A kinetic study of the early steps in the oxidation of chlorophenols by hydrogen peroxide catalyzed by a water-soluble iron(III) porphyrin

Article abstract of DOI:10.1039/b400482e

The kinetics and mechanism of the initial steps in the oxidation of 2,4,6-trichlorophenol by hydrogen peroxide using iron(III) meso-tetra(4- sulfonatophenyl)porphine chloride as a catalyst were studied in this work. The first oxidation step is the formation of a substituted 1,4-benzoquinone. This step was also studied using a selection of differently substituted chlorophenols. It was shown that the rate constants characteristic for the oxidation of the substrate do not follow the pattern of pK_aS, but correlate well with the ¹³C chemical shifts of the carbon atoms directly bonded to the oxygen in chlorophenols. The kinetics of the catalyzed and uncatalyzed oxidation of 2,6-dichloro-1,4-benzoquinone by hydrogen peroxide was also studied. The catalyzed and uncatalyzed pathways give different products.

Full text of DOI:10.1039/b400482e

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P A P E R
A kinetic study of the early steps in the oxidation of chlorophenols by
hydrogen peroxide catalyzed by a water-soluble iron(III) porphyriny
a
b
´
Gabor Lente* and James H. Espenson
a
Department of Inorganic and Analytical Chemistry, University of Debrecen,
H-4010, Debrecen, Hungary. E-mail: lenteg@delfin.klte.hu; Fax: þ36 52 489-667;
Tel: þ36 52 512-900. Ext.: 2373.
b
Ames Laboratory, Iowa State University, Ames, IA 50011, USA.
E-mail: espenson@ameslab.gov; Fax: +1 515 294-5233; Tel: þ1 515 294-5370
Received (in Montpellier, France) 12th January 2004, Accepted 11th March 2004
First published as an Advance Article on the web 14th June 2004
The kinetics and mechanism of the initial steps in the oxidation of 2,4,6-trichlorophenol by hydrogen peroxide
using iron(III) meso-tetra(4-sulfonatophenyl)porphine chloride as a catalyst were studied in this work. The first
oxidation step is the formation of a substituted 1,4-benzoquinone. This step was also studied using a selection
of differently substituted chlorophenols. It was shown that the rate constants characteristic for the oxidation
of the substrate do not follow the pattern of pKas, but correlate well with the 13C chemical shifts of the carbon
atoms directly bonded to the oxygen in chlorophenols. The kinetics of the catalyzed and uncatalyzed oxidation
of 2,6-dichloro-1,4-benzoquinone by hydrogen peroxide was also studied. The catalyzed and uncatalyzed
pathways give different products.
because of the inherent errors in concentrations and the
time-consuming analysis steps.
Introduction
Polychlorinated phenols are used as wood preservatives, pesti-
cides, fungicides, herbicides, insecticides or disinfectants, and
they are also present in the waste from paper mills. These com-
pounds are highly toxic, persistent, and regarded as priority
pollutants for which efficient chemical treatment processes
are needed.1–3 Oxidative degradation is probably the most
advantageous reaction type for this purpose and several differ-
ent methods have been reported. Chemical methods usually
use H2O2 or KHSO5 as a stoichiometric oxidant and iron com-
plexes involving mostly N-donor ligands as catalysts.1,2,4–11
TiO2-based systems for photodegradation,12–16 ozonization,17
heterogeneous catalysis18,19 and bacterial methods20,21 have
also been thoroughly studied.
In this paper, we present our results using an insightful selec-
tion of different chlorophenols, and also report detailed kinetic
studies on the second step in the oxidation of 2,4,6-trichloro-
phenol (TCP), which is one of the most significant pollutants
among chlorinated phenols and is often used to test the effi-
ciency of oxidation methods.1,2 It is also our goal to show that
on-line methods (UV-vis spectroscopy and ion-selective poten-
tiometry) can also be used to follow the kinetics of the oxida-
tion process. The chemical structures of the catalyst and the
most important two substrates are given in Chart 1.
Despite the large number of papers published in this field,
very few high quality kinetic studies can be found in the litera-
ture. Mechanisms for parts of the oxidation process are often
proposed, but they are usually based on using chemical com-
mon sense and previous knowledge about similar reactions.
