Steric Effects in Acid-Catalyzed Decomposition and Base-Catalyzed Cyclization of 1-(2-Alkoxycarbonylphenyl)-3-phenyltriazenes

1996 ◽  
Vol 61 (5) ◽  
pp. 751-763 ◽  
Author(s):  
Oldřich Pytela ◽  
Aleš Halama
Keyword(s):  
Rate Constant ◽  
Nmr Spectra ◽  
Steric Effects ◽  
Catalytic Rate Constant ◽  
Rate Limiting Step ◽  
Acid Catalyzed ◽  
Catalytic Rate ◽  
Rate Limiting ◽  
Kinetics Of ◽  
Derivatives Of

Eight derivatives of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazene (R = methyl, ethyl, propyl, isopropyl, butyl, isobutyl, hexyl, and allyl) have been synthesized and their UV-VIS, IR, 1H and 13C NMR spectra measured. The NMR spectra have been interpreted in detail. The kinetics of acid-catalyzed decomposition and base-catalyzed cyclization of the title compounds have been measured in 52.1% w/w methanol at 25.0 °C. The unit reaction order has been verified and the cyclization product has been identified. The pH-profiles obtained have been used to calculate the catalytic rate constants kA (acid-catalyzed decomposition) and kB (base-catalyzed cyclization) of all the derivatives; the constants have been interpreted with regard to inductive and steric effects. The catalytic rate constant kA has been found to be independent of the substituents. The catalytic rate constant kB depends statistically significantly upon both inductive and steric effects, the sensitivity to the former being more significant. The experimental results and their interpretation confirm the base-catalyzed cyclization mechanism with formation of tetrahedral intermediate as the rate-limiting step.

Effect of Medium on Acid-Catalyzed Decomposition of N-(Phenylazo)-Substituted Nitrogen Heterocycles

1999 ◽  
Vol 64 (10) ◽  
pp. 1654-1672 ◽  
Author(s):  
Miroslav Ludwig ◽  
Iva Bednářová ◽  
Patrik Pařík
Keyword(s):  
Rate Constant ◽  
Principal Component ◽  
Catalyst Particle ◽  
Pivalic Acid ◽  
Linear Regression Method ◽  
Catalytic Rate Constant ◽  
Rate Limiting Step ◽  
Acid Catalyzed ◽  
Rate Limiting ◽  
Kinetics Of

Four N-(phenylazo)-substituted saturated nitrogen heterocyclics were synthesized and their structure was confirmed by 1H and 13C NMR spectroscopy. The kinetics of their acid-catalyzed decomposition were studied at various concentrations of the catalyst (pivalic acid) in 40, 30, and 20% (v/v) aqueous ethanol at 25 °C. The values obtained for the observed rate constants were processed by the non-linear regression method according to the suggested kinetic models and by the method of principal component analysis (PCA). The interpretation of the results has shown that the acid-catalyzed decomposition of the heterocyclics under the conditions used proceeds by the mechanism of general acid catalysis, the proton being the dominant catalyst particle of the rate-limiting step. The decrease in the observed rate constant at higher concentrations of the catalyst was explained by the formation of a non-reactive complex composed of the undissociated acid and the respective N-(phenylazo)heterocycle. The effect of medium and steric effect of the heterocyclic moiety on the values of catalytic rate constant are discussed.

Kinetics of cyclization of S-ethoxycarbonylmethylisothiouronium chloride to 2-imino-4-thiazolidone

1979 ◽  
Vol 44 (3) ◽  
pp. 912-917 ◽  
Author(s):  
Vladimír Macháček ◽  
Said A. El-bahai ◽  
Vojeslav Štěrba
Keyword(s):  
Hydrochloric Acid ◽  
Dilute Hydrochloric Acid ◽  
Base Catalysis ◽  
General Base ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Kinetics Of Formation ◽  
Acid Catalyzed ◽  
Rate Limiting ◽  
Kinetics Of

Kinetics of formation of 2-imino-4-thiazolidone from S-ethoxycarbonylmethylisothiouronium chloride has been studied in aqueous buffers and dilute hydrochloric acid. The reaction is subject to general base catalysis, the β value being 0.65. Its rate limiting step consists in acid-catalyzed splitting off of ethoxide ion from dipolar tetrahedral intermediate. At pH < 2 formation of this intermediate becomes rate-limiting; rate constant of its formation is 2 . 104 s-1.

