scholarly journals Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

1971 ◽  
Vol 49 (7) ◽  
pp. 1032-1035 ◽  
Author(s):  
G. E. Dunn ◽  
Gordon K. J . Lee
Keyword(s):  
Aqueous Solution ◽  
Ionic Strength ◽  
Carboxylic Acid ◽  
Isotope Effect ◽  
Anthranilic Acid ◽  
Kinetic Isotope Effect ◽  
Pyrrole Ring ◽  
First Order ◽  
Kinetic Isotope ◽  
Ph Range

The decarboxylation of pyrrole-2-carboxylic acid in aqueous buffers at 50° and ionic strength 1.0 has been found to be first order with respect to substrate at a fixed pH. As the pH is decreased, the rate constant increases slightly in the pH range 3–1, then rises rapidly from pH 1 to 10 M HCl. The 13C-carboxyl kinetic isotope effect is 2.8% in 4 M HClO4 and negligible at pH ~ 3. These observations can be accounted for by a mechanism, previously proposed for the decarboxylation of anthranilic acid, in which the species undergoing decarboxylation is the carboxylate ion protonated at the 2-position of the pyrrole ring. This intermediate can be formed both by ring-protonation of the carboxylate anion and by ionization of the ring-protonated acid. At low acidities ring-protonation is rate determining, but at higher acidities the rate of protonation exceeds that of decarboxylation.

Ketonization equilibria of phenol in aqueous solution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 605-613 ◽  
Author(s):  
Marco Capponi ◽  
Ivo G Gut ◽  
Bruno Hellrung ◽  
Gaby Persy ◽  
Jakob Wirz
Keyword(s):  
Aqueous Solution ◽  
Proton Transfer ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Flash Photolysis ◽  
Equilibrium Constants ◽  
Thermodynamic Cycle ◽  
Acidity Constant ◽  
Kinetic Isotope ◽  
Carbon Acids

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

ORTHO PARTICIPATION IN THE CONVERSION OF syn-BENZALDOXIME ESTERS TO NITRILES

1965 ◽  
Vol 43 (12) ◽  
pp. 3178-3187 ◽  
Author(s):  
Robert J. Crawford ◽  
Charles Woo
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Alcoholic Solution ◽  
First Order ◽  
Rate Determining Step ◽  
First Order Kinetics ◽  
Kinetic Isotope ◽  
Marked Enhancement

Substituted syn-benzaldoxime esters are transformed, in an alcoholic solution, to the corresponding nitriles according to first-order kinetics. All ortho substituents were observed to accelerate the rate of nitrile formation relative to the corresponding para derivative. While the ko/kp ratios for the bromo, chloro, fluoro, methoxy, and methyl substituents fall within the range of 2 to 9, the iodo and methylthio substituents are 119 and 11 000 respectively. Isotopic replacement of the aldoximino hydrogen by deuterium gives rise to a kinetic isotope effect, kH/kD being 5.21 for syn-o-chlorobenzaldoxime p-toluenesulfonate, 1.22 for syn-o-iodobenzaldoxime p-toluenesulfonate, and 1.23 for syn-o-methylthiobenzaldoxime o-iodobenzoate. The marked enhancement of rate and the absence of an appreciable isotope effect are considered to be associated with sulfur and iodine participation in the rate-determining step. A mechanism which is capable of explaining the results observed is suggested.

Kinetic evidence for the importance of solvent-separated ion pairs during the solvolyses of adamantylideneadamantyl derivatives

2004 ◽  
Vol 82 (9) ◽  
pp. 1336-1340
Author(s):  
Xicai Huang ◽  
Andrew J Bennet
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Salt Effects ◽  
Kinetic Isotope

The aqueous ethanolysis reactions of adamantylideneadamantyl tosylate, -bromide, and -iodide (1-OTs, 1-Br and 1-I) were monitored as a function of ionic strength. Special salt effects are observed during the solvolyses of both homoallylic halides, but not in the case of the tosylate 1-OTs. The measured α-secondary deuterium kinetic isotope effects for the solvolysis of 1-Br in 80:20 and 60:40 v/v ethanol–water mixtures at 25 °C are 1.110 ± 0.018 and 1.146 ± 0.009, respectively. The above results are consistent with the homoallylic halides reacting via a virtual transition state in which both formation and dissociation of a solvent-separated ion pair are partially rate-determining. While the corresponding transition state for adamantylideneadamantyl tosylate involves formation of the solvent-separated ion pair.Key words: salt effects, kinetic isotope effect, internal return, solvolysis, ion pairs.

