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.

Studies on Sulphamic Acid and Related Compounds. II. Kinetics and Mechanism of Hydrolysis of Trimethylamine- and of Triethylamine Sulphur Trioxide Addition Compounds

1962 ◽  
Vol 15 (2) ◽  
pp. 251 ◽  
Author(s):  
BE Fleischfresser ◽  
I Lauder
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Aqueous Acetone ◽  
Second Order ◽  
Fluoride Ions ◽  
Order Rate ◽  
Mechanism Of Hydrolysis ◽  
Reaction With Water ◽  
Sulphamic Acid ◽  
Hydrolysis Of

The kinetics of hydrolysis of trimethylamine- and of triethylaminesulphur trioxide addition compounds have been studied in water and in aqueous acetone. Reaction occurs according to the equation,������������� f - + R,N.SO,+H,O-tR,XH+HSO~.The solvolysis reactions are first-order and are not catalyzed by acids. The halide ions, Cl', Br', and 1', show only a normal salt effect on the rate of hydrolysis of + - (CH,),N.SO, but in the presence of fluoride ions, the rate constant for the production + - of acid from (C,H,),N.SO, in water at 95 OC is about one-seventh of that in the absence of fluoride under the same conditions. It is suggested that the fluorosulphonate ion is formed rapidly, and that this ion then undergoes slow hydrolysis :�In the presence of alkali, using water as the solvent, second-order kinetics are observed, the equation for the reaction being,�������������� + - R,N.SO,+~OH-+R,X+SO~= +H,O. Assuming the reaction with water is bimolecular, the ratio of the (bimolecular) rate constants at 35 OC, ko~-/k~,o is approximately lo8 for each complex. In aqueous acetone, at low water concentrations, the hydrolysis reactions of the trialkylaminesulphur trioxide complexes show second-order kinetics. At 35 OC for the hydrolysis of + - (CH,),N.SO, the ratio of the second-order rate constant in aqueous acetone to the + - (calculated) second-order rate constant in water is approximately 550 ; for (C,H,),N.SO, the same ratio is 6900. It is considered that hydrolysis occurs in water and in aqueous acetone via a bi- molecular attack at sulphur.

scholarly journals The reactions of D-glyceraldehyde 3-phosphate with thiols and the holoenzyme of D-glyceraldehyde 3-phosphate dehydrogenase and of inorganic phosphate with the acyl-holoenzyme

1976 ◽  
Vol 159 (3) ◽  
pp. 513-527 ◽  
Author(s):  
J M Armstrong ◽  
D R Trentham
Keyword(s):  
Rate Constant ◽  
Active Sites ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Second Order ◽  
Rate Equations ◽  
Buffer System ◽  
Exponential Process ◽  
Order Rate ◽  
Rate Profile

D-Glyceraldehyde 3-phosphate forms adducts with thiols. These adducts, which are presumed to be hemithioacetals, equilibrate rapidly with the unhydrated form of the aldehyde, which is the subtrate for D-glyceraldehyde 3-phosphate dehydrogenase. The adduct provides a substrate buffer system whereby a constant low free aldehyde concentration can be maintained during the oxidation of aldehyde by the enzyme and NAD+. With this system, the kinetics of the association of the aldehyde with the enzyme were examined. The rate profile for this reaction is a single exponential process, showing that all four active sites of the enzyme have equivalent and independent reactivity towards the aldehyde, with an apparent second-order rate constant of 5 × 10(7)M-1-S-1 at pH8.0 and 21 degrees C. The second-order rate constant becomes 8 × 10(7)M-1-S-1 when account is taken of the forward and reverse catalytic rate constants of the dehydrogenase. The pH-dependence of the observed rate constant is consistent with a requirement for the unprotonated form of a group of pK 6.1, which is the pK observed for second ionization of glyceraldehyde 3-phosphate. The rate of phosphorolysis of the acyl-enzyme intermediate during the steady-state oxidative phosphorylation of the aldehyde was studied, and is proportional to the total Pi concentration up to at least 1 mM-Pi at pH 7.5. The pH-dependence of the rate of NADH generation under these conditions can be explained by the rate law d[NADA]/dt = k[acy] holoenzyme][PO4(3-)-A1, where thioester bond, although kinetically indistinguishable rate equations for the reaction are possible. The rates of the phosphorolysis reaction and of the aldehyde-association reaction decrease with increasing ionic strength, suggesting that the active site of the enzyme has cationic groups which are involved in the reaction of the enzyme with anionic substrates.