The largest problem with obtaining high-quality kinetic
information is the complexity of the system, which makes
the application of on-line detection methods (UV-vis, NMR)
complicated. Off-line methods such as LC or GC are some-
times used to analyze the time-dependent composition of reac-
tion mixtures during chlorophenol oxidation.22 These methods
usually require quenching of the original reaction and some
kind of sample treatment or derivatization before analysis,
both of which are major possible sources of unknown second-
ary chemical reactions. Used with care, these methods usually
provide valuable information on the identity of the intermedi-
ate(s) or product(s) present in relatively high concentrations,
but it is seldom possible to determine high-quality kinetics
y Electronic supplementary information (ESI) available: figures
providing additional data on the catalytic oxidation of TCP, as well
as the calibration curve for the chloride ion selective electrode. See
http://www.rsc.org/suppdata/nj/b4/b400482e/
Chart 1 Structural formulas of the catalyst, 2,4,6-trichlorophenol
(TCP), and 2,6-dichloro-1,4-benzoquinone (DCQ).
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2
847

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dissolved oxygen increases slightly during the first 20 min of
Experimental
the catalyzed reaction, but the overall change was less than
10% (see ESI). The increase in O2 concentration was somewhat
faster when no TCP was present in the solutions, so this effect
was rationalized as decomposition of H2O2 slightly catalyzed
by Fe(TPPS)þ. In conclusion, the presence of oxygen does
not influence the oxidation reaction.
The pH was also followed in a few experiments. In an oxida-
tion experiment, the original pH of a TCP test solution (5.2)
decreased to a final pH of about 2.8 in 15 h (see ESI). This con-
firmed that the reaction produces acid. However, as will be
explained later, the rate of the reaction did not depend on
pH in this region and no buffers were used in this study.
Materials
Chlorophenols and 2,6-dichloro-1,4-benzoquinone were pur-
chased from Aldrich and Lancaster and purified by vacuum
sublimation. Iron(III) meso-tetra(4-sulfonatophenyl)porphine
chloride was used as received from Frontier Scientific
(www.porphyrin.com). 2,4,6-Trichlorophenol-3-5-d2 was syn-
thesized from phenol-d6 , HCl and H2O2 using a literature
method reported for the preparation of undeuterated 2,4,6-tri-
chlorophenol.23 Ion-exchanged and ultrafiltered water from a
Millipore MILLI-Q purification system was used to prepare
the solutions. The concentration of hydrogen peroxide stock
solutions was determined iodometrically.
Oxidation of chlorophenols to benzoquinones
Instrumentation and computations
The first step of the oxidation of a chlorophenol is the forma-
tion of a substituted 1,4-benzoquinone,22,27 which is a detect-
able intermediate in the overall oxidation process. Eqn. (1)
gives the stoichiometry of the reaction:
NMR spectra were recorded in CDCl3 and referenced to the
residual proton signal of the deuterated solvent. The pKa
values were determined by a combined pH-metric and UV-
vis spectrophotometric technique.24 In potentiometric exp-
eriments, a combination of pH and chloride ion selective
electrodes was used, connected to a precision pH-meter.
The electrodes were calibrated daily using standard buffers
and NaCl solutions. Kinetic experiments with the chloride
ion selective electrode were always carried out with a small
amount of NaCl added prior to the experiment (referred to
as [Clꢀ]0 in the figure captions) in order to avoid badly defined
voltage readings at the beginning of the kinetic curves. Con-
stant ionic strength was maintained with 0.1 M NaNO3 in both
potentiometric and spectrophotometric studies. An oxygen
monitor was used to measure the concentration of dissolved
oxygen. All experiments were carried out in the dark because
light (even fluorescent room light) significantly accelerates
further oxidation.25 Our experience has shown that the mono-
chromatic analyzing light beam of a scanning spectrophot-
ometer or stopped-flow instrument is not intense enough to
influence the reaction, but the white light used in a diode-array
spectrophotometer corrupts the measurements. Nonlinear
least-squares fitting was carried out using the software package
Scientist.26
ð1Þ
The reactions shown in Scheme 1 allow one to interpret the
kinetic observations for all of the chlorophenol substrates used
in this study.