Reactions of Coordinated Ligands. The Bromination of 2,4-dimethyl-3H-imidazole and 3H-imidazole Coordinated (at N1) to Pentaamminecobalt(III)

1991 ◽  
Vol 44 (7) ◽  
pp. 981 ◽  
Author(s):  
AG Blackman ◽  
DA Buckingham ◽  
CR Clark
Keyword(s):  
Aqueous Solution ◽  
Kinetic Isotope Effect ◽  
Ph Dependence ◽  
Acidic Aqueous Solution ◽  
Proton Abstraction ◽  
Kinetic Isotope ◽  
Diffusion Controlled ◽  
Base Form ◽  
Ph Range ◽  
Conjugate Base

The pH dependence of the bromination of R-2,4-Me2ImH3+ [R=(NH3)5Co] by Br2 in acidic aqueous solution, giving R-2,4-Me2-5-BrImH3+, suggests that proton abstraction from a bromine-substituted Wheland intermediate is rate determining for pH < 3 (both OH-- catalysed and spontaneous paths are observed), while in the pH range 3-5, bromine addition is rate determining. For pH > 5 the reaction may be interpreted to occur through rate-determining Br2 addition to the conjugate base from of the reactant. Rate constants for bromine addition to R-2,4-Me2ImH3+ and R-2,4-Me2Im2+ are respectively 1.1×104 and 3.4×1010 dm3 mol-1 s-1 at 25.0°C, I = 1.0 (NaClO4). Bromination of RimH3+, to give R-4-BrImH3+ initially, appears to occur largely via the conjugate base form of the reactant (k2 = 3.6×109 dm3 mol-1 s-1), but for Ph < 2, addition of Br2 to RImH3+ contributes to the observed rate (k1 = 0.68 dm3 mol-1 s-1). Ea = 61�2 kJ mol-1 for the former process, and the primary kinetic isotope effect, kH/kD, increases from 1.3 in 0.05 M H+ to 2.4 in 5.42 M HClO4. These observations are discussed in terms of diffusion-controlled, or preassociation -type mechanisms.

Aspects of the hydrolysis of formamide: revisitation of the water reaction and determination of the solvent deuterium kinetic isotope effect in base

2002 ◽  
Vol 80 (10) ◽  
pp. 1343-1350 ◽  
Author(s):  
H Slebocka-Tilk ◽  
F Sauriol ◽  
Martine Monette ◽  
R S Brown
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Base Hydrolysis ◽  
Kinetic Isotope ◽  
Acid And Base ◽  
Acid Reaction ◽  
Ph Range ◽  
Hydrolysis Of

A study of the hydrolysis of formamide is reported with the aims of isolating the water reaction for hydrolysis from the acid and base hydrolysis terms and determining the solvent deuterium kinetic isotope effect (dkie) on base-catalyzed hydrolysis. Respective activation parameters (ΔH‡ and ΔS‡) of (17.0 ± 0.4) kcal mol–1 and (–18.8 ± 1.3) cal mol–1 K–1 for the acid reaction and (17.9 ± 0.2) kcal mol–1 and (–11.1 ± 0.5) cal mol–1 K–1 for the base reaction were determined from Eyring plots of the second-order rate constants over the range of 27–120°C. Kinetic studies at the minima of the pH/rate profiles in the pH range from 5.6 to 6.2 in MES buffers at 56°C, and in the pH range of 4.25–6.87 in acetate and phosphate buffers at 120°C are reported. At 56°C the available data fit the expression k56obs = 0.00303[H3O+] + 0.032[HO–] + (3.6 ± 0.1) × 10–9, while at 120°C the data fit k120obs = (0.15 ± 0.02)[H3O+] + (3.20 ± 0.24)[HO–] + (1.09 ± 0.29) × 10–6. Preliminary experimental estimates of Ea (ln A) of 22.5 kcal mol–1 (15.03) for the water rate constant (kw) are calculated from an Arrhenius plot of the 56 and 120°C data giving an estimated kw of 1.1 × 10–10 s–1 (t1/2 = 199 years) at 25°C. Solvent dkie values of kOH/kOD = 1.15 and 0.77 ± 0.06 were determined at [OL–] = 0.075 and 1.47 M, respectively. The inverse value is determined under conditions where the the first step of the reaction dominates and is analyzed in terms of a rate-limiting attack of OL–.Key words: formamide, activation parameters, water reaction, acid and base hydrolysis, solvent kinetic isotope effect.

