An anomalous effect of methyl group on acidity of acylthioureas

1987 ◽  
Vol 52 (8) ◽  
pp. 1992-1998 ◽  
Author(s):  
Jaromír Kaválek ◽  
Josef Jirman ◽  
Vladimír Macháček ◽  
Vojeslav Štěrba
Keyword(s):  
Hydrogen Atom ◽  
Rate Constant ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Acetyl Derivative ◽  
Methyl Groups ◽  
Anomalous Effect ◽  
Methyl Group ◽  
Methyl Derivatives

Dissociation constants and methanolysis rate constants have been measured of 1-acetyl- and 1-benzoylthioureas and their N-methyl derivatives. Replacement of hydrogen atom at N(1) (next to the acyl group) by methyl group increases the acidity of the benzoyl derivative by one order, that of the acetyl derivative by as much as two orders of magnitude. Replacement of both hydrogens at N(3) by methyl groups lowers the methanolysis rate constant by more than two orders, whereas the replacement of hydrogen atom at N(1) by methyl group increases the methanolysis rate by the factor of 30.

Radiolysis studies on the corrosion inhibitors 1 -hexyn-3-ol, cinnamonitrile, and 1,3-diethyl-2-thiourea

1995 ◽  
Vol 73 (12) ◽  
pp. 2137-2142 ◽  
Author(s):  
A.J. Elliot ◽  
M.P. Chenier ◽  
D.C. Ouellette
Keyword(s):  
Hydrogen Atom ◽  
Hydroxyl Radical ◽  
Temperature Dependence ◽  
Rate Constant ◽  
Rate Constants ◽  
Corrosion Inhibitors ◽  
Hydrated Electron

In this publication we report: (i) the rate constants for reaction of the hydrated electron with 1-hexyn-3-ol ((8.6 ± 0.3) × 108 dm3 mol−1 s−1 at 18 °C), cinnamonitrile ((2.3 ± 0.2) × 1010 dm3 mol−1 s−1 at 20 °C), and 1,3-diethyl-2-thiourea ((3.5 ± 0.3) × 108 dm3 mol−1 s−1 at 22 °C). For cinnamonitrile and diethylthiourea, the temperature dependence up to 200 °C and 150 °C, respectively, is also reported; (ii) the rate constants for the reaction of the hydroxyl radical with 1-hexyn-3-ol ((5.5 ± 0.5) × 109 dm3 mol−1 s−1 at 20 °C), cinnamonitrile ((9.2 ± 0.3) × 109 dm3 mol−1 s−1 at 21 °C), and diethylthiourea ((8.0 ± 0.8) × 108 dm3 mol−1 s−1 at 22 °C). For cinnamonitrile, the temperature dependence up to 200 °C is also reported; (iii) the rate constant for the hydrogen atom reacting with 1-hexyn-3-ol ((4.3 ± 0.4) × 109 dm3 mol−1 s−1 at 20 °C). Keywords: radiolysis, corrosion inhibitors, rate constants.

Kinetics of acid-catalyzed cyclization of substituted hydantoinamides to substituted hydantoins

1987 ◽  
Vol 52 (8) ◽  
pp. 1999-2004 ◽  
Author(s):  
Jaromír Kaválek ◽  
Vladimír Macháček ◽  
Gabriela Svobodová ◽  
Vojeslav Štěrba
Keyword(s):  
Hydrogen Atom ◽  
Nitrogen Atom ◽  
Hydrochloric Acid ◽  
Dissociation Constants ◽  
Amide Group ◽  
The Other ◽  
Methyl Group ◽  
Acid Catalyzed ◽  
Cyclization Rate ◽  
Kinetics Of

The kinetics of acid-catalyzed cyclization of the hydantoinamides type R3-N(5)H-CO-N(3)R2-CH2-CO-N(1)HR1 (R1, R2, R3 = H and/or CH3) has been studied in 0·5 to 5 mol l-1 hydrochloric acid. The cyclization rate is limited by the rate of the attack of nitrogen atom N(5) on the carbon atom of the protonated amide group. The dissociation constants of the protonated hydantoinamides and rate constants of their cyclizations have been determined. Replacement of hydrogen atom by methyl group at the N(5) nitrogen atom accelerates the cyclization about two times., the same substitution at N(3) accelerates about 50x, whereas at N(1) it results in a 300 fold retardation. With the hydantoinamides having R3 = CH3, the cyclization rate of the protonated hydantoinamide increases with increasing concentration of hydrochloric acid, whereas with the other derivatives this value is independent of the acid concentration.