This scheme is an extension of an earlier kinetic model pos-
tulated using the same catalyst in an oxidation process28 and is
based on measuring chloride ion concentrations using an ion
selective electrode and UV-vis measurements carried out at
395 nm, corresponding to the intense Soret band of the catalyst
Fe(TPPS)þ. Standard steady-state treatment for Cat0 in this
scheme, assuming constant concentrations of S (substrate)
and H2O2 , gives the following time-dependent expression for
the concentration of Clꢀ:29
ꢀ
ꢁ
ꢂ
ꢃ
k2
½Clꢀꢁ ¼ ½Sꢁ0 FeðTPPSÞþ ꢂ 1 ꢀ eꢀk
ð2Þ
Ct
0
k3
Results and discussion
where the pseudo-first-order rate constant kC is given as:
General observations
k1½H2O2ꢁ
An earlier study on the oxidation of TCP by H2O2 using
Fe(TPPS)þ as a catalyst found that DCQ was the final product
of the oxidation.7 These experiments were done in citrate buf-
fered CH3CN. With unbuffered aqueous solution, as explained
later, the oxidation is much deeper. Total organic carbon mea-
surements showed that about 40% of the organic carbon con-
tent was lost after the completion of the process (about 24 h),
indicating the formation of carbon dioxide. Almost all chlorine
( > 97%) present initially in TCP was transformed onto chlor-
ide ion, according to measurements with a chloride ion selec-
tive electrode. Citric acid had a slight inhibiting effect on the
oxidation of TCP under acidic conditions. Treatment of
DCQ with H2O2 in the presence of Fe(TPPS)þ also produced
chloride ions, showing that DCQ cannot be a stable final
product of TCP oxidation under these conditions.
kC
¼
ð3Þ
ðk2k3Þ½Sꢁ þ 1
The initial rate for the concentration of Fe(TPPS)þ can also be
calculated from:
!
ꢀ
ꢁ
ꢄ
ꢀ
ꢅ
d FeðTPPSÞþ
k1½H2O2ꢁ0
¼ ꢀ
dt
ðk2k3Þ½Sꢁ0þ1
t¼0
ꢁ
ꢂ FeðTPPSÞþ
ð4Þ
0
The chemical sense of the mechanism given in Scheme 1 is
that the catalyst reacts with H2O2 , first forming a highly oxi-
dizing intermediate, Cat0, which oxidizes the substrate (k2)
and gives the original Cat form back, or decomposes to a much
less active form, Cat00 (k3). Catalyst decomposition occurs
The oxidation process was followed by UV-vis and a chlor-
ide ion selective electrode. It was shown that no oxidation
occurs in solutions containing hydrogen peroxide and chloro-
phenols without catalysts. The catalytic experiments were
usually carried out in air-saturated solutions. The reaction
was tested for the effect of oxygen. The UV-vis curves were
the same under air-free and aerobic conditions [see Electronic
supplementary information (ESI)]. Experiments with an
oxygen measuring system showed that the concentration of
Scheme 1
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
848
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2

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in competition with oxidation of the substrate and typically
10–15% of the substrate is oxidized in the TCP system before
practically all of the catalyst is transformed to the Cat00 form.
As pointed out above in this paper, there is evidence for
further slow oxidation, but the process is not significant until
after the initial period, which usually takes 5–15 min. Under
steady-state conditions, one cannot hope to determine all three
rate constants appearing in Scheme 1 independently;29 only k1
and the ratio k2/k3 can be resolved. Another interesting aspect
of the scheme is that the reaction rate is independent of the pH
in the pH range 2–5, in agreement with experimental results.
At higher pH, Fe(TPPS)þ forms a m-oxo dimer,30 which seems
to be catalytically inactive.
Scheme 1 features only one rate constant, k2 , that is depen-
dent on the identity of the substrate used. As discussed earlier,
the value of k2 cannot be determined, only the k2/k3 ratio. The
value of this ratio is characteristic of the substrate used and the
values were calculated using two different methods. From
chloride ion kinetic data, the overall concentration of chloride
ion produced ([Clꢀ]final) in the first stage can be used to
estimate the k2/k3 value:
The values listed in Table 1 span a range of two orders of
magnitude. It is interesting to note that 2,6-dichlorophenol
and 2,4,6-trichlorophenol (TCP) have almost the same k2/k3
values. This may be somewhat unexpected because the substi-
tuent in the para position must be directly involved in the che-
mical reaction. However, the ortho substituents, although not
involved in the reaction directly, seem to have a large effect on
the k2/k3 values. TCP-d2 and TCP also have values that are the
same within the error limits, showing the absence of primary
deuterium isotope effects in agreement with expectations.
