Kalyan Kali 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 some aldoses by vanadium(v) in perchloric acid medium

Abstract The kinetics of oxidation of some aldoses by vanadium(V) in perchloric acid media have been investigated. Each reaction is first order with respect to both [Vanadium(V)] and [Aldose]. The reactions are catalysed by acid. The addition of sodium perchlorate accelerates the rate of reaction. Kinetic evidence for the formation of an intermediate compound between vanadium(V) and aldoses is insignificant, and a mechanism is suggested in which vanadium(V) reacts with the aldoses by a fast step to form a transition state, followed by the decomposition of the latter to give the products of reaction in a slow step. The formation of free-radical intermediates has been demonstrated, and one-electron reduction of vanadium(V) by aldoses seems to be the most plausible mechanism. The oxidation rates follow the order: xyloses arabinose galactose mannose. The activation parameters are reported.

Kinetic behaviour and relative reactivities of some aldoses, amino sugars, and methylated sugars towards platinum(IV) in alkaline medium

Abstract The kinetic behaviour and relative reactivities of some carbohydrates (aldoses, amino sugars and methylated sugars) towards platinum(IV) in alkaline medium have been investigated. The reactions are first order with respect to [substrate] and Pt IV . The rates increase with the increase in OH − . The reactions show pseudo-first-order dependence on OH − . The oxidation rates in alkaline medium follow the order triose>tetrose>pentose>hexose. Activation parameters of the reactions have been calculated. Mechanisms have been proposed for the reactions.

Kinetics of the oxidation of hydroxylamine by platinum(IV)

Abstract The kinetics of the oxidation of hydroxylamine by platinum(IV) have been studied spectrophotometrically in sodium acetate-acetic acid buffer in the temperature range 25–40°C. The reaction is first order in both hydroxylamine and platinum(IV). The rate increases with decreasing acidity and dielectric constant. Different activation parameters of the reaction have been evaluated. Some possible reaction steps have been suggested.

Kinetics and mechanism of the oxidation of some aldoses, amino sugars and methylated sugars by tris(pyridine-2-carboxylato)manganese(III) in weakly acidic medium

Abstract The kinetics of oxidation of some aldoses, amino sugars and methylated sugars by tris(pyridine-2-carboxylato)manganese(III) have been studied spectrophotometrically in sodium picolinate–picolinic acid buffer medium. The reactions are first-order with respect to both manganese(III) and sugar concentrations, but independent with respect to sodium picolinate–picolinic acid buffer medium. The mechanisms for the reactions are discussed.

Kinetics and mechanism of oxidation of d-erythrose and dL-glyceraldehyde by chromium(VI) and vanadium(V) in perchloric acid medium

Abstract The kinetics of oxidation of d -erythrose and dL -glyceraldehyde by chromium (VI) and vanadium(V) in perchloric acid medium have been investigated spectrophotometrically. Each reaction was first-order with respect to [oxidant] and [substrate]. The reactions were catalysed by acid, but their dependence on acidity was complex. Sodium perchlorate accelerated the rate of each reaction. The oxidation rates follow the order glyceraldehyde > erythrose. The activation parameters were calculated and mechanisms consistent with the experimental observations are proposed.

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.

Reactivities of osmium(VIII), iridium(IV) and platinum(IV) towards glycolaldehyde

Oxidations of glycolaldehyde (GA) to glyoxal by osmium(VIII), iridium(IV) and platinum(IV) have been investigated in aqueous solution, and the orders with respect to each [reactant] determined. The reaction involving iridium(IV) takes place through intermediate formation of free radicals in a MeCO2Na-MeCO2H buffer medium, whereas a one-step two-electron transfer process occurs in the oxidations by OsVIII and PtIV in an alkaline medium. Mechanisms consistent with the experimental findings are proposed.