Effect of changing pH upon the kinetic 13C-isotope effect in the decarboxylation of 4-methoxyanthranilic acid

1968 ◽  
Vol 46 (4) ◽  
pp. 563-566 ◽  
Author(s):  
G. E. Dunn ◽  
John Buccini
Keyword(s):  
Ionic Strength ◽  
Aqueous Solutions ◽  
Isotope Effect ◽  
Aromatic Ring ◽  
Kinetic Isotope Effect ◽  
13C Isotope ◽  
Kinetic Isotope ◽  
The One

The 13C-carboxyl kinetic isotope effect on decarboxylation of 4-methoxyanthranilic acid has been determined in aqueous solutions of ionic strength 0.50 at 60 °C. The isotope effect, 100(k12/k13 −1), is found to be 4.2% at pH −0.3, 1.4% at pH 1.3, and zero at pH 4.0. This information makes it possible to decide between two mechanisms previously proposed in favor of the one in which both anion and neutral acid are protonated at carbon-1 of the aromatic ring but only the protonated anion decarboxylates.

The mechanism of the thermal decarbonylation of 2,2-dimethyl-3-butenal

1977 ◽  
Vol 55 (22) ◽  
pp. 3951-3954 ◽  
Author(s):  
Robert J. Crawford ◽  
Stuart Lutener ◽  
Hirokazu Tokunaga
Keyword(s):  
Carbon Monoxide ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
First Order ◽  
First Order Kinetics ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium ◽  
Thermal Decarbonylation

The thermal decarbonylation of 2,2-dimethyl-3-butenal is shown to be an intramolecular extrusion of carbon monoxide concerted with the transfer of hydrogen (deuterium) to the γ-position. The reaction displays a kinetic isotope effect of 2.8 (at 296.9 °C) and follows first order kinetics (Ea = 44.2 ± 0.2 kcal mol−1, log A = 13.4 ± 0.3).

scholarly journals Solvent kinetic isotope effects of human placental alkaline phosphatase in reverse micelles

1998 ◽  
Vol 330 (1) ◽  
pp. 267-275 ◽  
Author(s):  
Ter-Mei HUANG ◽  
Hui-Chih HUNG ◽  
Tsu-Chung CHANG ◽  
Gu-Gang CHANG
Keyword(s):  
Aqueous Solution ◽  
Isotope Effect ◽  
Reverse Micelles ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Solvent System ◽  
Placental Alkaline Phosphatase ◽  
K Value ◽  
Kinetic Isotope ◽  
Human Placental Alkaline Phosphatase