High field 1H NMR Studies of Sol-Gel Kinetics

1986 ◽  
Vol 73 ◽  
Author(s):  
Bruce D. Kay ◽  
Roger A. Assink
Keyword(s):  
Equilibrium Distribution ◽  
Sol Gel ◽  
Nmr Studies ◽  
H Nmr ◽  
Rate Limiting Step ◽  
Rate Of Hydrolysis ◽  
Acid Catalyzed ◽  
Product Species ◽  
Rate Limiting ◽  
Kinetics Of

ABSTRACTHigh resolution 1H NMR spectroscopy at high magnetic fields is employed to study the reaction kinetics of the Si(OCH3)4:CH3OH:H2O sol-gel system. Both the overall extent of reaction as a function of time and the equilibrium distribution of species are measured. In acid catalyzed solution, condensation is the rate limiting step while in base catalyzed solution, hydrolysis becomes rate limiting. A kinetic model in which the rate of hydrolysis is assumed to be independent of the adjacent functional groups is presented. This model correctly predicts the distribution of product species during the initial stages of the sol-gel reaction.

Solvent effects on kinetics and mechanism of acid-catalyzed decomposition of 1,3-bis(4-methylphenyl)-triazene II. Reactions in aprotic solvents

1990 ◽  
Vol 55 (1) ◽  
pp. 156-164 ◽  
Author(s):  
Oldřich Pytela ◽  
Taťjana Nevěčná ◽  
Miroslav Ludwig
Keyword(s):  
Rate Constant ◽  
Acid Catalyst ◽  
Kinetics And Mechanism ◽  
Aprotic Solvents ◽  
Catalytic Rate Constant ◽  
Negative Effect ◽  
Acid Catalyzed ◽  
Catalytic Rate ◽  
Positive Effect ◽  
Solvent Acidity

The effect of aprotic solvents (hexane, cyclohexane, dichloromethane, 1,2-dichloroethane, benzene, acetonitrile, acetone, 1,2-dimethoxyethane, ethyl acetate, dioxane) on kinetics and mechanism of acid-catalyzed decomposition of 1,3-bis(4-methylphenyl)triazene has been studied with trichloroacetic acid as the acid catalyst. It has been found that beside the non-dissociated monomer of the acid also its dimer acts as the catalytic species. With regard to the results obtained in protic solvents (the catalysis by proton and general acid) three cases can be encountered of the dependence of observed rate constant on analytical concentration of the acid. The effect of solvents (inclusive of the protic ones) on the catalytic rate constant of the reaction with the non-dissociated monomer of acid is best interpreted by the equation suggested by Koppel and Palm and by the solvent scale suggested by us earlier. The solvent acidity and polarity have positive effect, whereas its basicity has negative effect. The catalytic rate constant of the reaction with the acid dimer decreases with increasing solvent basicity and polarity, due predominantly to the decrease in the equilibrium constant of dimerization.

The Kinetics of Reactions in and Near the Cytochrome b/f Complex of Chloroplast Thylakoids. I. Proton Deposition

1988 ◽  
Vol 15 (5) ◽  
pp. 695 ◽  
Author(s):  
AB Hope ◽  
J Liggins ◽  
DB Matthews
Keyword(s):  
Rate Constant ◽  
Bimolecular Reaction ◽  
Plastoquinone Pool ◽  
Rate Limiting Step ◽  
Fixed Concentration ◽  
Wide Range ◽  
Entropy Of Activation ◽  
Electron Donation ◽  
Rate Limiting ◽  
Kinetics Of

