Reactions of 1,3-diacylthioureas with methoxide ion and with amines

1987 ◽  
Vol 52 (1) ◽  
pp. 120-131 ◽  
Author(s):  
Jaromír Kaválek ◽  
Josef Jirman ◽  
Vojeslav Štěrba
Keyword(s):  
Carbonyl Group ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Equilibrium Constants ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Order Of Magnitude ◽  
Acetyl Group ◽  
Methoxide Ion ◽  
Rate Limiting

Rate constants of base-catalyzed methanolysis and dissociation constants in methanol have been determined for benzoylthiourea (II), 1,3-diacetylthiourea (III), 1,3-dibenzoylthiourea (IV), and 1-acetyl-3-benzoylthiourea (V). With the diacyl derivatives III and IV, the reaction of methoxide ion with the neutral substrate is accompanied by that of methoxide with the substrate anion (at higher alkoxide concentrations). Above 0.1 mol l-1 CH3O(-), the rate constants are also affected by medium. The rate of the reaction of neutral diacyl derivative is decreased, and that of the reaction of methoxide with the substrate anion is rapidly increased. The dissociation constant of II is higher than that of acetylthiourea (I) by about one order of magnitude, but the attack of methoxide on the carbonyl group of II is about three times slower than that in I. The benzoyl group at the N1 nitrogen exhibits a greater activating influence (in both the rate and the equilibrium constants) on the other NHCOR group than the acetyl group does. With V the ratio of methanolysis rate constants is 9 : 1 in favour of the acetyl group. The reaction of diacetyl derivative III with 1-butanamine has been followed in butanamine buffers. At the lowest butanamine concentrations, the reaction is second order in the amine, and the rate-limiting step is the proton transfer from the intermediate to the second amine molecule. At the highest butanamine concentrations the reaction becomes first order in the amine, and the rate-limiting step changes to the attack of butanamine on the carbonyl group of diacetyl derivative III.

Kinetics and mechanism of methanolysis of benzoyl derivatives of substituted phenylureas and phenylthioureas

1984 ◽  
Vol 49 (9) ◽  
pp. 2103-2110 ◽  
Author(s):  
Jaromír Kaválek ◽  
Said El Bahaie ◽  
Vojeslav Štěrba
Keyword(s):  
Nitrogen Atom ◽  
Steric Hindrance ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Kinetics And Mechanism ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Order Of Magnitude ◽  
Rate Limiting ◽  
Derivatives Of

The methanolysis rate constants and dissociation constants have been measured of benzoyl derivatives of substituted phenylureas and phenylthioureas. The dissociation constants of the thio derivatives are higher by 1 order of magnitude and the rate constants are higher by 2 orders of magnitude than the respective values of the oxygen analogues. Logarithms of the rate and dissociation constants have been correlated with the Hammet σ constant; the ρ constant of the methanolysis of the oxygen derivatives is almost 2x higher than that of the thio derivatives, which is explained by a change in the rate-limiting step. Methylation of the phenyl nitrogen atom increases the acidity by almost 2 orders of magnitude. This effect is due obviously to steric hindrance to the conjugation with the adjacent carbonyl or thiocarbonyl group.

Reactions of 1-substituted 2,4,6-trinitrobenzenes with nucleophiles

1979 ◽  
Vol 44 (5) ◽  
pp. 1453-1459 ◽  
Author(s):  
Jaromír Kaválek ◽  
Ahmad Ashfaq ◽  
Vojeslav Štěrba
Keyword(s):  
Methyl Ether ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Nucleophilic Aromatic Substitution ◽  
Aromatic Substitution ◽  
Phenyl Ester ◽  
Methyl Derivative ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

Rate constants have been determined of nucleophilic aromatic substitution of 2,4,6-trinitrophenyl methyl ether (Ia), 2,4,6-trinitrophenyl ethanoate (Ic), 2,4,6-trinitrochlorobenzene (Ib), 2,4,6-trinitrodiphenyl ether (Id), 2,4,6-trinitro-4'-bromodiphenyl ether (Ie), 2,3',4,6-tetranitrodiphenyl ether (If) and 2,4,4',6-tetranitrodiphenyl ether (Ig) with methoxide, ethanoate and methyl cyanoethanoate (II) anions in methanol. For the compounds Ia,b rate and equilibrium constants of addition of the anion II(-) at positions 3 and 5 have been measured, too. In reactions of the compounds Ia to Ig with ethanoate anion the first (rate-limiting) step produces the phenyl ester Ic which reacts with a further ethanoate anion to give 2,4,6-trinitrophenol (Ih) and ethanoic anhydride. In reactions of the bromo derivative Ie and, to a still larger extent, compound Id the methyl derivative Ia is formed besides the compound Ih.