Fig. 1 shows that the k2/k3 values do not correlate with the
pKa values of the chlorophenols. Fig. 2 shows that the correla-
tion is much better with the 13C chemical shift of the carbon
atom directly bonded to the oxygen in the chlorophenol
(Chart 2).
One can but speculate that the first step of phenol oxidation
might be hydrogen atom abstraction. If this is indeed the case,
the k2/k3 values should be related to the homolytic O–H bond
dissociation energies, which generally do not follow the pattern
of pKas. Unfortunately, no detailed literature data are avail-
able on the O–H bond strengths. However, studies of dioxin
formation in incinerators suggested that gradual chlorination
of phenol should decrease the O–H bond energies and it has
been estimated that the O–H bond energy in TCP is about 5
kcal molꢀ1 less than that of the parent phenol.33 These results
are in qualitative agreement with the trend of the k2/k3 values
observed here; more detailed data would be needed for quan-
tification. The successful correlation shown in Fig. 2 might
suggest that 13C chemical shifts, which should be related to
the electron density around the carbon nucleus, are in some
way indicative of the O–H bond strengths, although this
conclusion is highly speculative at this point.
k2
k3
½Clꢀꢁfinal
ꢀ
ꢁ
¼
ð5Þ
½Sꢁ0 FeðTPPSÞþ
0
From UV-vis curves, the initial rate affords a reliable way to
determine k2/k3 . The initial rate of absorbance change in the
presence of substrate (v0) is measured and then compared to
the initial rate measured in a reference experiment, where
H2O2 and the catalyst is used in the same concentration with-
out any substrate added [v0(ref)]:
ꢄ
ꢅ
k2
1
v0ðrefÞ
¼
ꢀ 1
ð6Þ
k3 ½Sꢁ0
v0
Uncatalyzed oxidation of 2,6-dichloro-1,4-benzoquinone
The k2/k3 values determined for different chlorophenols are
summarized in Table 1. It should be noted that 2,6-dichloro-
phenol does not have a chlorine substituent in the para posi-
tion. However, the experimental data showed that the
oxidation product in this case is also the corresponding substi-
tuted p-benzoquinone, that is 2,6-dichloro-1,4-benzoquinone,
which was identified from its UV-vis spectrum. No chloride
ion was formed in the first stage of the reaction, but k2/k3
could be determined spectrophotometrically. 2,20,6,60-Tetra-
chlorodiphenoquinone is another possible, although not parti-
cularly likely, product of the oxidation of 2,6-dichlorophenol
through the symmetric oxidative coupling of two aromatic
rings.31 This compound absorbs strongly at 400 nm32 and
UV-vis experiments showed that it was not formed at all.
We attempted to oxidize unsubstituted phenol as well and
determine its k2/k3 value, but the product was not the expected
1,4-benzoquinone. The product, most likely diphenoquinone,
had a large absorption at the wavelength used for the UV-
vis experiments, making quantitative evaluation impossible.
Previous work and our observations showed without doubt
that the first step of the oxidation of TCP is the formation
of DCQ.7,22,25,27 We also confirmed that DCQ was not a final
product and made efforts to identify the next step in the oxida-
tion sequence and study its kinetics.
1,4-Benzoquinone is oxidized to 2-hydroxy-1,4-benzoqui-
none by hydrogen peroxide in basic aqueous solution.34 Our
observations showed that the same product is formed in acidic
solution as well. In addition, oxidation of DCQ by H2O2 in
acidic solution also gave the analogous substituted product,
3-hydroxy-2,6-dichloroquinone [eqn. (7)]:
ð7Þ
Table 1 Values of k2/k3 determined for different substrates at 25.0 ꢃC
Substrate
k2/k3/104 Mꢀ1
Methoda
13C d
pKa
2,4,6-Trichlorophenol-d2
2,4,6-Trichlorophenol (TCP)
2,6-Dichlorophenol
16 ꢄ 2
14 ꢄ 1
a
a
b
c
147.1
147.1
148.1
150.3
—
6.10
6.15
6.77
7.83
—
13 ꢄ 1
2,4-Dichlorophenol
3.7 ꢄ 0.3
2.1 ꢄ 0.3
1.6 ꢄ 0.1
0.32 ꢄ 0.02
0.27 ꢄ 0.01
2,6-Dichloro-1,4-benzoquinone (DCQ)
2,4,5-Trichlorophenol
3,4-Dichlorophenol
a
a
c
150.8
154.6
153.8
6.97
8.56
9.34
4-Chlorophenol
a
a
Key for methods: (a) Clꢀ and UV-vis methods together, (b) UV-vis method only, (c) Clꢀ method only.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2
849

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Chart 2
concentration of H2O2 (Fig. 4). The initial rate showed an
inverse dependence on the hydrogen ion concentration (Fig.