The kinetics of proton deposition in the intrathylakoid spaces of pea chloroplasts were measured under a wide range of conditions. With duroquinol added to reduce the plastoquinone pool, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea added to inhibit photosystem II, but no ionophore present, the proton deposition, attributed to plastoquinol oxidation, was biphasic. About half the deposition had an apparent rate constant (k) of 150-200 s-1, the other half about 10 s-1. Valinomycin or nonactin (<0.1 �M) plus potassium ions made the deposition almost monophasic, with k = 140 s-1. When the state of reduction of the plastoquinone pool was varied by the addition of varied concentrations of duroquinol, in the presence of 1 �M nonactin, k for proton deposition varied from about 20 (0.01 mM duroquinol) up to a maximum of 140 s-1 (0.5 mM duroquinol). When temperature was varied between 4 and 23°C, with 1 �M nonactin, an Arrhenius plot of ln(k) for proton deposition was linear; the activation enthalpy was 67 kJ mol-1, the entropy of activation, 23 J K-1 mol-1. The data are analysed in terms of a bimolecular reaction between a varying concentration of plastoquinol and a fixed concentration of oxidised Rieske centre. The results are consistent with a rate constant, for the first electron donation by plastoquinol, of 28 s-1 (the rate-limiting step), followed by a relatively fast second electron donation to cytochrome b563 (low potential), followed by deposition of two protons. The speed of the second proton deposition is dependent on the membrane potential difference.

Stereochemistry and kinetics of amines addition to acetylenic esters

1991 ◽  
Vol 69 (9) ◽  
pp. 1445-1449 ◽  
Author(s):  
Saber M. Sharaf ◽  
Samir K. El-Sadany ◽  
Ezzat A. Hamed ◽  
Abdel-Hamid A. Youssef
Keyword(s):  
Key Words ◽  
Nucleophilic Addition ◽  
Nmr Spectra ◽  
Acetylenic Esters ◽  
H Nmr ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting ◽  
Kinetics Of ◽  
Step Mechanism

The reactions of a series of methyl para-substituted phenylpropiolates 1a–e with piperidine, morpholine, and diethylamine in methanol and dimethylformamide (DMF) have been studied and their rates measured. The products were methyl β-(N,N-dialkylamino)-p-substituted cinnamates 2–4a–e. 1H NMR spectra were used to determine the configuration of the products. The ρ values in methanol ranged between 0.34 and 1.24 whereas in DMF they were between 0.85 and 1.88. The values of ΔS≠ favor a bimolecular rate-limiting step mechanism. Key words: nucleophilic addition to acetylenic esters.

Coupling Kinetics of Benzenediazonium Ions with 2,6-Dioxo-3-(p-substituted phenylhydrazono)-1,2,3,6-tetrahydropyridine-4-carboxylic Acid

1992 ◽  
Vol 57 (9) ◽  
pp. 1915-1927
Author(s):  
Jaroslava Horáčková ◽  
Vojeslav Štěrba
Keyword(s):  
Base Catalysis ◽  
Partial Replacement ◽  
Tetrahedral Intermediate ◽  
Buffer Solutions ◽  
General Base ◽  
Rate Limiting Step ◽  
Bisazo Compounds ◽  
Rate Limiting ◽  
Kinetics Of ◽  
Derivatives Of

The kinetics have been measured of the reactions of 4-nitro-, 4-chloro-, and 4-methoxybenzenediazonium ions with substituted phenylazo derivatives of citrazinic acid in buffer solutions, and the pKa values of the corresponding monoazo and bisazo compounds have been estimated. The reactions of 4-nitrobenzenediazonium ion with 4-chloro- and 4-methoxyphenylazo derivatives and of 4-chlorobenzenediazonium ion with 4-methoxyphenylazo derivative were accompanied by a partial replacement of the substituted phenylazo group by the 4-nitro- and 4-chlorophenylazo groups, respectively. The reactions of 4-chloro- and 4-methoxybenzenediazonium ions are subject to general base catalysis, the rate-limiting step consisting in the splitting off of the proton from the tetrahedral intermediate; with 4-nitrobenzenediazonium ion the reaction rate is limited by the formation of the tetrahedral intermediate.