Reactivity of Silicates 1. Kinetic Studies of the Hydrolysis of Linear and Cyclic Siloxanes as Models for Defect Structure in Silicates

1986 ◽  
Vol 73 ◽  
Author(s):  
Carol A. Balfe ◽  
Kenneth J. Ward ◽  
David R. Tallant ◽  
Sheryl L. Martinez
Keyword(s):  
Rate Constant ◽  
Kinetic Studies ◽  
Order Rate Constant ◽  
Hydrolysis Rate ◽  
Rate Data ◽  
Cyclic Siloxanes ◽  
Highly Strained ◽  
Kinetics Of ◽  
Hydrolysis Of ◽  
Tetrahydrofuran Solution

ABSTRACTThe kinetics of hydrolysis of hexamethylcyclotrisiloxane and di-t-butyldimesitylcyclodisiloxane in tetrahydrofuran solution have been determined and compared to hydrolysis rates of silica defects. In the presence of sufficient excess witer, the first-order rate constant of the cyclotrisiloxine, k= 3.8 × 10−3 min is similar to the rate constant, k = 5.2 × 10−1 min, of the disappearance of the D2 Raman silica defect band it has been proposed to model. Limited hydrolysis rate data for the cyclodisiloxane suggests that it hydrolyzes at least four times faster than does the cyclotrisiloxane. These data are consistent with rate data available for silica crack growth and support the assignment of highly strained siloxane bonds at the crack tip to cyclodisiloxanes. Infrared spectra determined for the cyclodisiloxanes lend further support to this model.

The base hydrolysis of coordinated fluorophosphate

1984 ◽  
Vol 37 (10) ◽  
pp. 1999 ◽  
Author(s):  
II Creaser ◽  
RV Dubs ◽  
AM Sargeson
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Second Order ◽  
Base Hydrolysis ◽  
Order Rate ◽  
Measured Rate ◽  
Hydrolysis Of

[(NH3)5CoO3PF]+ undergoes base hydrolysis in 0.1-0.3 M NaOH, � = 1.0, at 25�C to generate free FPO32-, F- and [(NH3)4Co(OH)(NH2PO3)] with a second-order rate constant k2 3.3 × 10 mol-1 s-1 where k2 is the composite rate constant for FPO32- release and hydrolysis of F- with the two products being formed in approximately equal amounts. The measured rate constitutes an estimated enhancement of about 1010 over the base hydrolysis of the uncoordinated FPO32- ion under the same conditions when the concentration of the coordinated nucleophile is taken into account.

Solvolysis of exo- and endo-Tricyclo[3,3,0,02,8]oct-4-yl p-Toluenesulphonates

1975 ◽  
Vol 28 (6) ◽  
pp. 1311 ◽  
Author(s):  
RG Buckeridge ◽  
KJ Frayne ◽  
BL Johnson
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Aqueous Acetone ◽  
Subsequent Formation ◽  
First Order ◽  
Order Rate ◽  
Hydrolysis Of

