ground state (GS) is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. The effect of medium on reactivity has been dissected into the GS and TS contributions through combination of the kinetic data with the transfer enthalpies (ΔΔHtr) from H2O to DMSO-H2O mixtures for OH‒ ion.

Solvolysis of isopropyl bromide in ethanol–water mixtures. Dissection into initial state and transition state contributions

1976 ◽  
Vol 54 (24) ◽  
pp. 3944-3948 ◽  
Author(s):  
Wiendelt Drenth ◽  
Michael Cocivera
Keyword(s):  
Vapor Pressure ◽  
Transition State ◽  
Thermodynamic Parameters ◽  
Activation Parameters ◽  
Solvent Composition ◽  
Initial State ◽  
Partial Vapor Pressure ◽  
State Variation

Rates were determined for the solvolysis of isopropyl bromide in ethanol–water mixtures (20 to 80% by volume of ethanol) at 50 and 75 °C and the corresponding activation parameters calculated. From the partial vapor pressure of isopropyl bromide over the various solutions at 50 and 75 °C, the variations in its initial state thermodynamic parameters were calculated. Thus, the variation in the activation parameters with solvent composition could be analyzed in terms of initial and transition state contributions. The initial state variation dominates according to a unimolecular as well as to a bimolecular treatment of data.

Solvent effect on the initial state and transition state of anation of pentaammine-aquachromium(III) ion by thiocyanate

1993 ◽  
Vol 18 (1) ◽  
pp. 110-112 ◽  
Author(s):  
Olga A. ◽  
J�n Benko ◽  
Ol'ga Voll�rov� ◽  
Vladislav Holba
Keyword(s):  
Transition State ◽  
Solvent Effect ◽  
Initial State

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

Initial state and transition state contributions to solvent effects on activation enthalpies for substitution reactions of tris(1,10-phenanthroline)iron(II) in aqueous methanol

1983 ◽  
Vol 8 (3) ◽  
pp. 148-152 ◽  
Author(s):  
Michael J. Blandamer ◽  
John Burgess ◽  
Timothy Digman ◽  
Philip P. Duce ◽  
John P. McCann ◽  
...  
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Aqueous Methanol ◽  
Substitution Reactions ◽  
Initial State
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