• V Jagannadham

      Articles written in Journal of Chemical Sciences

    • Lifetimes ofα-halo andα-azidobenzyl carbocations in aqueous solution

      R Sanjeev V Jagannadham

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      The title cations were produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-dihalides and benzyl-gem-diazides. The solvolysis reactions of benzyl-gem-dihalides and benzyl-gem-diazides in water proceed by a stepwise mechanism through α-halobenzyl carbocation and α-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratiokx/ks(M−1) for partitioning of the carbocation between reaction with halide/azide ion and reaction with water is determined by analysis of halide/azide common ion inhibition of the solvolysis reaction. The rate constantsks(s-1) for the reaction of the cation with solvent water were determined from the experimental values ofkx/ks andksolv, for the solvolysis of the benzyl-gem-dihalides and benzyl-gem-diazides respectively, usingkx = 5 × 109M−1 s−1 for diffusion-limited reaction of halide/azide ion with α-substituted benzyl carbocations. The values of 1/ks are thus the lifetimes of the α-halobenzyl carbocations and α-azidobenzyl carbocations respectively.

    • Kinetics and mechanism of oxidation of dimethyl sulphoxide by sodium bromate-sodium bisulphite reagent in aqueous medium

      J Viroopakshappa V Jagannadham

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      Rates of oxidation of dimethyl sulphoxide (DMSO) by HOBr producedin situ from sodium bromate-sodium bisulphite reagent have been studied iodometrically in aqueous medium. The order in [DMSO] is one when [DMSO] < 001 mol dm-3, fractional when [DMSO] is between 0.01 and 0.5mol dm-3 and zero when (DMSO) > 0.5 mol dm-3. Different rate laws are operative under these three conditions though HOBr is the effective oxidizing species in all the cases. A mechanism involving an intermediate four-membered cyclic transition state between DMSO and HOBr (formation constantK), which decomposes in a slow step with a rate constant(k) has been proposed. Thermodynamic parameters for the adduct formation step and activation parameters for the first-order decomposition of the adduct step have been evaluated in the temperature range 308–323 K. Activation parameters have also been determined while the orders in [DMSO] are unity and zero. The reaction product has been identified as dimethyl sulphone (DMSO2).

    • Carbocation lifetimes and entropy of water addition to carbocations dependent on their stability

      V Jagannadham

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      Iminodiazonium ions (α-azidobenzyl carbocations) were produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-diazides. Solvolysis reactions of benzyl-gem-diazides in water proceed by a stepwise mechanism through a-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratiokaz/ks (M-1) for partitioning of the carbocation between reaction with azide ion and reaction with water was determined by analysis of azide common ion inhibition of the solvolysis reaction. Rate constantsks (s-1) for the reaction of the cation with solvent water were determined from the experimental values ofkaz/ks andksolv, for the solvolysis of the benzyl-gem-diazides, usingkaz = 5 X 109 M-1 s-1 for diffusion-limited reactions of azide ion with a-azidobenzyl carbocations. Values of1/ks were thus the lifetimes of the a-azidobenzyl carbocations. From the study of effect of temperature onksolv andks the activation parameters are determined and discussed.

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