The structure of endo-tricyclo[3,3,0,02,8]octan-4-ol (3; X = OH) has been confirmed by hydrogenolysis which affords the known alcohols endo- (equatorial)-bicyclo[3,2,1]octan-2-(11) and cis-bicyclo[3,3,0]octan- anti-2-ol (12). Hydrolysis of derived p-toluenesulphonate (3; X = OTs) in 70% aqueous acetone at 21.6� proceeds with a first-order rate constant of 6.67�0.21x10-4s-1, and under buffered conditions yields endo- tricyclo[3,3,0,02,8]octan-4-ol (3; X = OH) as the only observable product. The results suggest that ionization of (3; X = OTs) proceeds with participation of the C 1 to C2 bonding electrons to give the intermediate trishomocyclopropenyl cation (4) which suffers stereospecific solvent capture to yield (3; X = OH). The results obtained with the monodeuterated isotopomer (17; X = OTs) are consistent with this mechanism. Hydrolysis of exo- tricyclo[3,3,0,02,8]oct-4-yl p-toluenesulphonate (5; X = OTs) is a little slower than its epimer(3; X = OTs), and proceeds with a first-order rate constant of (1.9�0.04)x 10-4s-1 at 49.9� in 70% aqueous acetone. The mechanism in this instance appears to involve anchimerically assisted ionization and subsequent formation of the intermediate tricyclo[3,2,1,02,7]oct-6-yl cation (24)which yields a characteristic mixture of products consisting of endo-tricyclo[3,2,1,02,7]octan-6-ol(20; X = OH) (mainly), its epimer (21; X = OH), exo-bicyclo[3,2,1]oct-6-en- 2-ol (18; X =OH)and exo-bicyclo[2,2,2]oct-5-en-2-ol (19; X = OH).��� A reinvestigation of the buffered acetolysis of exo- tricyclo[3,2,1,02,7]oct-6-yl p-nitrobenzoate(21; X = OPnb) has shown that, contrary to previous conclusions, there is no leakage from the L series to the G series in this system.

The kinetics of the reaction of N,N-dimethyl-2-phenylaziridinium ion with bovine erythrocyte acetylcholinesterase

1970 ◽  
Vol 48 (3) ◽  
pp. 244-250 ◽  
Author(s):  
Jocelyn E. Purdie ◽  
R. M. Heggie
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Bovine Erythrocyte ◽  
Acidic Group ◽  
Ph Values ◽  
Order Rate ◽  
Decreasing Ph ◽  
Ph Range ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the hydrolysis of N,N-dimethyl-2-phenylaziridinium ion (DPA) have been studied over the pH range 5.5–8.0 as have the kinetics of the interaction of DPA with bovine erythrocyte acetyl-cholinesterase. The enzyme is initially inhibited reversibly and subsequently irreversibly towards acetylcholine hydrolysis. The hydrolysis of DPA was found to be pH independent over the range studied while the reversible noncompetitive inhibition increased with increasing pH, the data suggesting the requirement for a basic group on the enzyme with a pKa of about 6.5.Between pH values of 6.0 and 8.0 the kinetics of the irreversible inhibition are consistent with either of two kinetically indistinguishable mechanisms, one involving transformation of the initial reversible complex and the other an independent attack on the uncomplexed enzyme. The first mechanism gives rise to a first-order rate constant which is comparable with that for the hydrolysis of DPA but which increases with decreasing pH; an acidic group on the enzyme with pKa between 6.0 and 7.0 may be involved. The second-order rate constant arising from the second treatment goes through a maximum at pH 7.3. At pH 5.5 the kinetics are not consistent with either mechanism.

Spontaneous hydrolysis of heroin in buffered solution

1978 ◽  
Vol 56 (4) ◽  
pp. 665-667 ◽  
Author(s):  
Paul T. Smith ◽  
M. Hirst ◽  
C. W. Gowdey
Keyword(s):  
Liquid Chromatography ◽  
Rate Constant ◽  
Phosphate Buffer ◽  
Internal Standard ◽  
Order Rate Constant ◽  
Gas Liquid Chromatography ◽  
First Order ◽  
Order Rate ◽  
Spontaneous Hydrolysis ◽  
Hydrolysis Of

Electron-capture gas–liquid chromatography was used to study the spontaneous hydrolysis of heroin in phosphate buffer (pH 6.4 and pH 7.4) at 23 °C. Aliquots of solution were taken over a 24-h period. After extraction at pH 8.9 into propan-2-ol (10%) – ethyl acetate, deacetylated products were made into heptafluorobutyrate derivatives which were analyzed quantitatively using nalorphine as the internal standard. Heroin decomposes to O6-monoacetylmorphine (O6-MAM) under these conditions. Further decomposition to morphine was not observed. Spontaneous hydrolysis was faster at pH 7.4 (first-order rate constant, 9.6 × 10−5 min−1) than at pH 6.4 (first-order rate constant, 3.0 × 10−5 min−1). In 24 h, the decomposition to O6-MAM was 13 and 4%, respectively.

