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Issue 18, 2003
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Alkaline hydrolysis of trinitrotoluene, TNT

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The kinetics of the alkaline hydrolysis of trinitrotoluene, TNT, in an aqueous solution is a possible approach to destroying the active agent in unwanted munitions. The kinetics are shown to have a rapid initial step, step A, in which a highly coloured species, X (λmax = 450 nm) is formed via an equilibrium reaction: TNT + OH ⇔ X. The bimolecular rate constant for the forward part of this equilibrium process, k1, is: 0.099 ± 0.004, 0.32 ± 0.02 and 1.27 ± 0.05 dm3 mol−1 s−1, at 25, 40 and 60 °C, respectively. The activation energy for the forward process is 60 kJ mol−1. The first-order rate constant for the reverse of this process, k−1, is: (5.3 ± 2.6) × 10−4, (1.2 ± 1.0) × 10−3 and (7.7 ± 2.9) × 10−3 s−1 at 25, 40 and 60 °C, respectively. The activation energy for the overall equilibrium process (k1/k−1) is ca. −5 kJ mol−1. The subsequent alkaline hydrolysis of X to form the final product P, i.e. step B, is much slower than step A and appears to comprise two processes coupled in series, i.e. steps B1 (X + 2 OH → Z) and B2 (Z + OH → P). At 25 °C, Step B1 appears rate determining throughout the decay process. At 45 °C and, more so, at 60 °C, step B appears increasingly biphasic with increasing alkaline concentrations, as step B2 begins to compete with step B1 for position as the rate determining step. The trimolecular rate constant for step B1 is: 0.017 ± 0.001, 0.0085 ± 0.0002 and 0.0011 ± 0.0001 dm6 mol−2 s−1 at 25, 40 and 60 °C, respectively, and the process has an activation energy of 64 kJ mol−1. The transition from uniform kinetics, described by step B1, to mixed kinetics, described by steps B1 and B2, as the reaction temperature and alkali concentration are increased most likely occurs because (a) step B2 has a lower activation energy than B1, although it was not possible to measure the former parameter, and (b) step B2 has a lower (1st) order dependence upon [OH] compared with that of step B1 (2nd). The bimolecular rate constant for step B2 is 0.0035 ± 0.03 dm3 mol−1 s−1 at 60 °C. A brief NMR study of the initial hydrolysis product in water, acetone and chloroform, coupled with UV/visible spectra, provides evidence that species X is a Meisenheimer complex.

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Publication details

The article was received on 29 Apr 2003, accepted on 14 Jul 2003 and first published on 18 Aug 2003

Article type: Paper
DOI: 10.1039/B304616H
Phys. Chem. Chem. Phys., 2003,5, 3921-3927

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    Alkaline hydrolysis of trinitrotoluene, TNT

    A. Mills, A. Seth and G. Peters, Phys. Chem. Chem. Phys., 2003, 5, 3921
    DOI: 10.1039/B304616H

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