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Transition Met Chem (2011) 36:707–719
study the catalytic action of Ru(III), we chose RuCl3 as a
catalyst. While studying the present reaction in the pres-
ence of mercury(II), a bromo-complexing metal ion, some
interesting results were obtained. Ru(III)-catalyzed bro-
mate oxidation of malonic acid (MA) and methyl malonic
acid (MMA) exhibit an induction period [11], whose
duration is inversely proportional to [substrate], [catalyst]
and [acid], and independent of [bromate]. After induction,
the reaction is first order in both [bromate] and [catalyst]
but shows less than unity order in [substrate] and an inverse
fractional order in [acid]. However, the present reaction
does not show any induction period over a wide concen-
tration range of bromate, Ru(III), oxo acid and H2SO4. The
kinetic behavior of the present reactants and catalyst is
entirely different from the earlier reports of Ru(III)-cata-
lyzed oxidation reactions [6–11]. The mechanism neither
resembles catalysis by bromide anions [12] nor that of
Ru(III)-catalyzed oxidation of organic substrates with the
oxidants [6–11].
identification of organic intermediates in the reaction were
performed using HPLC. The experiments were performed
with a Shimadzu instrument using an ion-exchange column
at 45 °C and a UV detector working at 220 nm. The
intermediates and products were identified from their
retention time (tr). A Shimadzu multipurpose recording
double beam UV–visible spectrophotometer equipped with
a temperature controller was used for absorption studies.
Kinetic measurements
All the kinetic measurements were carried out in black-
coated vessels at constant temperature ( 0.1 °C) under
pseudo-first-order conditions with [oxo acid] ꢁ [bromate].
The reaction was initiated by the rapid addition of tem-
perature-equilibrated oxidant of the required concentration
to reaction mixtures containing the required amounts of
substrate, catalyst, Hg(OAc)2, H2SO4, AcOH and water, in
glass-stoppered Pyrex boiling tubes that were thermally
equilibrated for 1 h. The progress of the reaction was
monitored by iodometric determination of unconsumed
[bromate] in known aliquots of the reaction mixtures at
different time intervals. Before adopting the iodometric
method, it was ensured that the presence of oxo acid in the
quenching solution of KI did not change the bromate titer
value. The course of the reaction was studied for at least
three half-lives. The rate constants (k, s-1) were deter-
mined from the pseudo-first-order plots of log [oxidant]
against time. The pseudo-first-order plots were linear
(R2 C 0.99) for more than 80% completion of the reaction,
and the k (s-1) values were reproducible to within 5%.
Rate constants did not alter in a nitrogen atmosphere,
and all the rate constants reported in this paper were
obtained without nitrogen. Freshly prepared solutions of
oxo acids in purified acetic acid were used throughout. The
uncatalyzed reaction is very slow under these conditions
and is first order in both [bromate] and [oxo acid], and
second order in [H?]. However, the reaction rate is
appreciably faster (40–80 times) in the presence of a
minute quantity (10-5 mol dm-3) of RuCl3. All the rate
constants for the catalyzed reactions were obtained as:
kcatalyzed = koverall – kuncatalyzed and represented as k (s-1).
In the present study, we describe the kinetics and
mechanism of oxidation of substituted 4-oxo-4-arylbuta-
noic acids by bromate in sulphuric acid medium containing
mercury(II) and Ru(III), including the active species of the
substrate, catalyst and oxidant, and the oxidation products.
We have also studied the catalytic efficiency of Ru(III) and
evaluated the related kinetic and thermodynamic parame-
ters of the reaction, along with the linear free-energy and
isokinetic relationships.
Experimental
All the oxo acids were obtained from Aldrich and recrys-
tallized twice from water before use. The purity of sub-
strates was checked by their melting points, UV, IR and
NMR spectra. KBrO3 (Reidal), H2SO4 and Hg(OAc)2
(Merck) were of analytical reagent grade and used as
received. D2O (99.4% pure) was obtained from the Baba
Atomic Research Centre, Mumbai, India. Acetic acid
(BDH) was purified by refluxing with chromic acid and
acetic anhydride for 6 h and then distilled. Ruthenium
trichloride hydrate (Johnson Matthey) was purified by
evaporation with conc. HCl. A stock solution of Ru(III)
was prepared by dissolving a weighed quantity of RuCl3 in
dilute HCl of known strength. Mercury was added to this
solution, to detect the formation of any Ru(IV) during its
preparation, and the solution was kept for 1 day. The
obtained solution was assayed by EDTA titration [13] and
stored in a glass-stoppered flask, painted black, at *5 °C.
Dilute solutions of Ru(III) were made from the stock
solution as required. Other solutions were prepared with
either doubly distilled water or purified acetic acid and
were standardized by known methods. Separation and
Results
Characterization of the reaction kinetics
Molecular bromine production anticipated from the bro-
mate–bromide reaction and its subsequent reaction with the
oxo acid was eliminated by the addition of the bromo-
complexing agent Hg(II) [14]. The added mercury acetate
had no effect on the rate of reaction over a wide
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