scholarly journals Beyond the Active Site: Mechanistic Investigations of the Role of the Secondary Coordination Sphere and Beyond on Multi-Electron Electrocatalytic Reactions and Their Relations Between Kinetics and Thermodynamics

2020 ◽  
Author(s):  
Vincent Wang
Keyword(s):  
Thermodynamic Parameters ◽  
Coordination Sphere ◽  
Active Site ◽  
Rate Constants ◽  
Reduction Reaction ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Secondary Coordination Sphere ◽  
Catalytic Rate ◽  
Rate Limiting

<p>The development of an electrocatalyst with a rapid turnover frequency, low overpotential and long-term stability is highly desired for fuel-forming reactions, such as water splitting and CO<sub>2</sub> reduction. The findings of the scaling relationships between the catalytic rate and thermodynamic parameters over a wide range of electrocatalysts in homogeneous and heterogeneous systems provide useful guidelines and predictions for designing better catalysts for those redox reactions. However, such relationships also suggest that a catalyst with a high catalytic rate is typically associated with a high overpotential for a given reaction. Inspired by enzymes, the introduction of additional interactions through the secondary coordination sphere beyond the active site, such as hydrogen-bonding or electrostatic interactions, have been shown to offer a promising avenue to disrupt these unfavorable relationships. Herein, we further investigate the influence of these cooperative interactions on the faster chemical steps, in addition to the rate-limiting step widely examined before, for molecular electrocatalysts with the structural and electronic modifications designed to facilitate the dioxygen reduction reaction, CO<sub>2</sub> reduction reaction and hydrogen evolving reaction. Based on the electrocatalytic kinetic analysis, the rate constants for faster chemical steps and their correlation with the corresponding thermodynamic parameters are evaluated. The results suggest that the effects of the secondary coordination sphere and beyond on these fuel-forming reactions are not necessarily beneficial for promoting all chemical steps and no apparent relation between rate constants and thermodynamic parameters are found in some cases studied here, which may implicate the design of electrocatalysts in the future. Finally, these analyses demonstrate that the characteristic features for voltammograms and foot-of-the-wave-analysis plots are associated with the specific kinetic phenomenon among these multi-electron electrocatalytic reactions, which provides a useful framework to probe the insights of chemical and electronic modifications on the catalytic steps quantitatively (i.e. kinetic rate constants) and to optimize some of critical steps beyond the rate-limiting step.</p>

scholarly journals Control analysis applied to single enzymes: can an isolated enzyme have a unique rate-limiting step?

1993 ◽  
Vol 294 (1) ◽  
pp. 87-94 ◽  
Author(s):  
G C Brown ◽  
C E Cooper
Keyword(s):  
Rate Constants ◽  
General Assumption ◽  
Control Analysis ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Steady State Rate ◽  
Carbamate Kinase ◽  
State Rate ◽  
Rate Limiting ◽  
Control Coefficients

Control analysis is used to analyse and quantify the concept of a rate-limiting step within an enzyme. The extent to which each rate constant within the enzyme limits the steady-state rate of the enzyme and the levels of enzyme intermediate species are quantified as flux and concentration control coefficients. These coefficients are additive and obey summation theorems. The control coefficients of triose phosphate isomerase, carbamate kinase and lactate dehydrogenase are calculated from literature values of the rate constants. It is shown that, contrary to previous assumption, these enzymes do not have a unique rate-limiting step, but rather flux control is shared by several rate constants and varies with substrate, product and effector concentrations, and with the direction of the reaction. Thus the general assumption that an enzyme will have a unique rate-limiting step is unjustified.

Structure reactivity relationship in the accelerated formation of 2,3-diarylquinoxalines in the microdroplets of a nebuliser

2019 ◽  
Vol 25 (6) ◽  
pp. 457-462
Author(s):  
Nadia Hayat ◽  
Nathan W Fenwick ◽  
Amie Saidykhan ◽  
Richard Telford ◽  
William HC Martin ◽  
...  
Keyword(s):  
Carbonyl Group ◽  
Nucleophilic Addition ◽  
Negative Charge ◽  
Appreciable Amount ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Structure Reactivity Relationship ◽  
Rate Limiting ◽  
Hammett Relationship