5). Thus, the experimental rate equation of the process is:
½DCQꢁ½H2O2ꢁ
v ¼ kuncat
ð8Þ
½Hþꢁ
The value kuncat ¼ (2.7 ꢄ 0.1) ꢂ 10ꢀ7 sꢀ1 was determined with
least-squares fitting at 25.0 ꢃC and 0.1 M ionic strength
(NaNO3 or acetate buffer). The deprotonation of DCQ is very
unlikely and our NMR observation also showed that DCQ
does not exchange its hydrogens in D2O. Therefore, the depro-
tonation of H2O2 is most likely responsible for the inverse
hydrogen ion dependence. H2O2 is known to have a pKa of
about 12 at 25.0 ꢃC.37 Because pKa > pH over the pH range
(2–5.5), HO2ꢀ is present at low concentration levels. However,
with pKa ca. 12, one can calculate the actual second-oꢀrder rate
constant for the reaction between DCQ and HO2 as ca.
Fig. 1 Ratio of rate constants k2/k3 as a function of the pKa of the
chlorophenols (25.0 ꢃC).
To our knowledge, this compound was only characterized in a
single previous work as a product of a photochemical reac-
tion.35 3-Hydroxy-2,6-dichloroquinone was found to be fairly
stable in dilute aqueous solution, but could not be isolated
in a pure form due to its reactivity; it is a relatively strong acid
with pKa about 1.6 and the corresponding dissociated anion
has a characteristic UV-vis absorption (lmax ¼ 524 nm;
e ¼ 2530 Mꢀ1 cmꢀ1),35 which was used for identification in
this work.
3 ꢂ 105 Mꢀ1
s
ꢀ1, which is clearly not approaching the diffu-
sion-controlled limit and therefore appears to be a reasonable
value for the process.
The kinetics of the reaction could be easily followed by mon-
itoring the peak of the product anion. The process was com-
plete within 10 s under neutral or basic conditions, but was
slower in acidic solutions. Sample kinetic curves are given in
Fig. 3. Some absorbance decrease was also detectable at longer
times, indicating that the product is not completely stable, in
agreement with earlier data obtained for other hydroxybenzo-
quinone derivatives.36 The early parts of the kinetic traces (up
to 15 minutes), where DCQ was oxidized, were well-separated
from the later unexplored secondary reactions (hours) and
could be treated independently. Experiments were done at
large H2O2 excess. These curves gave an excellent fit to an
exponential function, indicating that the process is first-order
with respect to the limiting reagent DCQ. The pseudo-first-
order rate constant at constant pH was proportional to the
Catalytic oxidation of 2,6-dichloro-1,4-benzoquinone
In the presence of the Fe(TPPS)þ catalyst, the oxidation of
DCQ with H2O2 gave a product different from the one
obtained in the uncatalyzed process. As shown in Fig. 6, chlor-
ide ion was clearly produced during the reaction. Fig. 7 shows
absorbance traces at two wavelengths: 395 nm, where primar-
ily the catalyst is monitored, and 524 nm, where 2,6-dichloro-
3-hydroxy-1,4-benzoquinone is the dominant absorbing
species but the catalyst also provides some absorbance
contribution. It can again be seen that 2,6-dichloro-3-hydroxy-
1,4-benzoquinone is not a product of the catalyzed pathway.
If it were, an absorbance increase of about 0.2 units would
Fig. 3 Kinetic traces taken during the reaction of 0.28 mM 2,6-
dichloro-1,4-benzoquinone with a [H2O2] of (a) 37 and (b) 187 mM
at pH ¼ 5.25, m ¼ 0.1 M (acetate buffer), 25.0 ꢃC, l ¼ 524 nm and
path length of 1 cm.
Fig. 2 Ratio of rate constants k2/k3 as a function of the 13C chemical
shift of the carbon atom bonded to the oxygen in chlorophenols
(25.0 ꢃC).
