Puttaswamy et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 113–121
121
From slow/rate-determining step of Scheme 2:
pared to alkaline medium. This is confirmed by the magnitudes of
activation energies (Tables 2 and 4). The different oxidizing species
of BAB involved in two media may be responsible for the differ-
ence in reactivity. This fact makes us to conclude that the conjugate
acid, PhSO2NHBr is a more efficient reactive species than the anion,
PhSO2N−Br, in the oxidation of TA by BAB.
d[BAB]t
Rate = −
= k6[PhSO2N−Br] [Pd(II)]
(21)
dt
By substituting for [PhSO2N−Br] from Eq. (20) into Eq. (21), the
following rate law is obtained:
d[BAB]t
K5k6[BAB]t[OH−][Pd(II)]
[H2O] + K5[OH−]
Rate = −
=
(22)
4. Conclusions
dt
This rate law satisfactorily fits well to the experimental results
wherein a first-order dependence of rate on each [BAB]o and
[Pd(II)], less than unity order on [OH−] and the lack of effect of
the [TA]o and [BSA] on the rate of reaction was noticed. In alkaline
medium, the solvent isotope studies show that kꢀ (H2O)/kꢀ (D2O) < 1.
This is generally correlated with the fact that OD− ion is a stronger
base than OH− ion and in base catalyzed reactions, enhancement
of rate in D2O medium can be expected according to the concept
of solvent isotope theory [29,30]. However, the small magnitude
of the isotopic effect (0.82) can be attributed to the less than unity
order dependence on [OH−]. The mechanism proposed is congru-
ous with the observed negligible effects of benzenesulfonamide,
halide ions and also on the activation parameters.
It is concluded that the stoichiometry and oxidation products of
TA by BAB in acid and alkaline media are the same, but their kinetic
characteristics are different. Activation parameters have been com-
puted. Reactions of TA with BAB are 10-fold faster in acid medium in
comparison with alkaline medium. However, oxidation of TA with
BAB in alkaline medium will become facile in the presence of a
micro quantity of Pd(II) catalyst and reaction rates showed that
Pd(II) catalyzed reactions are 6-fold faster than uncatalyzed reac-
tions. This justifies the need of Pd(II) catalyst in the facile oxidation
of TA–BAB redox system in alkaline medium. The obtained kinetic
data have been explained by elegant mechanisms and the relevant
rate laws.
It was also considered worthwhile to compare the reactivity of
at different temperatures (293–313 K) and from the Arrhenius plot
of log kꢀ versus 1/T (r = 0.9810), values of activation parameters for
the uncatalyzed reaction were computed. These data are recorded
in Table 4. It is seen from the Table 4 that the Pd(II) catalyzed reac-
tion rate is found to be 6-fold faster than the uncatalyzed reaction.
Thus the observed rate of oxidation in the presence of Pd(II) cata-
lyst justifies the need of a catalyst for a facile oxidation of the TA
by BAB in alkaline medium. The activation parameters evaluated
the oxidizing species, which increases the oxidizing property of the
oxidant and makes the oxidation reaction facile. Further, the cat-
alyst Pd(II) favourably modifies the reaction path by lowering the
energy of activation (Table 4).
Acknowledgements
One of the authors (AS) thanks the Principal and Management of
M.E.S. College, Bangalore, for their encouragement. We thank Prof.
M.A. Pasha for helpful discussions.
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=
(24)
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that obtained in the absence of Pd(II) catalyst (ko = 0.79 × 10−4 s−1
)
(Table 4). This clearly establishes the above relationship.
Under comparable set of experimental conditions, the TA–BAB
oxidation reaction rate is about 10-fold faster in acid medium com-