Human placental alkaline phosphatase was embedded in a reverse micellar system prepared by dissolving the surfactant sodium bis(2-ethylhexyl) sulphosuccinate (Aerosol-OT) in 2,2,4-trimethylpentane. This microemulsion system provides a convenient instrumental tool to study the possible kinetic properties of the membranous enzyme in an immobilized form. The pL (pH/p2H) dependence of hydrolysis of 4-nitrophenyl phosphate has been examined over a pL range of 8.5-12.5 in both aqueous and reverse micellar systems. Profiles of log V versus pL were Ha-bell shaped in the acidic region but reached a plateau in the basic region in which two pKa values of 9.01-9.71 and 9.86-10.48, respectively, were observed in reverse micelles. However, only one pKa value of 9.78-10.27 in aqueous solution was detected. Profiles of log V/K versus pL were bell-shaped in the acidic region. However, they were wave-shaped in the basic region in which a residue of pKa 9.10-9.44 in aqueous solution and 8.07-8.78 in reverse micelles must be dehydronated for the reaction to reach an optimum. The V/K value shifted to a lower value upon dehydronation of a pKa value of 9.80-10.62 in aqueous solution and 11.23-12.17 in reverse micelles. Solvent kinetic isotope effects were measured at three pL values. At pL 9.5, the observed isotope effect was a product of equilibrium isotope effect and a kinetic isotope effect; at pL 10.4, the log V/K value was identical in water and deuterium. The deuterium kinetic isotope effect on V/K was 1.14 in an aqueous solution and 1.16 in reverse micelles. At pL 11.0 at which the log V values reached a plateau in either solvent system, the deuterium kinetic isotope effect on V was 2.08 in an aqueous solution and 0.62 in reverse micelles. Results from a proton inventory experiment suggested that a hydron transfer step is involved in the transition state of the catalytic reaction. The isotopic fractionation factor (ϕ) for deuterium for the transition state (ϕT) increased when the pH of the solution was raised. At pL 11.0, the ϕT was 1.07 in reverse micelles, which corresponds to the inverse-isotope effect of the reaction in this solvent system. Normal viscosity effects on kcat and kcat/Km were observed in aqueous solution, corresponding to a diffusional controlled physical step as the rate-limiting step. We propose that the rate-limiting step of the hydrolytic reaction changes from phosphate releasing in aqueous solution to a covalent phosphorylation or dephosphorylation step in reverse micelles.

Kinetics and Mechanism of the Oxidation of Some α-Hydroxyacids by Morpholinium Chlorochromate

2008 ◽  
Vol 33 (4) ◽  
pp. 393-405
Author(s):  
Neha Malani ◽  
Manju Baghmar ◽  
Preeti Swami ◽  
Pradeep Kumar Sharma
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Ion Transfer ◽  
Hydrogen Ions ◽  
Solvent Isotope Effect ◽  
First Order ◽  
Hydride Ion ◽  
Kinetic Isotope ◽  
Solvent Isotope ◽  
Hydride Ion Transfer

The oxidation of glycollic, lactic, malic and a few substituted mandelic acids by morpholinium chlorochromate (MCC) in dimethylsulfoxide (DMSO) leads to the corresponding oxoacids. The reaction is first order each in MCC and hydroxyacid. The reaction failed to induce the polymerisation of acrylonitrile. The oxidation of α–deuteriomandelic acid shows a primary kinetic isotope effect ( kH/ kD = 5.63 at 298 K) but does not exhibit a solvent isotope effect. The reaction is catalysed by hydrogen ions according to: kobs = a + b[H+]. The oxidation of p-methyl mandelic acid has been studied in 19 different organic solvents and the solvent effect analysed using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed.

scholarly journals Kinetics and Mechanism of the Oxidation of α-Hydroxy Carboxylic Acids by Bromine

1973 ◽  
Vol 28 (7-8) ◽  
pp. 450-453 ◽  
Author(s):  
Kalyan K. Banerji
Keyword(s):  
Isotope Effect ◽  
Hydroxy Acid ◽  
Kinetic Isotope Effect ◽  
Activation Parameters ◽  
Probable Mechanism ◽  
Molecular Bromine ◽  
Solvent Isotope Effect ◽  
First Order ◽  
Kinetic Isotope ◽  
Solvent Isotope

The oxidation of glycollic, lactic, u-hydroxybutyric, and 2-phenyllactic acids by aqueous bromine has been studied. The reaction is of first order with respect to the oxidant and the anion of the hydroxy acid respectively. The active oxidising species is molecular bromine. The oxidation of α,α-dideuterioglycollic acid indicated a kinetic isotope effect, kH/kD=4.62 at 25°C. The reaction does not show any appreciable solvent isotope effect. The activation parameters arc evaluated. A probable mechanism has been suggested.

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