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Pre-Reactive Complexes and a Bifurcation

2019 ◽  
Author(s):  
Khoa T. Lam ◽  
Curtis J. Wilhelmsen ◽  
Theodore Dibble
Keyword(s):  
Hydrogen Atom ◽  
Quantum Chemistry ◽  
Transition State ◽  
Rate Constant ◽  
Rate Constants ◽  
Minimum Energy ◽  
Atmospheric Oxidation ◽  
Minimum Energy Path ◽  
Oh Radical ◽  
Hydrogen Atoms

Models suggest BrHgONO to be the major Hg(II) species formed in the global oxidation of Hg(0), and BrHgONO undergoes rapid photolysis to produce the thermally stable radical BrHgO•. We previously used quantum chemistry to demonstrate that BrHgO• can, like OH radical, readily can abstract hydrogen atoms from sp<sup>3</sup>-hybridized carbon atoms as well as add to NO and NO<sub>2</sub>. In the present work, we reveal that BrHgO• can also add to C<sub>2</sub>H<sub>4</sub> to form BrHgOCH<sub>2</sub>CH<sub>2</sub>•, although this addition appears to proceed with a lower rate constant than the analogous addition of •OH to C<sub>2</sub>H<sub>4</sub>. Additionally, BrHgO• can readily react with HCHO in two different ways: either by addition to the carbon or by abstraction of a hydrogen atom. The minimum energy path for the BrHgO• + HCHO reaction bifurcates, forming two pre-reactive complexes, each of which passes over a separate transition state to form a different product.

Peptide Free-Radicals: The Reactions of OH Radicals with Glycine Anhydride and its Methyl Derivatives Sarcosine and Alanine Anhydride. A Pulse Radiolysis and Product Study

1989 ◽  
Vol 44 (8) ◽  
pp. 959-974 ◽  
Author(s):  
Oliver J. Mieden ◽  
Clemens von Sonntag
Keyword(s):  
Hydrogen Atom ◽  
Dose Rate ◽  
Rate Constant ◽  
Pulse Radiolysis ◽  
Solute Concentration ◽  
Oh Radicals ◽  
Initial Attack ◽  
Product Analysis ◽  
H Nmr ◽  
Methyl Derivatives

The reactions of radiolytically generated OH radicals and H atoms with the cyclic dipeptides of glycine, alanine and sarcosine in deoxygenated aqueous solutions and the subsequent reactions of the transient peptide radicals were studied in the absence and presence of K3Fe(CN)6 as oxidant by pulse radiolysis and product analysis.Hydroxyl radicals and H atoms react with glycine anhydride and alanine anhydride by abstracting an H atom bound at C-3; there is no evidence for any other site of attack at these two peptides. The resulting radicals have pKa values of 9.8 and 10.6, respectively.In the absence of an oxidant the radicals decay by second order (2k = 7.0×108 dm3 mol-1 s-1 and 2k = 4.4×108 dm3 mol-1 s-1, resp.), the main fraction (94% of the glycine anhydride-derived radicals, 90% of the alanine anhydride-derived radicals) yielding dehydrodimers (G = 0.58 μmol J-1 and 0.56 µmol J-1 (in monomer units), resp.). A small portion however disproportionates via abstraction of a C-6-bound Η atom followed by isomerization to 2,5-dihydroxypyrazines (pKa values of the parent 2,5-dihydroxypyrazine at about 7.9 and 10.1) and subsequent addition of water to 2,5-diketo-3-hydroxypiperazines, thus indicating that the transfer of a carbon-bound hydrogen atom is prefered to the transfer of a nitrogen-bound hydrogen atom.No disproportionation products but three different dehydrodimers (G = 0.36, 0.18 and 0.04 µmol J-1 (in monomer units)) were found after irradiation of sarcosine anhydride. In this case a dose rate and solute concentration dependence of dehydrodimer formation indicates a radical-solute reaction converting part of the N-methyl radicals (21% of ‘initial’ attack) into the C-3-yl radicals. A rate constant of k = 600 ± 50 dm3 mol-1 s-1 was obtained for this reaction by measuring and computing the dehydrodimer yields as a function of dose rate and solute concentration. Thus the observed transient spectrum accounts only for about 79% of the radicals from the ‘initial’ attack at C-3.The rate of oxidation of the glycine anhydride-derived radicals by Fe(CN)63- reflects the pKa of the transient radical. The rate constant for oxidation of the (protonated) radical derived from glycine anhydride is: k = 1.0x 108 dm3 mol-1, the corresponding radical anion is oxidized with k = 3.1 × 108 dm3 mol-1 s-1. No change with pH was observed in the case of the alanine anhydridederived radicals (k = 7.9x 108 dm3 mol-1 s-1). In contrast to the disproportionation, oxidation by Fe(CN)63- leads to the removal of a proton from the heteroatom, a carbocation being the intermediate. The resulting dehydropiperazines rapidly add water to yield the corresponding 2,5-diketo-3-hydroxypiperazines (G = 0.61 μmol J-1 after oxidation of the glycine anhydride-derived radicals, G = 0.58 µmol J-1 after oxidation of the alanine anhydride-derived radicals). The radicals derived from sarcosine anhydride are readily oxidized with k = 4.0×108 dm3 mol-1 s-1, independent of pH.1H and 13C{1H} NMR-spectroscopic and mass-spectroscopic data of the products are given.

