Détermination cinétique de la nature du conformère réactif dans l'hydrolyse des orthoesters

1983 ◽  
Vol 61 (12) ◽  
pp. 2643-2650 ◽  
Author(s):  
Gérard Lamaty ◽  
Claude Moreau ◽  
Zéphirin Mouloungui
Keyword(s):  
Rate Constants ◽  
Rate Determining Step ◽  
Catalytic Rate ◽  
Hydrolysis Of

The rates of hydrolysis of the following orthoesters, 6 or 7-substituted 1,1-diethoxy 3,4-dihydrobenzo-2-pyrans (R = 6-MeO, H, 7-MeO, 7-NO2 and 6-NO2) were determined at 27 °C in water/dioxan (2:1 by volume). The catalytic rate constants as well as the Hammett ρ constants show that the rate-determining step is the formation of a cyclic carboxonium ion intermediate and constitute a kinetic piece of evidence for the determination of the nature of the reactive conformer in hydrolysis of 2,2-dialkoxy tetrahydropyran orthoesters.

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.

Substituent Effects on the Hydrolysis of p-Substituted Benzonitriles in Sulfuric Acid Solutions at (25.0± 0.1) °C

2008 ◽  
Vol 63 (9) ◽  
pp. 603-608 ◽  
Author(s):  
Khamis A. Abbas
Keyword(s):  
Sulfuric Acid ◽  
Rate Constants ◽  
Inductive Effect ◽  
Substituent Effects ◽  
Acid Solutions ◽  
Rate Determining Step ◽  
Inductive Effects ◽  
High Concentrations ◽  
Sulfuric Acid Solutions ◽  
Hydrolysis Of

The rate constants of the hydrolysis of p-substituted benzonitriles with sulfuric acid solutions (18.2 M to 10 M) have been determined spectrophotometrically at (25.1±0.1) °C. It was found that the catalytic activity of sulfuric acid was strongly inhibited by water. The logarithms of the observed rate constants were correlated with different substituent inductive (localized) and resonance (delocalized) constants. The results of the correlation studies indicated that the rate-determining step of the hydrolysis of benzonitriles in 18.2 M sulfuric acid was the addition of a nucleophile, and the hydrolysis was clearly enhanced by the electron-withdrawing inductive effect, while the rate-determining step of the hydrolysis of p-substituted benzonitriles in 10.0 M sulfuric acid was most probably the protonation of benzonitriles, and the rate constants increased by both electron-donating resonance and inductive effects. A mixture of the two mechanisms most probably occurred in 15.3 to 17.0 M sulfuric acid. HSO4 − rather thanwater most probably acted as nucleophile in the hydrolysis of benzonitriles especially at high concentrations of sulfuric acid solutions.

The retroaldol reaction of 3-methyl-2-butenal

1983 ◽  
Vol 61 (1) ◽  
pp. 171-178 ◽  
Author(s):  
J. Peter Guthrie ◽  
Brian A. Dawson
Keyword(s):  
Sodium Hydroxide ◽  
Equilibrium Constant ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Aqueous Sodium Hydroxide ◽  
Initial Increase ◽  
Aqueous Sodium ◽  
Rate Determining Step ◽  
Kinetics Of

In aqueous sodium hydroxide solutions at 25 °C, 3-methyl-2-butenal, 1c, undergoes retroaldol cleavage to acetone and acetaldehyde. The kinetics of the retroaldol reaction were followed spectrophotometrically at 242 nm and showed simple first order behavior. When 3-methyl-3-hydroxybutanal, 2c, was added to aqueous sodium hydroxide solutions at 25 °C, there was an initial increase in absorbance at 242 nm, attributed to formation of 1c, followed by a 20-fold slower decrease; the rate of the slow decrease matches the rate of disappearance of 1c under the same conditions. Analysis of the kinetics allows determination of the three rate constants needed to describe the system: khyd = 0.00342; kdehyd = 0.00832; kretro = 0.0564; all M−1 s−1. The equilibrium constant for enone hydration is 0.41. Rate constants for the analogous reactions for acrolein and crotonaldehyde could be obtained from the literature. There is a reasonable rate–equilibrium correlation for the retroaldol step. For the enone hydration step, rate and equilibrium constants respond differently to replacement of hydrogen by methyl. It is proposed that this results from release of strain after the rate-determining step by rotation about a single bond; this decrease in strain is reflected in the equilibrium constant but not in the rate constant.

scholarly journals The linkage between binding of the C-terminal domain of hirudin and amidase activity in human α-thrombin

1993 ◽  
Vol 289 (2) ◽  
pp. 475-480 ◽  
Author(s):  
R de Cristofaro ◽  
B Rocca ◽  
B Bizzi ◽  
R Landolfi
Keyword(s):  
Rate Constants ◽  
Binding Constant ◽  
Amidase Activity ◽  
Equilibrium Binding ◽  
Viscosity Effects ◽  
The Individual ◽  
Individual Rate ◽  
Hydrolysis Of ◽  
Equilibrium Binding Constant