scholarly journals Reactions of l-ergothioneine and some other aminothiones with 2,2′- and 4,4′-dipyridyl disulphides and of l-ergothioneine with iodoacetamide. 2-Mercaptoimidazoles, 2- and 4-thiopyridones, thiourea and thioacetamide as highly reactive neutral sulphur nucleophiles

1974 ◽  
Vol 139 (1) ◽  
pp. 221-235 ◽  
Author(s):  
Jan Carlsson ◽  
Marek P. J. Kierstan ◽  
Keith Brocklehurst
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Base Catalysis ◽  
The Other ◽  
Deuterium Isotope Effect ◽  
General Base ◽  
Nucleophilic Reactivity ◽  
Ph Range

1. The reactions of 2,2′- and 4,4′-dipyridyl disulphide (2-Py–S–S–2-Py and 4-Py–S–S–4-Py) with l-ergothioneine (2-mercapto-l-histidine betaine), 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, thiourea, thioacetamide, 2-thiopyridone (Py–2-SH) and 4-thiopyridone (Py–4-SH) were investigated spectrophotometrically in the pH range approx. 1–9. 2. These reactions involve two sequential reversible thiol–disulphide interchanges. 3. The reaction of l-ergothioneine with 2-Py–S–S–2-Py and/or with the l-ergothioneine–Py–2-SH mixed disulphide, both of which provide Py–2-SH, is characterized by at least three reactive protonic states. This provides definitive evidence that neutral l-ergothioneine is a reactive nucleophile, particularly towards the highly electrophilic protonated disulphides. 4. A similar situation appears to obtain in the reactions of l-ergothioneine and Py–2-SH with 4-Py–S–S–4-Py and in the reactions of the other 2-mercaptoimidazoles, thiourea and Py–4-SH with 2-Py–S–S–2-Py. The nucleophilic reactivity of Py–4-SH suggests that general base catalysis provided by the disulphide in a cyclic or quasi-cyclic transition state is not necessary to generate nucleophilic reactivity in the other amino-thiones whose geometry could permit such catalysis. 5. The existence of a positive deuterium isotope effect in the l-ergothioneine–2-Py–S–S–2-Py system at pH6–7 provides no evidence for general base catalysis but is in accord with a mechanism involving specific acid catalysis and post-transition-state proton transfer. 6. The pH-dependences of the overall equilibrium positions of the various thiol–disulphide interchanges are described. 7. Reaction of thioacetamide with a stoicheiometric quantity of 2-Py–S–S–2-Py at pH1 provides 2 molecules of Py–2-SH per molecule of thioacetamide and elemental sulphur; these findings can be accounted for by thiol–disulphide interchange to provide a thioacetamide–Py–2-SH mixed disulphide followed by fragmentation to provide CH3CN, S and Py–2-SH. 8. Provision of high reactivity in the neutral forms of the members of this series of sulphur nucleophiles by electron donation by the amino group is compared with the well known α effect that provides enhanced nucleophilicity in compounds containing an electronegative atom adjacent to the nucleophilic atom. 9. The decrease in the u.v. absorption of l-ergothioneine at 257nm consequent on transformation of its aminothione moiety into an S-alkyl-2-mercaptoimidazole moiety provides a convenient method of following the alkylation of l-ergothioneine by iodoacetamide. 10. The pH dependence of the extinction coefficient of l-ergothioneine at 257nm is described by ε257={8×103/(1+Ka/[H+]} +6×103m−1·cm−1 in which pKa=10.8. 11. In the pH range 3–11 the reaction is characterized by two reactive protonic states (X and XH). 12. The X state, reaction of the ionized 2-mercaptoimidazole moiety of the l-ergothioneine dianion with neutral iodoacetamide, is characterized by the second-order rate constant 4.0m−1·s−1 (25.0°C, I=0.05). The XH state, characterized by the second-order rate constant 0.03m−1·s−1, is interpreted as reaction of the thione form of the neutral 2-mercaptoimidazole moiety of the l-ergothioneine monoanion with neutral iodoacetamide. 13. The XH state of the alkylation reaction does not exhibit a deuterium isotope effect.

scholarly journals The determination of specificity constants in enzyme-catalysed reactions