The mechanism of the pyrolysis of 2, 2, 3, 3-tetramethylbutane

1978 ◽  
Vol 363 (1715) ◽  
pp. 503-523 ◽  
Keyword(s):  
Hydrogen Atom ◽  
Rate Constant ◽  
Rate Constants ◽  
Volume Ratio ◽  
Reaction Scheme ◽  
Secondary Source ◽  
Kinetic Characteristics ◽  
Hydrogen Atoms ◽  
Reaction Conditions ◽  
Total Product

The pyrolysis of 2, 2, 3, 3-tetramethylbutane (TMB) was investigated in the ranges 699-735 K and 3-19 Torr (0.4-2.5 kPa) at up to 4% decomposition. The reaction is strongly self-inhibited and sensitive to the surface/volume ratio of the reaction vessel. A simple Rice-Herzfeld chain terminated by the heterogeneous removal of hydrogen atoms is proposed for the initial, uninhibited reaction generating isobutene and hydrogen in a 2:1 ratio. Self-inhibition is due to abstraction by hydrogen atoms of hydrogen atoms from product isobutene giving resonance-stabilized 2-methylallyl radicals which participate in homogeneous termination reactions. The kinetic characteristics of the major primary products (> 95% on a mole basis), isobutene and hydrogen, are accounted for when reasonable values are assumed for the rate constants for hydrogen atom abstraction by hydrogen atoms from TMB and from isobutene and for initiation and heterogeneous termination of the chain reaction. The kinetic characteristics of the formation of methane and propene (2-4% of total product) are accounted for by the secondary reaction scheme H + i-C 4 H 8 → i-C 4 H 9 , i-C 4 H 9 → CH 3 + C 3 H 6 , CH 3 + TMB → CH 4 + C 8 H 17 , when a reasonable value for the rate constant for the hydrogen atom addition to isobutene is assumed. The kinetic characteristics of the formation of ethene ( ca . 0.1% of total product) are accounted for by the tertiary reaction scheme H + C 3 H 6 → n -C 3 H 7 n -C 3 H 7 → CH 3 + C 2 H 4 , when a reasonable value for the rate constant for the hydrogen atom addition to propene is assumed. The kinetic characteristics of the formation of isobutane ( ca . 1% of total product) are much less affected by an increase in surface/volume ratio of the reactor than are those of the other products. A heterogeneous, secondary source is suggested, viz. 1/2H 2 ( g ) ⇌ H (wall), H (wall) + t-C 4 H 9 ( g ) ⇌ i-C 4 H 10 ( g ), which can generate the observed dependence of the isobutane yield on the reaction conditions but the reasonableness or otherwise of the values of the equilibrium and rate constants it is necessary to postulate is impossible to assess without further work designed specifically to investigate this problem.