A method derived from the analysis of viscosity effects on the hydrolysis of the amide substrates D-phenylalanylpipecolyl-arginine-p-nitroaniline, tosylglycylprolylarginine-p-nitroanaline and cyclohexylglycylalanylarginine-p-nitroalanine by human alpha-thrombin was developed to dissect the Michaelis-Menten parameters Km and kcat into the individual rate constants of the binding, acylation and deacylation reactions. This method was used to analyse the effect of the C-terminal hirudin (residues 54-65) [hir-(54-65)] domain on the binding and hydrolysis of the three substrates. The results showed that the C-terminal hir-(54-65) fragment affects only the acylation rate, which is increased approx. 1.2-fold for all the substrates. Analysis of the dependence of acylation rate constants on hirudin-fragment concentration, allowed the determination of the equilibrium binding constant of C-terminal hir-(54-65) (Kd approximately 0.7 microM). In addition this peptide was found to competitively inhibit thrombin-fibrinogen interaction with a Ki which is in excellent agreement with the equilibrium constant derived from viscosity experiments. These results demonstrate that binding of hir-(54-65) to the fibrinogen recognition site of thrombin does not affect the equilibrium binding of amide substrates, but induces only a small increase in the acylation rate of the hydrolysis reaction.

Cyclic voltammetric study of acid hydrolysis of tris(bipyridine) chromium(II): a critical appraisal of CV determination of kinetic parameters

1985 ◽  
Vol 63 (10) ◽  
pp. 2732-2735 ◽  
Author(s):  
Shachar D. Nadler ◽  
James G. Dick ◽  
Cooper H. Langford
Keyword(s):  
Critical Appraisal ◽  
Kinetic Studies ◽  
Cyclic Voltammetric ◽  
Residual Currents ◽  
Cyclic Voltammetric Study ◽  
Rate Determining Step ◽  
Voltammetric Study ◽  
Anodic Peak Current ◽  
Hydrolysis Of

Cyclic voltammetry has been exploited to reinvestigate discrepancies in the reported rate constant for the reaction:[Formula: see text]which follows electrode reduction of the substitution inert [Formula: see text]. It is shown that the problems arise from oversimplified treatment of residual currents when estimating the anodic peak current, ia. A "two scan" procedure for estimating the background currents is introduced which appears to resolve discrepancies. The kinetic studies were extended to a methyl-substituted bipyridine. Since ring methylation has very little effect on kf, it appears that the aquation reaction is initiated by a rate-determining step which requires a minimum of bipyridine rearrangement.

Kinetics and Mechanism of Acid Catalyzed Decomposition of 1-Phenyl-3,3-dialkyltriazenes

1994 ◽  
Vol 59 (2) ◽  
pp. 401-411 ◽  
Author(s):  
Miroslav Ludwig ◽  
Pavla Valášková ◽  
Oldřich Pytela
Keyword(s):  
Rate Constants ◽  
Steric Effect ◽  
Aqueous Ethanol ◽  
Pivalic Acid ◽  
Decomposition Kinetics ◽  
Methyl Ethyl ◽  
Rate Determining Step ◽  
Alkyl Groups ◽  
Acid Catalyzed ◽  
Catalytic Rate

Five model 1-phenyl-3,3-dialkyltriazenes (methyl, ethyl, 2-propyl, butyl, cyclohexyl) have been synthesized and their acid-catalyzed decomposition kinetics have been investigated spectrophotometrically in aqueous ethanol (40 vol.%) with pivalic acid as the catalyst. The results show that the rate-determining step is catalyzed by the proton. The decrease in the observed rate constant at higher concentrations of pivalic acid is explained by the formation of an unreactive complex of the nondissociated acid and respective triazene. The steric effect of alkyl groups on the catalytic rate constants is discussed.

Polarographic determination of the catalytic rate constants of dehydration of some dicarbonyl compounds in strong acid medium

1984 ◽  
Vol 29 (10) ◽  
pp. 1493-1494 ◽  
Author(s):  
J.M. Rodriguez-Mellado ◽  
L. Camacho ◽  
J.J. Ruiz
Keyword(s):  
Acid Medium ◽  
Rate Constants ◽  
Strong Acid ◽  
Polarographic Determination ◽  
Dicarbonyl Compounds ◽  
Catalytic Rate

Determination of relative rate constants of hydrolysis of poly[(dimethylamino)ethyl methacrylate]

1983 ◽  
Vol 16 (6) ◽  
pp. 1009-1010 ◽  
Author(s):  
Yves Merle ◽  
Liliane Merle-Aubry
Keyword(s):  
Rate Constants ◽  
Relative Rate ◽  
Ethyl Methacrylate ◽  
Relative Rate Constants ◽  
Hydrolysis Of

scholarly journals Optimization and a Kinetic Study on the Acidic Hydrolysis of Dialkyl α-Hydroxybenzylphosphonates

Molecules ◽  
2020 ◽  
Vol 25 (17) ◽  
pp. 3793
Author(s):  
Nikoletta Harsági ◽  
Zita Rádai ◽  
Áron Szigetvári ◽  
János Kóti ◽  
György Keglevich
Keyword(s):  
Kinetic Study ◽  
Rate Constants ◽  
Acidic Hydrolysis ◽  
First Order ◽  
Order Rate ◽  
Rate Determining Step ◽  
Pseudo First Order ◽  
Hydrolysis Of ◽  
Electron Withdrawing Substituents

The two-step acidic hydrolysis of α-hydroxybenzylphosphonates and a few related derivatives was monitored in order to determine the kinetics and to map the reactivity of the differently substituted phosphonates in hydrolysis. Electron-withdrawing substituents increased the rate, while electron-releasing ones slowed down the reaction. Both hydrolysis steps were characterized by pseudo-first-order rate constants. The fission of the second P-O-C bond was found to be the rate-determining step.

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