Shipra Sen Gupta

Kinetics and mechanism of oxidation of some aldoses by chromic acid in perchloric acid medium

Abstract The kinetics of chromic acid oxidation of d -galactose, d -mannose, l -arabinose, and d -xylose have been studied in perchloric acid media. The reactions show first-order dependence on both chromium (VI) and aldose concentrations. The order with respect to hydrogen-ion concentration in each reaction is complex, and the hypotheses for the mechanism of acid catalysis have been tested. Different activation parameters have been evaluated. Aldoses are oxidized mainly to the aldonic acids when the aldose substrate is in excess. A tentative reaction mechanism has been suggested.

Kinetics and mechanism of oxidation of d-fructose and l-sorbose by chromium(VI) and vanadium(V) in perchloric acid medium

Abstract Kinetic data for the oxidations of d -fructose and l -sorbose by chromium(VI) and vanadium(V) in perchloric acid medium are reported. The addition of perchloric acid and sodium perchlorate increases the pseudo-first-order rate constants. Change of the reaction medium from water to deuterium oxide appreciably affects the rates of chromium(VI) oxidations, but does not affect those of vanadium(V) oxidations. The activation parameters are ΔH ‡ = 46.6 ±3.4 (fructose) and 50.6 ±6.3 (sorbose) kJ.mol −1 , and ΔS ‡ = −105 ±11 (fructose) and −100 ±20 (sorbose) J.deg −1 .mol −1 for chromium(VI) oxidations, and, for the other reactions, ΔH ‡ = 53.2 ±4.2 (fructose) and 52.3 ±6.3 (sorbose) kJ.mol −1 , and ΔS ‡ = −139.0 ±14 (fructose) and −137 ±20 (sorbose) J.deg −1 .mol −1 . The kinetics of the oxidations of ketohexoses by chromium(VI) indicate no intermediate-complex formation, whereas those for vanadium(V) indicate the formation of a 1:1 intermediate complex between ketohexoses and vanadium(V).

Evidence of esterification during the oxidation of some aromatic aldehydes by chromium (VI) in acid medium and the mechanism of the oxidation process

Abstract The oxidation kinetics of some aromatic aldehydes by chromium (VI) have been studied in perchloric acid medium. Kinetic and spectrophotometric results indicate the formation of a 1:1 intermediate ester between the reactive chromium (VI) species and protonated benzaldehyde. The rate is directly proportional to the square of hydrogen ion concentrations. The rate of oxidation decreases with the presence of electron donating groups and increases with electron withdrawing groups on the aromatic ring. The activation parameters associated with the rate determining step and the thermodynamic values associated with the equilibrium step have been computed. An attempt has been made to compare the results obtained with those for the oxidations of some aliphatic aldehydes which do not have enolizable hydrogen.

Kinetics and mechanism of the oxidation of thiosulphate by hexachloroplatinate(IV)

SummaryRate constants for the oxidation of thiosulphate by hexachloroplatinate(IV) have been measured. The kinetics of the oxidation of thiosulphate follow a second-order rate law, first order with respect to thiosulphate and first order with respect to platinum(IV). The influence of pH is small. The rates are found to depend on the nature and concentration of the cations and follow the order: Cs+>Rb+>K+>Na+>Li+. The activation parameters calculated from the temperature studies are: ΔH‡=42.9 k J mol−1 and ΔS‡=−102 JK−1 mol−1. A mechanism of the reaction in terms of intermediate formation of free radicals followed by the formation of tetrathionate is postulated to explain the kinetic behaviour.

Kinetics and Mechanism of the Oxidation of Some Aldoses by Cerium(IV)

Abstract The kinetics of oxidation of some aldoses by cerium(IV) have been studied spectrophotometrically in sulphuric acid medium. The reactions are second order; first order with respect to both cerium(IV) and aldose concentration. The reaction rate decreases with increase in sulphuric acid concentration. The mechanisms for the reactions are discussed.

Kinetics of the oxidation of sulphite by tetrachloroaurate(III)

SummaryThe reaction between sulphite and gold(III) in hydrochloric acid medium has been studied; an initial complex is formed prior to electron transfer. The reaction rate is inversely proportional to the hydrogen ion concentration and the reaction appears to proceed through the intermediate formation of a free radical which reacts with gold(III) to give the products. Evidence for the formation of dithionate together with sulphate is presented.

Kinetics of the oxidation of hydrazine by chromium(VI)

Abstract Kinetic studies of the oxidation of hydrazine by chromic acid were made under various experimental conditions. The enthalpy and entropy of activation have been calculated to be 10·5 ± 0·5 kcal mole −1 and −24·5 ± 1·0 (e.u.) respectively and an attempt has been made to correlate activation parameters with those of the chromic acid oxidations of some phosphorus(III) and arsenic(III) compounds. The data suggest that oxidation follows formation of a complex between N 2 H 5 + and the reacting species of chromium(VI) in the rate determining step. The decomposition of the complex by a fast step gives the reaction products. Various mechanistic possibilities have been suggested.

Kinetics of oxidation of sulphite by hexachloroplatinate(IV) in acidic solution

SummaryThe kinetics of the oxidation of sulphite by hexachloroplatinate(IV) has been studied over wide range of experimental conditions. The reaction is first-order in substrate and in platinum(IV). The rate decreases with the increase in acidity. The effect of salt and of changing dielectric constants on the reaction rate have been studied. Values of ΔH‡ and ΔS‡ have been calculated and are 26.3 kJ mol−1 and −35.9 JK−1 mol−1, respectively. On the basis of experimental evidence, a two-electron reduction mechanism is proposed.

Kinetics and mechanism of oxidation of D-glucopyranose 6-phosphate and D-ribofuranose 5-phosphate by chromium(VI) in perchloric acid media

Abstract The kinetics of oxidation of d -glucopyranose 6-phosphate and d -ribofuranose 5-phosphate by chromium(VI) in perchloric acid media have been studied by u.v. spectroscopy. Each reaction is first order with respect to [chromium(VI)] and [substrate]. The reactions are acid-catalysed and accelerated by the addition of sodium perchlorate. The mechanism of the reactions is discussed.

Kinetics of oxidation of ethanol by potassium permanganate in perchloric acid medium

The oxidation of ethanol by permanganate has been found to obey the rate law v=k[MeCHO][MnO4−][H+]. The results are compared with those found for chromium (VI) oxidation of the same substrate.AbstractСкорость окисления ацетальдегида перманганатом подчиняется следующему уравнению V=k[MeCHO][MnO4−][H+]. Результаты сравниваются с данными, полученными при окислении того же субстрата хромом (VI).