Synthesis, characterization, and investigation of the antioxidant activity of some 1,2,4-benzothiadiazine-1,1-dioxides bearing sulfonylthioureas moieties

2019 ◽  
Vol 97 (12) ◽  
pp. 824-832 ◽  
Author(s):  
Kamel Harrouche ◽  
Asma Lahouel ◽  
Mebrouk Belghobsi ◽  
Bernard Pirotte ◽  
Smail Khelili
Keyword(s):  
Lipid Peroxidation ◽  
Antioxidant Activity ◽  
Ascorbic Acid ◽  
Hydrogen Atom ◽  
Crucial Role ◽  
Reducing Power ◽  
Methyl Groups ◽  
Methyl Group ◽  
The Future ◽  
Reducing Power Assay

A series of 1,2,4-benzothiadiazine-1,1-dioxides bearing a sulfonylthiourea moiety were synthesized, characterized, and screened for their antioxidant activity, using six antioxidant analytical assays comparatively to reference compounds, ascorbic acid and quercetin. The results indicated that several compounds demonstrated strong antioxidant activity in DPPH, ABTS, H2O2, and lipid peroxidation assays where some of them were either as active as or more active than reference compounds. However, all compounds were largely less active than references compounds in the reducing power assay. The results indicated that the thiourea moiety probably played a crucial role in the antioxidant activity of the target compounds, as a thiolate ion. The most favorable R1 groups were the hydrogen atom and methyl group, followed by phenyl and benzyl groups, whereas the most favorable R2 group was iPr, followed by the phenyl and methyl groups. The combination of benzothiadiazine ring with sulfonylthiourea moieties led to valuable new antioxidants, which could be used in the treatment or the prevention of certain diseases or in the field of cosmetics, which needs further investigations in the future.

Flame structure and flame reaction kinetics - V. Investigation of reaction mechanism in a rich hydrogen+nitrogen+oxygen flame by solution of conservation equations

1970 ◽  
Vol 317 (1529) ◽  
pp. 235-263 ◽  
Keyword(s):  
Reaction Mechanism ◽  
Hydrogen Atom ◽  
Reaction Zone ◽  
Rate Constant ◽  
Rate Constants ◽  
Time Dependent ◽  
Flame Velocity ◽  
Atom Concentration ◽  
Comparison With Experiment ◽  
Oxygen Flame

A powerful combination of two computational methods has been used to investigate the reaction mechanism in a fuel-rich hydrogen+nitrogen+oxygen flame. The first of these involves the solution of the time-dependent heat conduction and diffusion equations by finite difference methods. It allows a preliminary assessment of reaction mechanisms and rate constants which must be used to reproduce the observed flame velocity. However, the transport fluxes are only represented approximately in this time-dependent model, so that a precise calculation of flame profiles cannot be made. The second computational method uses a Runge–Kutta procedure to calculate the steady-state flame profiles, and is an extension of the methods discussed by Dixon-Lewis (1968). It incorporates detailed transport property calculations, and thus allows computation of detailed flame profiles for comparison with experiment. Application of the methods to the rich hydrogen+nitrogen+oxygen flame and subsequent comparison with experiment has established the participation of hydroperoxyl in the flame mechanism, and has shown the principal reactions in the flame to be: OH + H 2 = H 2 O + H, (i) H + O 2 =OH + O, (ii) O + H 2 =OH + H, (iii) H + O 2 + M = HO 2 + M, (iv) H + HO 2 = OH + OH, (vii) H + HO 2 = H 2 + O 2 , (xii) H+ H + M = H 2 + M. (xv) It was found that the interplay between these reactions is such that it is impossible to use the atmospheric pressure flame for an independent, precise determination of the hydrogenoxygen chain branching-rate constant k 2 . Another property of the mechanism is that the hydrogen atom concentration profile in the flame is not very dependent on the precise rate constants employed, so that the profile itself can be computed probably to better than ±10%. The reaction zone of the very rich flame commences at about 550 K, the maximum overall reaction rate is at about 900 K, and the maximum hydrogen atom concentration is at 1030 to 1040 K. The rate constant ratio k 7 / k 12 is found to lie in the range 5±1, assumed independent of temperature over the reaction zone. Assuming equal efficiencies of all the molecules in the flame as third bodies in the hydrogen atom recombination, the rate constant k 15 is estimated to lie in the range 4.5±1.5 x 10 15 cm 6 mol -2 s -1 .

