Prediction of the Rate Constants for Proton Abstraction from Carbon Acids, Using a Simple Model and Multidimensional Marcus Theory

1997 ◽  
Vol 119 (5) ◽  
pp. 1151-1152 ◽  
Author(s):  
J. Peter Guthrie
Keyword(s):  
Simple Model ◽  
Rate Constants ◽  
Marcus Theory ◽  
Proton Abstraction ◽  
Carbon Acids

scholarly journals Kinetics and Mechanism of the Change of the 4-Nitrobenzylthio-System into 4,4′-Diformylazoxybenzene in Alkaline Dioxane-Water Media

1985 ◽  
Vol 40 (11) ◽  
pp. 1128-1132
Author(s):  
Y. Riad ◽  
Adel N. Asaad ◽  
G.-A. S. Gohar ◽  
A. A. Abdallah
Keyword(s):  
Acetic Acid ◽  
Sodium Hydroxide ◽  
Rate Constants ◽  
Main Product ◽  
Reaction Medium ◽  
Aqueous Dioxane ◽  
Proton Abstraction ◽  
First Order ◽  
Water Media ◽  
Different Temperatures

Sodium hydroxide reacts with α -(4-nitrobenzylthio)-acetic acid in aqueous-dioxane media to give 4,4'-diformylazoxybenzene as the main product besides 4,4'-dicarboxyazoxybenzene and a nitrone acid. This reaction was kinetically studied in presence of excess of alkali in different dioxane-water media at different temperatures. It started by a fast reversible a-proton abstraction step followed by two consecutive irreversible first-order steps forming two intermediates (α -hydroxy, 4-nitrosobenzylthio)-acetic acid and 4-nitrosobenzaldehyde. The latter underwent a Cannizzaro's reaction, the products of which changed in the reaction medium into 4,4'-diformylazoxybenzene and 4,4'-dicarboxyazoxybenzene. The rate constants and the thermodynamic parameters of the two consecutive steps were calculated and discussed. A mechanism was put forward for the formation of the nitrone acid.Other six 4-nitrobenzyl, aryl sulphides were qualitatively studied and they gave mainly 4,4'-diformylazoxybenzene beside 4,4'-dicarboxyazoxybenzene or its corresponding azo acid.

Kinetic hydrogen isotope effects in the ionization of some carbon acids

1970 ◽  
Vol 318 (1535) ◽  
pp. 421-440 ◽  
Keyword(s):  
Aqueous Solutions ◽  
Rate Constants ◽  
Hydrogen Isotope ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope ◽  
Pyridine Bases ◽  
Carbon Acids ◽  
Ethyl Nitroacetate ◽  
Basic Strength

Rate constants in aqueous solutions are reported for proton and deuteron abstraction by a variety of bases from tricarbomethoxymethane, the propan-2-one-1-sulphonate ion, 2-acetylcyclohexanone and ethyl nitroacetate. The rates of ionization were measured by using bromine or iodine as scavengers to remove the anions, and, for ethyl nitroacetate by direct observation of the rate of appearance of the anion. The kinetic isotope effects vary from k H / k D = 2.5 to k H / k D = 10.3, and confirm the regularities previously found (Bell & Crooks 1965; Bell & Goodall 1966). In particular, the results for the reaction of ethyl nitro-acetate with nine bases show clearly that with increasing basic strength the isotope effect passes through a well-marked maximum. Sterically hindered pyridine bases give rise to abnormally high isotope effects, probably attributable to increased tunnel corrections.

Towards a complete account of diastereotopic hydrogen isotope exchange of carbon acids. III. Base-catalyzed exchange of sulfoxides. Evidence for exchange by inversion

1985 ◽  
Vol 63 (8) ◽  
pp. 2100-2109 ◽  
Author(s):  
Nick Henry Werstiuk ◽  
Sujit Banerjee
Keyword(s):  
Steady State ◽  
Rate Constants ◽  
Hydrogen Isotope ◽  
Isotope Exchange ◽  
Hydrogen Isotope Exchange ◽  
Upper Limit ◽  
Steady State Assumption ◽  
Carbon Acids ◽  
State Assumption ◽  
Complete Account

The rate constants for deuteroxide and hydroxide catalyzed H → D and D → H exchange of benzodihydrothiophene oxide (1a) and several of its deuterated analogs 1b, 1c, and 1d at 30.00 ± 0.05 °C are reported along with the rate constants for D → H exchange of deuterated benzyl methyl sulfoxides 2b and 2c. Application of the steady-state assumption to schemes involving equilibrating pyramidal anions yield equations which are used to fit experimentally determined (kf/ks)H → D and (kf/ks)D → H ratios. The analysis supports our view that exchange of the diastereotopic protons (deuterons) occurs by inversion. The inversion component contributes significantly to exchange of the "slow" proton (deuteron) of a diastereotopic pair and this accounts for the observation that (kf/ks)D → H < (kf/ks)H → D. This study establishes an upper limit of 6 kcal for the barrier to inversion of the carbanions derivable from 1 and 2, if pyramidal anions are formed.

Kinetics of Proton Transfer Reactions of Carbon Acids. II. Reaction of Di-(4-nitrophenyl)methane with Alkoxide Bases

1972 ◽  
Vol 50 (1) ◽  
pp. 24-30 ◽  
Author(s):  
A. Jarczewski ◽  
K. T. Leffek
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Rate Constants ◽  
Activation Parameters ◽  
Transfer Reactions ◽  
Activation Process ◽  
Proton Transfers ◽  
Proton Transfer Reactions ◽  
Carbon Acids ◽  
Kinetics Of

The second-order rate constants have been measured over a range of temperatures for the proton-transter reactions from di-(4-nitrophenyl)methane to ethoxide, isopropoxide, and t-butoxide ions in solvents consisting of the corresponding alcohols containing 10% toluene by volume. The activation parameters ΔH≠ and ΔS≠ have been calculated and an interpretation of them is given in terms of solvation effects during the activation process. A comparison between the activation parameters for proton transfers and E2 olefin-forming β-elimination reactions is made and discussed with respect to transition state character of the latter reactions.

Marcus theory and the existence or non-existence of transition states in gas phase deprotonation of carbon acids

1984 ◽  
Vol 62 (8) ◽  
pp. 1465-1469 ◽  
Author(s):  
Saul Wolfe
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Transition States ◽  
Marcus Theory ◽  
Energy Change ◽  
Transfer Reactions ◽  
Gas Phase Reactions ◽  
Proton Transfer Reactions ◽  
Carbon Acids ◽  
Computational Level

At the 3-21G (3-21G*) computational level, the intrinsic barriers associated with proton transfer between XCH2− and CH3X have been found to be essentially constant (ca. 10 kcal/mol) for X = H, F, SH, Cl. According to the Marcus rate-equilibrium treatment of proton transfer reactions, this result means that transition states should not exist for gas phase reactions [Formula: see text], when the energy change exceeds 20 kcal/mol. This prediction has been confirmed for two cases (X = H, F) in which the energy change is less than 20 kcal/mol, and two cases (X = SH, Cl) in which the energy change is greater than 20 kcal/mol.

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