1986 ◽  
Vol 239 (1) ◽  
pp. 221-224 ◽  
Author(s):  
I E Crompton ◽  
S G Waley
Keyword(s):  
Rate Constant ◽  
Competitive Inhibition ◽  
Order Rate Constant ◽  
Beta Lactamase ◽  
Initial Concentration ◽  
First Order ◽  
Order Rate ◽  
Accurate Procedure ◽  
Hydrolysis Of

A convenient and accurate procedure for determining the kinetic parameter Vmax./Km is described. This avoids the error in the usual method of taking the observed first-order rate constant of an enzymic reaction at low substrate concentration as Vmax./Km. A series of reactions is used in which the initial concentration of substrate is below Km (e.g. from 5% to 50% of Km). Measurements are taken over the same extent of reaction (e.g. 70%) for each member of the series, and treated as if the kinetics were truly first-order. The reciprocal of the observed first-order rate constant is then plotted against the initial concentration of substrate: the reciprocal of the ordinate intercept is Vmax./Km. The procedure, as well as being applicable to simple reactions, is shown to be valid when there is competitive inhibition by the product, or when the reaction is reversible, or when there is competitive or mixed inhibition. The hydrolysis of cephalosporin C by a beta-lactamase from Pseudomonas aeruginosa is used to illustrate the method.

The hydrolysis of an activated ester by a tris(4,5-di-n-propyl-2-imidazolyl)phosphine-Zn2+ complex in neutral micellar medium as a model for carbonic anhydrase

2002 ◽  
Vol 80 (2) ◽  
pp. 183-191 ◽  
Author(s):  
Terry B Koerner ◽  
R S Brown
Keyword(s):  
Carbonic Anhydrase ◽  
Rate Constant ◽  
Titration Curve ◽  
Order Rate Constant ◽  
Second Order ◽  
Slow Step ◽  
Order Rate ◽  
Brij 35 ◽  
Triton X ◽  
Hydrolysis Of

The properties of tris(4,5-di-n-propyl-2-imidazolyl)phosphine–M2+ complexes (3–M2+, M = Zn, Co) in neutral micellar media of Brij-35 and Triton X-100 have been studied in water with respect to their quantitative potenti metric titration, Co2+-visible absorption spectra, and ability of the 3–Zn2+ complex to promote the hydrolysis of the activated ester, p-nitrophenyl acetate (PNPA). Potentiometric titration of the 3–M2+(CIO4–)2 complexes in 20 mM Brij-35 media yields a steep titration curve indicative of the cooperative consumption of two hydroxides, with computed pK1 and pK2 values of 8.75 and 6.25, respectively, and the midpoint of the titration curve (pKapp) being 7.50. A similar titration of the Co2+ complex also indicates cooperative consumption of two HO–, and this is tied to the formation of a 4- or 5-coordinate complex, pKapp ~ 7.3–7.4. The cooperativity is explained in terms of sequential replacement of the two CIO4– ions associated with the 3–M2+ to eventually yield 3–M2+–HO–/(HO–(H2O)n) having the first hydroxide ligated to the metal ion and the second associated as an ion pair. The 3–Zn2+ complex catalyzes the hydrolysis of PNPA in 20 mM Brij-35 and 40 mM Triton X-100. Plots of the observed second order rate constant (k2) vs. pH in Brij-35 increase linearly with pH and plateau to a value of k2max = 0.86 M–1 s–1, with a kinetic pKa of 8.7. These data are analyzed by a process wherein the 3–Zn2+–HO– is kinetically active in the rate-limiting step of the reaction, while the ion-paired (HO–(H2O)n) exists as a spectator to the slow step, possibly promoting rapid breakdown of a tetrahedral intermediate. Analysis of the kinetic data in terms of a model that accounts for the partitioning of PNPA between water and hydrophobic micellar pseudophase indicates that the second-order rate constant of the micelle-bound ester is augmented by 45-fold due to loading of the PNPA substrate into the micelle. Key words: Brij-35, TritonX-100, neutral micelle, carbonic anhydrase model, kinetics, potentiometric titrations, catalysis, p-nitrophenyl acetate hydrolysis.

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