Synthesis, kinetics and mechanism of cyclization of 1-(2-aryloxycarbonylphenyl)-3-phenyltriazenes

1990 ◽  
Vol 55 (11) ◽  
pp. 2692-2700 ◽  
Author(s):  
Oldřich Pytela ◽  
Zdeněk Bahník
Keyword(s):  
Kinetic Isotope Effect ◽  
Intramolecular Proton Transfer ◽  
Steric Effects ◽  
Aqueous Methanol ◽  
Kinetic Isotope ◽  
Reaction Constant ◽  
Acid Catalyzed ◽  
Rate Limiting ◽  
Kinetics Of ◽  
Substituted 1

Twelve substituted 1-(2-aryloxycarbonylphenyl)-3-phenyltriazenes have been synthetized and kinetics of their reactions have been measured in 52.1% (by mass) aqueous methanol at pH 3 to 11. Plots of kobs vs pH show three regions: noncatalyzed cyclization (pH 4 to 7), acid-catalyzed splitting of the triazene chain, and base-catalyzed cyclization. The non-catalyzed cyclization exhibits a kinetic isotope effect, the reaction constant ρ = 2.69 (σ-p), and β1g = 1.02, which indicates a mechanism of E1cB type with intramolecular proton transfer and a transient formation of a ketene intermediate. The base-catalyzed cyclization, on the other hand, exhibits the reaction constant ρ = 1.05 (σ-p), β1g = 0.4, and distinct steric effects, which indicates a cyclization by BAc2 mechanism with rate-limiting formation of the tetrahedral intermediate.

Low Temperature CVD of TiN from Ti(NR2)4 and NH3: FTIR Studies of the Gas-Phase Chemical Reactions

1993 ◽  
Vol 335 ◽  
Author(s):  
Bruce H. Weiller
Keyword(s):  
Reaction Time ◽  
Gas Phase ◽  
Rate Constant ◽  
Chemical Vapor ◽  
Rate Limiting Step ◽  
Tube Reactor ◽  
Ftir Spectrometer ◽  
Rate Limiting ◽  
Kinetics Of ◽  
Phase Chemical

AbstractThe gas-phase chemical reaction between Ti(NMe2)4 and NH3 is a critical step in the Metallorganic Chemical Vapor Deposition (MOCVD) of TiN at low temperatures. We have examined this reaction using a flow-tube reactor coupled to an FTIR spectrometer. A sliding injector provides control over the reaction time and the kinetics of reactive species can be measured as a function of the partial pressure of an added reagent. The disappearance of Ti(NMe2)4 was measured as a function of reaction time and NH3 pressure at 26°C. The resulting bimolecular rate constant is (1.1±0. 1) x 10-16 cm3molecules−1s−1 Dimethylamine is observed as a direct product from this reaction consistent with other studies. We have also measured the rate constant using ND3 and find a substantial isotope effect, kh/kd ≈2.4± 0.4. This indicates that H-atom transfer is involved in the rate limiting step. We show that these results can be explained by a mechanism comprised of transamination reactions with NH3.

Solvolysis kinetics of ethyl 3-ethoxy-3-iminopropanoate

1986 ◽  
Vol 51 (3) ◽  
pp. 677-683 ◽  
Author(s):  
Jaromír Kaválek ◽  
Josef Panchartek ◽  
Tomáš Potěšil ◽  
Vojeslav Štěrba
Keyword(s):  
Water Molecule ◽  
Rate Constant ◽  
Hydrolysis Rate ◽  
Nucleophilic Attack ◽  
Tetrahedral Intermediate ◽  
Hydrolysis Rate Constant ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting ◽  
Kinetics Of

Kinetics have been studied of hydrolysis and methanolysis of ethyl 3-ethoxy-3-iminopropanoate. The methanolysis rate constant is lower than the hydrolysis rate constant by about 3 orders of magnitude. The rate-limiting step of the hydrolysis consists in the nucleophilic attack of the protonated substrate by a water molecule, whereas that of the methanolysis consists in the decomposition of tetrahedral intermediate which is several orders of magnitude slower than the decomposition of the intermediate formed in the hydrolysis.

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