Solvent effect as a criterion of solvolysis mechanism. Solvolysis of 3-acyl-1,3-diphenyltriazenes

1981 ◽  
Vol 46 (9) ◽  
pp. 2091-2103 ◽  
Author(s):  
Oldřich Pytela ◽  
Petr Svoboda ◽  
Miroslav Večeřa
Keyword(s):  
Transition State ◽  
Solvent Effect ◽  
Mole Fraction ◽  
Rate Constant ◽  
Rate Constants ◽  
Proton Donor ◽  
Hydrolysis Rate ◽  
Solvent Mixtures ◽  
Solvent Dependence ◽  
Hydrolysis Of

Solvent dependence of hydrolysis rate constants of 3-acetyl-1,3-diphenyltriazene (I) and 3-(N-methylcarbamoyl)-1,3-diphenyltriazene (II) has been followed in the solvent mixtures ethanol-water, methanol-water, dioxane-water, and formamide-water within the mole fraction x = 0.0 to 0.5 at 25, 35 and 45 °C. A criterion has been suggested, based on sign of change of logarithm of the observed rate constant in dependence on change of the solvent composition, for evaluation of the reaction molecularity and, hence, participation of water in the hydrolysis mechanism. It has been found that water takes part as a proton donor in the transition state of hydrolysis of the substrates studied.

Kinetics and mechanism of the hydrolysis of a (5,6)-spirophosphorane. Thermodynamics of the hydrolysis of cyclic five-membered and six-membered phosphonium ions

1991 ◽  
Vol 69 (12) ◽  
pp. 2064-2074 ◽  
Author(s):  
Glenn H. McGall ◽  
Robert A. McClelland
Keyword(s):  
Free Energy ◽  
Transition State ◽  
Rate Constants ◽  
Strong Acid ◽  
Membered Ring ◽  
Phosphate Esters ◽  
Hydrolysis Rate ◽  
Activation Free Energy ◽  
Stabilizing Effect ◽  
Hydrolysis Of

The cyclic five-membered phosphonium ion 2b (2-(2′-hydroxyethoxy)-2-phenyl-1,3,2-dioxaphospholan-2-ylium) derived from ring-opening of the (5,5)-spirophosphorane 1b (5-phenyl-1,4,6,9-tetraoxa-5-phosphaspiro[4,4]nonane) has been observed in neat CF3SO3H and at >85% H2SO4. The cation undergoes hydrolysis in the latter solutions, and an extrapolation has been carried out to obtain an estimate for reactivity in 100% water. Hydrolysis rate constants for phenyltrialkoxyphosphonium ions in water are 107, 100, and 5 × 10−3 s−1 for cyclic five-membered, cyclic six-membered, and acyclic derivatives respectively; these show an excellent correlation with rate constants for a similar series of phosphate esters. An investigation of the hydrolysis of the (5,6)-spirophosphorane 5 (5-phenyl-8,8-dimethyl-1,4,6,10-tetraoxa-5-phosphaspiro[4,5]decane) provides a clue as to the origins of these rate differences. This phosphorane can in principle hydrolyze via two isomeric cyclic phosphonium ions, the six-membered 14 and the five-membered 15. The former is thermodynamically more stable, being the only cation observed under equilibrating conditions of strong acid. However, the hydrolysis of the spirophosphorane, as well as the hydrolysis of fully formed 14, channels through the cyclic five-membered 15. A thermodynamic breakdown reveals that the 9.5 kcal mol−1 difference in activation free energy for the hydrolysis of five- and six-membered cyclic phosphonium ions is due to a combination of a higher free energy (2.5–4.5 kcal mol−1) for the five-membered cation, and a lower free energy (7–5 kcal mol−1) for the pentacoordinate transition state with the five-membered ring. This analysis also shows that a (5,6)-spirophosphorane is 6–8 kcal mol−1 more stable than a (6,6)-spirophosphorane. Thus, a five-membered ring has a significant stabilizing effect on a pentacoordinated phosphorus structure. The accelerated hydrolysis of cyclic phosphonium ions and phosphate esters with five-membered rings is caused by a combination of this stabilizing effect in the transition state and a destabilizing effect in the ground state associated with ring strain. Key words: phosphorane, hydrolysis, phosphate, phosphonium.

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.