Competition experiments in which 1,2-phenylenediamine, C6H4(NH2)2, condenses with equimolar quantities of benzil, (C6H5CO)2, and a 3,3′- or 4,4′-disubstituted benzil (XC6H4CO)2 (X = F, Cl, Br, CH3 or CH3O) to form a mixture of 2,3-diphenylquinoxaline and the corresponding 2,3-diarylquinoxaline (Ar = XC6H4) in the microdroplets produced in a nebuliser allow a Hammett relationship with a ρ value of 1.85 to be developed for this accelerated condensation in the nebuliser. This structure reactivity relationship reveals that an appreciable amount of negative charge builds up on the carbon of the carbonyl group of the benzil during the rate-limiting step of the reaction, thus confirming that this process involves nucleophilic addition of the 1,2-phenylenediamine to the benzil. In general, the presence of an electron donating substituent, particularly in the 4 and 4′ positions, in the benzil retards the reaction, whereas an electron attracting substituent, especially in the 3 and 3′ position, accelerates it.

FLUXY: a simple code for computing steady-state metal fluxes at consuming (bio)interfaces, in natural waters

2008 ◽  
Vol 5 (3) ◽  
pp. 204 ◽  
Author(s):  
Zeshi Zhang ◽  
Jacques Buffle ◽  
Konstantin Startchev ◽  
Davide Alemani
Keyword(s):  
Steady State ◽  
Rate Constants ◽  
Natural Waters ◽  
Equilibrium Constants ◽  
Computer Time ◽  
Rate Limiting Step ◽  
Broad Size Distribution ◽  
Metal Flux ◽  
User Friendly ◽  
Rate Limiting

Environmental context. Until now there was no user-friendly code for metal flux computations in natural mixtures of aquatic complexants, which are however essential for prediction of metal bioavailability. The present paper describes the capabilities and limitations of one of the only two such codes presently available, called FLUXY. The results of FLUXY are compared with those of another code, and it is shown that it enables quick computation and is applicable to natural ligands under many environmental conditions. Abstract. The computation of metal fluxes at consuming interfaces like microorganisms or bioanalogical sensors is of great importance in ecotoxicology. The present paper describes the application of a simple code, FLUXY, for the computation of steady-state metal fluxes in the presence of a very large number of complexes, with broadly varying values of equilibrium constants, rate constants and diffusion coefficients. This code includes two major limiting assumptions, namely, (i) the existence of excess of ligand (L) compared with metal (M), and (ii) the fact that in a series of successive MLn complexes, the reaction is the rate-limiting step in flux computation. The domains of rate constants for which these assumptions are valid are tested systematically, and the corresponding errors are evaluated by comparison with the exact results given by another code: MHEDYN. FLUXY is then applied and compared with MHEDYN for case studies typical of aquatic systems, namely (i) a culture medium containing simple ligands; (ii) solutions of fulvic compounds including a broad distribution of complex stability and rate constants; and (iii) suspensions of aggregates with a broad size distribution. It is shown that FLUXY gives good results for cases (i) and (iii). Application to case (ii) (fulvic compounds) is also feasible under conditions that are clearly described. Altogether, FLUXY and MHEDYN are complementary. In particular, FLUXY only computes steady-state fluxes and requires the fulfilment of a few conditions, but when these are met, computations require much less computer time than MHEDYN.

scholarly journals Estimation of the dissociation constants of enzyme-substrate complexes from steady-state measurements. Interpretation of pH-independence of Km

1976 ◽  
Vol 153 (2) ◽  
pp. 455-461 ◽  
Author(s):  
A Cornish-Bowden
Keyword(s):  
Dissociation Constants ◽  
Michaelis Constant ◽  
Ph Profile ◽  
Substrate Complex ◽  
Enzyme Substrate ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Catalytic Constant ◽  
Almost All ◽  
Rate Limiting

If the Michaelis constant of an enzyme-catalysed reaction is independent of pH under conditions where the catalytic constant varies with pH, it is equal to the thermodynamic dissociation constant of the enzyme-substrate complex. This is true for realistic mechanisms in which binding and catalytic steps, are clearly distinguished, as well as for the simpler mechanisms that have been considered previously. It is also true for a mechanism in which a bell-shaped pH profile for the catalytic constant results from a change of rate-limiting step with pH. The relaxation time for ionization of a typical group in unbuffered solutions at 25 degrees C is of the order of 0.1 ms at the longest, and is much shorter in buffered solutions. Thus ionizations in almost all enzyme mechanisms can properly be treated as equilibria, provided that ionization is not accompanied by a slow, compulsory change in conformation.