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
850
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2

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Fig. 4 Pseudo-first-order rate constants as a function of the concen-
tration of H2O2 during the reaction of 2,6-dichloro-1,4-benzoquinone
with H2O2 : pH ¼ 5.25, m ¼ 0.1 M (acetate buffer), 25.0 ꢃC.
Fig. 6 Chloride ion concentration as a function of time during the
reaction of 2,6-dichloro-1,4-benzoquinone with H2O2 catalyzed by
Fe(TPPS)þ: [DCQ] ¼ 1.63 mM, [H2O2] ¼ 139 mM, [Fe(TPPS)þ] ¼
2.9 mM, m ¼ 0.1 M (NaNO3), 25.0 ꢃC, [Clꢀ]0 ¼ 48 mM.
be expected at 524 nm in the first 3 min rather than the small
decrease observed experimentally. At later times, relatively
slow formation of 2,6-dichloro-3-hydroxy-1,4-benzoquinone
produced in the uncatalyzed pathway is observed at 524 nm.
The kinetic observations in the catalytic oxidation of DCQ
are in agreement with Scheme 1. The value of k2/k3 was deter-
mined both with the chloride selective (2.3 ꢂ 104 Mꢀ1) and the
spectrophotometric (1.9 ꢂ 104 Mꢀ1) methods. The agreement
of the two values, which is not obvious in this case, shows that
one Clꢀ is produced for every DCQ consumed in the process.
The identity of the product formed in the catalytic reaction is
not very easily confirmed. One possibility is that one of the
chlorine atoms in DCQ might be substituted with a hydroxyl
group in this process. However, based on the UV-vis observa-
tions, the product is not a substituted hydroxy-1,4-benzoqui-
none, which has strong absorption in the visible range
around 500 nm.35,38 The product is only formed in relatively
small concentrations and the 2,6-dichloro-3-hydroxy-1,4-ben-
zoquinone formed simultaneously in the uncatalyzed pathway
hindered our attempts to characterize it more thoroughly.
Later steps in the oxidation of 2,4,6-trichlorophenol
From the data collected in Table 1, it is possible to draw
further conclusions for the catalytic oxidation of TCP, which
has a k2/k3 value that is about an order of magnitude larger
than that for DCQ. Consequently, when TCP is present in
the oxidation system, DCQ is not oxidized. In other words,
DCQ accumulates during the oxidation and becomes a detect-
able intermediate. However, DCQ is slowly oxidized by H2O2
under the somewhat acidic conditions of TCP oxidation in the
direct pathway without the involvement of the catalyst. There-
fore, 2,6-dichloro-3-hydroxy-1,4-benzoquinone, the oxidation
product of the uncatalyzed DCQ oxidation pathway, should
be observable in the system. An experimental confirmation
of this prediction is shown in Fig. 8. This figure shows an
experiment where a sample was taken 1 h after starting the
catalyzed oxidation of TCP in the dark; this sample was
titrated with strong acid as quickly as possible. The pH and
UV-vis spectra were measured simultaneously. The original
Fig. 5 Initial rate as a function of the concentration of hydrogen ion
during the reaction of 2,6-dichloro-1,4-benzoquinone with H2O2 :
[DCQ] ¼ 0.38 mM, [H2O2] ¼ 373 mM, m ¼ 0.1 M (acetate buffer or
NaNO3), 25.0 ꢃC, l ¼ 524 nm and path length of 1 cm.
Fig. 7 Kinetic traces during the reaction of 2,6-dichloro-1,4-benzo-
quinone with H2O2 catalyzed by Fe(TPPS)þ: [DCQ] ¼ 0.51 mM,
25.0 ꢃC and path length of 1 cm.
[H2O2] ¼ 75 mM, [Fe(TPPS)þ] ¼ 6.2 mM, m ¼ 0.1
M (NaNO3),
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2
851

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´
´ ´
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Acknowledgements
This research was supported by the Center for Catalysis, Insti-
tute for Physical Research and Technology, Iowa State
University. Some experiments were conducted with the use
of the facilities of the Ames Laboratory, which is operated
by Iowa State University of Science and Technology under
contract W-7405-Eng-82. We acknowledge helpful discussions
with Prof. W. S. Jenks and Mr. Youn-chul Oh. GL also wishes
to thank the Fulbright Program and Hungarian funding
agency OTKA (grant No. T042755) for financial support.
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
852
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 8 4 7 – 8 5 2