The mechanisms of hydrolysis of alkyl N-alkylthioncarbamate esters at 100 °C

2005 ◽  
Vol 83 (9) ◽  
pp. 1483-1491 ◽  
Author(s):  
Eduardo Humeres ◽  
Maria de Nazaré M. Sanchez ◽  
Conceição ML Lobato ◽  
Nito A Debacher ◽  
Eduardo P. de Souza
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Base Catalysis ◽  
Hydrolysis Rate ◽  
Negative Slope ◽  
General Base ◽  
Order Rate ◽  
Basic Hydrolysis ◽  
Rate Profile ◽  
Hydrolysis Of

The hydrolysis of ethyl N-ethylthioncarbamate (ETE) at 100 °C was studied in the range of 7 mol/L HCl to 4 mol/L NaOH. The pH–rate profile showed that the hydrolysis occurred through specific acid catalysis at pH < 2, spontaneous hydrolysis at pH 2–6.5, and specific basic catalysis at pH > 6.5. The Hammett acidity plot and the excess acidity plot against X were linear. The Bunnett–Olsen plot gave a negative slope indicating that the conjugate acid was less hydrated than the neutral substrate. It was concluded that the acid hydrolysis occurred by an A1 mechanism. The neutral species hydrolyzed with general base catalysis shown by the Brønsted plot with β = 0.48 ± 0.04. Water acted as a general base catalyst with (pseudo-)first-order rate constant, kN = 3.06 × 10–7 s–1. At pH > 6.5 the rate constants increased, reaching a plateau at high basicity. The basic hydrolysis rate constant of ethyl N,N-diethylthioncarbamate, which must react by a BAc2 mechanism, increased linearly at 1–3 mol/L NaOH with a second-order rate constant, k2 = 2.3 × 10–4 (mol/L)–1 s–1, which was 10 times slower than that expected for ETE. Experiments of ETE in 0.6 mol/L NaOH with an excess of ethylamine led to the formation of diethyl thiourea, presenting strong evidence that the basic hydrolysis occurred by the E1cb mechanism. In the rate-determining step, the E1cb mechanism involved the elimination of ethoxide ion from the thioncarbamate anion, producing an isothiocyanate intermediate that decomposed rapidly to form ethylamine, ethanol, and COS.Key words: alkylthioncarbamate esters, ethyl N-ethylthioncarbamate, ethyl N,N-diethylthioncarbamate, hydrolysis, mechanism.

Hydrolysis of aryl hydrogen maleate esters mediated by cyclodextrins — Effect on the intramolecular catalysis

2005 ◽  
Vol 83 (9) ◽  
pp. 1281-1286 ◽  
Author(s):  
Gabriel O Andrés ◽  
O Fernando Silva ◽  
Rita H de Rossi
Keyword(s):  
Maleic Anhydride ◽  
Transition State ◽  
Rate Constant ◽  
Association Constant ◽  
Kinetic Studies ◽  
Association Constants ◽  
Bulk Solution ◽  
Intramolecular Catalysis ◽  
Hydrolysis Of ◽  
Nonlinear Fashion

Kinetic studies of the hydrolysis of Z-aryl hydrogen maleates (Z = H, p-CH3, m-CH3, p-Cl, m-Cl) were carried out in the presence and absence of hydroxypropyl-β-cyclodextrin (HPCD) at variable pH from 1.00 to 3.00. The reaction involves the formation of maleic anhydride as an intermediate and the rate of its formation is strongly dependent on the pH. This is because the neighboring carboxylate group is a better catalyst than the carboxylic group. The rate constant for the formation of maleic anhydride decreases as the HPCD concentration increases in a nonlinear fashion. The results were interpreted in terms of the formation of a 1:1 inclusion complex of the esters with HPCD. The neutral (HA) and anionic (A) species of the substrate have different association constants (K[Formula: see text] and K[Formula: see text]). In all cases studied, K[Formula: see text] is higher than K[Formula: see text] for the same substrate. This difference is responsible for a decrease in the amount of the anionic substrate (reactive species) in the presence of HPCD, which results in a diminution of the observed rate constant. Besides, the rate constant for the reaction of the complexed substrate is smaller than that in the bulk solution indicating that the transition state of the cyclodextrin mediated reaction is less stabilized than the anionic substrate. The values of ΔΔG‡ are almost independent of the substituent on the aryl ring and range within 0.48 and 1.05 kcal mol–1 (1 cal = 4.184 J). There is no correlation between KTS and the association constant of the substrate indicating that the factors stabilizing the transition state are different from those that stabilize the substrate. Key words: cyclodextrins, intramolecular catalysis, hydrolysis, inhibition.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

The Hydrolysis of Bis(2-chloroethyl) Sulfide (Sulfur Mustard) in Aqueous Mixtures of Ethanol, Acetone and Dimethyl Sulfoxide