The methanolysis kinetics and dissociation constants of 1-(subst. benzoyl)-3-phenylthioureas

1988 ◽  
Vol 53 (3) ◽  
pp. 593-600
Author(s):  
Jaromír Kaválek ◽  
Josef Jirman ◽  
Vladimír Macháček ◽  
Vojeslav Štěrba
Keyword(s):  
Hydrogen Bond ◽  
Carbonyl Group ◽  
Intramolecular Hydrogen Bond ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Effect Of Substituents ◽  
Benzoyl Group ◽  
Methoxide Ion ◽  
Reaction Constants ◽  
Intramolecular Hydrogen

A series of seven 1-(subst. benzoyl)-3-phenylthioureas have been prepared and their dissociation constants and solvolysis rate constants have been measured in methanol at 25 °C. The reaction constants found show that the solvolysis rate is limited by the attack of methoxide ion on the benzoyl carbonyl group of the non-dissociated substrate. The polar effect of substituents in benzoyl group is extensively transferred also by the intramolecular hydrogen bond.

scholarly journals Beyond the Active Site: Mechanistic Investigations of the Role of the Secondary Coordination Sphere and Beyond on Multi-Electron Electrocatalytic Reactions and Their Relations Between Kinetics and Thermodynamics

2020 ◽  
Author(s):  
Vincent Wang
Keyword(s):  
Thermodynamic Parameters ◽  
Coordination Sphere ◽  
Active Site ◽  
Rate Constants ◽  
Reduction Reaction ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Secondary Coordination Sphere ◽  
Catalytic Rate ◽  
Rate Limiting

<p>The development of an electrocatalyst with a rapid turnover frequency, low overpotential and long-term stability is highly desired for fuel-forming reactions, such as water splitting and CO<sub>2</sub> reduction. The findings of the scaling relationships between the catalytic rate and thermodynamic parameters over a wide range of electrocatalysts in homogeneous and heterogeneous systems provide useful guidelines and predictions for designing better catalysts for those redox reactions. However, such relationships also suggest that a catalyst with a high catalytic rate is typically associated with a high overpotential for a given reaction. Inspired by enzymes, the introduction of additional interactions through the secondary coordination sphere beyond the active site, such as hydrogen-bonding or electrostatic interactions, have been shown to offer a promising avenue to disrupt these unfavorable relationships. Herein, we further investigate the influence of these cooperative interactions on the faster chemical steps, in addition to the rate-limiting step widely examined before, for molecular electrocatalysts with the structural and electronic modifications designed to facilitate the dioxygen reduction reaction, CO<sub>2</sub> reduction reaction and hydrogen evolving reaction. Based on the electrocatalytic kinetic analysis, the rate constants for faster chemical steps and their correlation with the corresponding thermodynamic parameters are evaluated. The results suggest that the effects of the secondary coordination sphere and beyond on these fuel-forming reactions are not necessarily beneficial for promoting all chemical steps and no apparent relation between rate constants and thermodynamic parameters are found in some cases studied here, which may implicate the design of electrocatalysts in the future. Finally, these analyses demonstrate that the characteristic features for voltammograms and foot-of-the-wave-analysis plots are associated with the specific kinetic phenomenon among these multi-electron electrocatalytic reactions, which provides a useful framework to probe the insights of chemical and electronic modifications on the catalytic steps quantitatively (i.e. kinetic rate constants) and to optimize some of critical steps beyond the rate-limiting step.</p>

Mechanism of base-catalyzed cyclization of ethyl N-(substituted aminocarbonyl)glycinates

1987 ◽  
Vol 52 (1) ◽  
pp. 156-161
Author(s):  
Jaromír Mindl ◽  
Vojeslav Štěrba
Keyword(s):  
Rate Constants ◽  
Ethoxy Group ◽  
Tetrahedral Intermediate ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Acid Catalyzed ◽  
Cyclization Rate ◽  
Rate Limiting ◽  
Cyclic Intermediate

The cyclization rate constants have been measured of substituted ethyl N-(phenylaminocarbonyl)-, N-(alkylaminocarbonyl)-, and N-(phenylaminothiocarbonyl)glycinates RNHCXNHCH2CO2.C2H5 (X = O, S). Logarithms of these constants increase with decreasing basicity of the amines down to the value of pKa(RNH2) = 5.5. The rate-limiting step of the reaction is formation of the tetrahedral intermediate. With ethyl N-(phenylaminocarbonyl)glycinates (whose pKa(RNH2) values are higher) this dependence, on the contrary, slightly decreases, and the acid-catalyzed splitting off of ethoxy group from the cyclic intermediate becomes rate-limiting. The cyclization rate of a series of ethyl N-(phenylaminothiocarbonyl)glycinates is practically independent of the pKa(RNH2) values, the change in the rate-limiting step would take place at pH about 9.

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