• B Thimme Gowda

      Articles written in Journal of Chemical Sciences

    • Mechanism of positive iodine reactions: Kinetics of oxidation of semicarbazide by iodamine-T, iodine monochloride and iodine

      B Thimme Gowda J Ishwara Bhat

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      Kinetics of oxidation of semicarbazide (SC) by iodamine-T (IAT), iodine monochloride and aqueous iodine has been studied in aqueous perchloric acid medium. The rate laws followed by the oxidation of SC were determined. The rates decreased slightly with increase in ionic strength of the medium in IAT and ICI oxidations, while the reverse trend was observed with I2. Decrease in dielectric constant of the medium increased the rates with IAT and ICI, while it decreased the rate in I2 oxidations. Addition of the reduced product,p-toluene-sulphonamide had no effect on the rate with IAT. Addition of I had slight negative and positive effects on the rates of oxidations with IAT and ICI, respectively, but the negative effect was considerable in I2 oxidations. Mechanisms consistent with the observed rate laws have been proposed and discussed. Rate determining steps have been identified and their coefficients calculated. These constants were used to predict the rate constants from the deduced rate laws as [SC], [H+] and [I] varied. Reasonable agreement between the calculated constants and experimental values provide support for the suggested mechanisms.

    • Kinetics and mechanism of oxidation of thiocarbohydrazide in the free state and in its metal complex and thiocarbohydrazone by chloramine-T and dichloramine-T

      B Thimme Gowda B S Sherigara

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      Kinetics of oxidation of thiocarbohydrazide (TCH) in the free state and as its metal complex, and as a hydrazone by chloramine-T (CAT) in aqueous HClO4 medium, and by dichloramine-T (DCT) in 1:1 (v/v) water-methanol medium in the presence of HClO4 have been studied. Rates of oxidation of TCH in the free state and in metal complex by CAT were determined. The rate law for the oxidation of TCH at high [H+ ] and for complex oxidations were identical to that for CAT oxidations. The conversion of TCH into its hydrazone changed the order in [H+] from a positive to a negative value, probably signalling the change of reaction site. The rate law for oxidation under these conditions was determined. Addition of the reduced product of the oxidants had no effect on the rate of oxidations. Variation in ionic strength of the medium had little positive effect, while decrease in dielectric constant of the medium decreased the rate in both the oxidations. Oxidation processes generally follow a Michaelis-Menten type of mechanism. Constants of the rate limiting steps have been calculated at different temperatures and these constants have been used to calculate the activation parameters from the Arrhenius plots. The proposed mechanisms are supported by investigations with HOC1 under identical reaction conditions. Metal complexation of the substrate decreased the reactivity, while conversion of TCH into its hydrazone changed the rate dependence on [H+].

    • A study of substituent effect on the oxidative strengths of sodium salts of N-bromo-arylsulphonamides: Kinetics and mechanism of oxidation of D-fructose and D-glucose in alkaline medium

      K M Usha B Thimme Gowda

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      N-Bromo-arylsulphonamides of different oxidizing strengths are used for studying the kinetics of oxidation of D-fructose and D-glucose in aqueous alkaline medium. The results are analysed and compared with those from the sodium salts of N-bromo-benzenesulphonamide and N-bromo-4-methylbenzenesulphonamide. The reactions show zero-order kinetics in [oxidant], fractional order in [Fru/Glu] and nearly first order in [OH-]. Rates of oxidation of fructose are higher than those for glucose with the same oxidant. Similarly,Ea values for glucose oxidations are higher than those for fructose. The results are explained by a suitable mechanism and the related rate law is deduced. The effective oxidising species in the reactions of N-bromo-arylsulphonamides is Br+. The oxidative strengths of the latter therefore depend on the ease with which Br+ is released from them. The ease with which Br+ is released from N-bromo-arylsulphonamides depends on the electron density on the nitrogen atom of the sulphonamide group, which in turn depends on the nature of the substituent on the benzene ring. The validity of the Hammett equation has also been tested for oxidation of both fructose and glucose. Enthalpies and entropies of activations of the oxidations by all the N-bromo-arylsulphonamides correlate well. The effect of substitution onEa and logA of the oxidations is also considered

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