1993 ◽  
Vol 46 (3) ◽  
pp. 293 ◽  
Author(s):  
RI Tilley
Keyword(s):  
Transition State ◽  
Dimethyl Sulfoxide ◽  
Rate Constants ◽  
Charge Separation ◽  
Sulfur Mustard ◽  
Transition States ◽  
Aqueous Mixtures ◽  
Sulfide Sulfur ◽  
Rate Of Hydrolysis ◽  
Hydrolysis Of

The rate of hydrolysis of bis (2-chloroethyl) sulfide (sulfur mustard) in aqueous mixtures of ethanol, acetone and dimethyl sulfoxide has been measured and compared with previously reported values. Rate constants in water at 25°C for the two consecutive hydrolysis reactions undergone by sulfur mustard were estimated to be (2.93�0.15)×10-3 and (3.87�0.14)×10-3 s-1. Charge separation of 0.42 in the transition states was indicated together with significant solvation of the positive end of the transition state dipoles.

THE STERIC FACTOR IN THE HYDROLYSIS OF β-1,4′-OLIGOGLUCOSIDES BY MYROTHECIUM CELLULASE

1956 ◽  
Vol 34 (1) ◽  
pp. 102-115 ◽  
Author(s):  
D. R. Whitaker
Keyword(s):  
Activation Energy ◽  
Rate Constant ◽  
Rate Constants ◽  
Activation Energies ◽  
Enzymic Activity ◽  
Degree Of Polymerization ◽  
Steric Factor ◽  
Collision Theory ◽  
Hydrolysis Of

A comparison of the rate constants and activation energies for the hydrolysis of cellobiose, cellotriose, cellotetraose, and cellopentaose by Myrothecium cellulase showed that while the rate constant was increased by a factor of about 450 as the degree of polymerization (D.P.) of the substrate was increased from two to five, the activation energy remained at about 12,000 cal. The results are interpreted, in terms of classical collision theory, as indicating that the increase in rate constant with D.P. is determined by an increase in the steric factor with D.P. Addition of a β-linked sorbityl group to an oligoglucoside increased the rate constant; the increase was less than that from addition of an anhydroglucose unit and, relative to the latter, diminished as the D.P. of the chain undergoing addition was increased. Exposing the enzyme to conditions favoring thermal or surface denaturation caused varying losses in enzymic activity towards the four oligoglucosides; wherever the loss in activity towards one oligoglucoside differed substantially from the loss in activity towards any other oligoglucoside, the greater loss was shown towards the substrate of lower D.P. The results are discussed.

Fluorodeoxyglucose rate constants, lumped constant, and glucose metabolic rate in rabbit heart

1987 ◽  
Vol 252 (4) ◽  
pp. H777-H787 ◽  
Author(s):  
J. Krivokapich ◽  
S. C. Huang ◽  
C. E. Selin ◽  
M. E. Phelps
Keyword(s):  
Metabolic Rate ◽  
Rate Constant ◽  
Rate Constants ◽  
Interventricular Septum ◽  
Hydrolysis Rate ◽  
Constant Infusion ◽  
Coincidence Counting ◽  
Paired Stimuli ◽  
Lumped Constant ◽  
Phosphorylation Rate

The isolated arterially perfused rabbit interventricular septum was used to measure myocardial metabolic rate for glucose (MMRGlc) and rate constants and lumped constant (LC) for the glucose analogue [18F]fluorodeoxyglucose (FDG) using a tracer kinetic model. FDG was delivered by constant infusion during coincidence counting of tissue 18F radioactivity. The MMRGlc was measured by the Fick method. Control septa were paced at 72 beats/min and perfused at 1.5 ml/min with oxygenated perfusate containing 5.6 mM glucose and 5 mU/ml insulin. The following conditions were tested: 3.0 and 4.5 ml/min; insulin increased to 25 mU/ml; insulin omitted; 2.8 mM and 11.2 mM glucose; 144 beats/min and 96 paired stimuli/min; and anoxia. Under all conditions studied the phosphorylation (hexokinase) reaction was rate limiting relative to transport. Compared with control conditions, the phosphorylation rate constant was significantly increased with 2.8 mM glucose as well as in anoxia. With 4.5 ml/min and 11.2 mM glucose, conditions that should increase glucose flux into tissue without increasing demand, the phosphorylation rate constant decreased significantly. With 11.2 mM glucose, 96 paired stimuli/min, and anoxia without insulin, a significant increase in the hydrolysis rate of FDG 6-phosphate was observed and suggests that hydrolysis is also an important mechanism for regulating the MMRGlc. Increased transport rate constants were observed with increased flow rates, 96 paired stimuli/min, and anoxia at 96 beats/min. The LC was not significantly different from control in 11 of 14 conditions studied. Therefore, under most conditions, an average LC can be used to calculate MMRGlc estimates.

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