Proton transfer reactions between ortho-methyl substituted derivatives of 4-nitrophenylphenylcyanomethane and nitrogen bases in acetonitrile solvent

1988 ◽  
Vol 66 (6) ◽  
pp. 1454-1458 ◽  
Author(s):  
Kenneth T. Leffek ◽  
Przemyslaw Pruszynski
Keyword(s):  
Proton Transfer ◽  
Isotope Effect ◽  
Rate Constants ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Activation Parameters ◽  
Transfer Reactions ◽  
Methyl Groups ◽  
Methyl Group ◽  
Proton Transfer Reactions

Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters have been measured for the proton transfer reactions in acetonitrile solvent of 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenyl-cyanomethane with tetramethylguanidine base and for the reactions of 2-methyl-4-nitrophenylphenylcyanomethane and 2,6-di-methyl-4-nitrophenylphenylcyanomethane with 1,5-diazabicyclo[5.4.0]undec-7-ene base. Introduction of the ortho-methyl groups in the substrate molecule caused significant reductions in the equilibrium and rate constants. The expected rise in the kinetic primary deuterium isotope effect was not observed when the first ortho-methyl group was introduced, but a 20% increase did accompany the introduction of the second ortho-methyl group. Enthalpy of activation measurements indicated that there was no increase in the proton tunnelling contribution to the isotope effect when the amount of steric hindrance is increased with ortho-methyl groups.

Kinetics and mechanism of hydrolysis of aryl N-methoxycarbamates and their derivatives

1983 ◽  
Vol 48 (3) ◽  
pp. 900-905 ◽  
Author(s):  
Jaromír Mindl ◽  
Vojeslav Štěrba
Keyword(s):  
Rate Constants ◽  
Dissociation Constants ◽  
Ion Concentration ◽  
Ph Dependency ◽  
Hydroxyl Ion ◽  
Mechanism Of Hydrolysis ◽  
Elimination Mechanism ◽  
Methyl Derivatives ◽  
Ph Range ◽  
Hydrolysis Of

Hydrolysis kinetics have been studied of 3- and 4-substituted phenyl N-methoxycarbamates and their N-methyl derivatives in aqueous buffers at 60 °C. The N-methyl derivatives show linear dependence of the rate constants on concentration of hydroxyl ion in the pH range measured. Hydrolysis of aryl N-methoxycarbamates is independent of hydroxyl ion concentration at higher pH values. Logarithms of the rate constants have been correlated with the substituent constants σ. The calculated values 0.9 for N-methyl derivatives, 4.5 and 3.3 for aryl N-methoxy-carbamates in the region of linear pH-dependency and pH-independency, respectively, suggest that the hydrolysis follows the BAC2 and ElcB mechanisms in the case of the N-methyl derivatives and aryl N-methoxycarbamates, respectively. Difference between the two ρ constants for the hydrolysis of aryl N-methoxycarbamates agrees with the found value ρ = 1.2 for dissociation constants of these compounds. The elimination mechanism has been confirmed by reaction of the isocyanate formed with added aniline to give the respective urea.

Hydrogen atom transfer reactions of ferrate(vi) with phenols and hydroquinone. Correlation of rate constants with bond strengths and application of the Marcus cross relation

2016 ◽  
Vol 45 (1) ◽  
pp. 70-73 ◽  
Author(s):  
Jianhui Xie ◽  
Li Ma ◽  
William W. Y. Lam ◽  
Kai-Chung Lau ◽  
Tai-Chu Lau
Keyword(s):  
Hydrogen Atom ◽  
Rate Constant ◽  
Rate Constants ◽  
Hydrogen Atom Transfer ◽  
Transfer Reactions ◽  
Atom Transfer ◽  
Bond Strengths ◽  
Oxidation Of Phenols ◽  
Atom Transfer Reactions

The oxidation of phenols (X-CH6H4OH) and hydroquinone (H2Q) by HFeO4− proceeds via a hydrogen atom transfer (HAT) mechanism. The Marcus cross relation can be used to predict the rate constant of HAT from H2Q to HFeO4−.

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