Kinetic study of the reactions of oxoiron(IV) with aromatic substrates in aqueous solutions

Article abstract of DOI:10.1002/kin.10076

The kinetics of the reactions of oxoiron(IV) (FeO²⁺) with phenol, nitrobenzene, m-, 0-, and p-nitrophenol in 1 M HClO₄ was investigated by the stopped-flow technique. The rate constants of these reactions decrease with increasing the one-electron reduction potentials of the corresponding radical cations of the substrates and with the Hammett parameter of the NO₂ group in the phenol ring. A reaction mechanism is proposed, which accounts for the observed trends and for the nature of the reaction products.

Full text of DOI:10.1002/kin.10076

Kinetic Study of the
Reactions of Oxoiron(IV)
with Aromatic Substrates
in Aqueous Solutions
1
1
2
2
´
DANIEL O. MARTIRE, PAULA CAREGNATO, JORGE FURLONG, PATRICIA ALLEGRETTI,
1
´
MONICA C. GONZALEZ
1
´
´
Instituto de Investigaciones Fisicoquımicas Teoricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad
Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina
2
´
LADECOR, Departamento de Quımica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina
Received 7 January 2002; accepted 30 April 2002
DOI 10.1002/kin.10076
ABSTRACT: The kinetics of the reactions of oxoiron(IV) (FeO2+) with phenol, nitrobenzene, m-,
o-, and p-nitrophenol in 1 M HClO4 was investigated by the stopped-flow technique. The rate
constants of these reactions decrease with increasing the one-electron reduction potentials of
the corresponding radical cations of the substrates and with the Hammett parameter of the NO2
group in the phenol ring. A reaction mechanism is proposed, which accounts for the observed
C
trends and for the nature of the reaction products.
Kinet 34: 488–494, 2002
2002 Wiley Periodicals, Inc. Int J Chem
years it has been discussed whether oxoiron(IV) or HO
radicals are formed in the Fenton reaction. Presently,
the consensus appears to be that at low pH the Fenton
reaction yields hydroxyl radicals [4,5] (reaction (3)).
Thus, formation of FeO2+ and its thermal reactions are
of importance in atmospheric aqueous phase environ-
ments.
INTRODUCTION
Oxoiron(IV), FeO2+, is known to be formed from the
reaction of Fe(II) with ozone [1–3] (reaction (1)); the
oxidation of Fe(II) being an important sink of ozone.
FeO2+ isalsoproposedasthereactivespeciesformedin
the reaction of Fe(II) with hydrogen peroxide (Fenton
reaction, reaction (2)) [4]. However, for more than 50
Fe2+ + O3 → FeO2+ + O2
1
k1 = 8.2 105 M 1 s
(1)
Dedicated to Prof. Andre´ Braun on the occasion of his 60th
birthday.
Fe2+ + H2O2 → Fe2+ H2O2
Correspondence to: Daniel O. Ma´rtire and Mo´nica C. Gonzalez;
e-mail: dmartire@inifta.unlp.edu.ar; gonzalez@inifta.unlp.edu.ar.
Mo´nica C. Gonzalez is a research member of CONICET.
Daniel O. Ma´rtire is a research member of Comisio´n de Investi-
gaciones Cient´ıficas de la Provincia de Buenos Aires (CIC).
Contract grant sponsor: Agencia Nacional de Promocio´n
Cient´ıfica y Tecnolo´gica, Argentina (ANPCyT).
→ FeO2+ + H2O
(2)
(3)
Fe2+ + H2O2 → Fe3+ + HO + HO
Contract grant sponsor: Consejo Nacional de Investigaciones
Cient´ıficas y Te´cnicas, Argentina (CONICET).
2002 Wiley Periodicals, Inc.
FeO2+ ions (E (FeO2+/Fe3+
were reported to react by electron transfer with
)
2.0 V vs. NHE [6])
c

KINETIC STUDY OF THE REACTIONS OF OXOIRON(IV)
489
4
5
10 M and [Fe2+] = 8
10 M showed no sig-
inorganic reactants and by H atom abstraction with
several organic substrates.
nificant signals in the wavelength range from 240 to
400 nm indicating that the contribution of the reaction
between H2O2 and Fe2+ to the yields of Fe3+ is negli-
gible under our experimental conditions.
Forexperimentsdoneinthepresenceoforganicsub-
strates, one of the syringes contained (NH4)2Fe(SO4)2
and the organic compound in 1 M HClO4 and the other
syringe contained the ozone solution in 1M HClO4.
Blank experiments showed that the thermal reaction of
Fe2+, Fe3+ or ozone and the organic substrates is neg-
ligible under our experimental conditions during the
time period of several minutes required for the perfor-
mance of the experiments.
The stopped-flow experiments yielded the time-
resolved difference absorbance at a given wavelength
, 1A (t, ) = A(t, ) A(tf, ), where t = 0 was taken
at the moment of initiating the mixture of the reactants
and at t = tf the reaction had reached completion. Ni-
trobenzene and nitrophenols absorbed in the 320–360
nm wavelength range and consequently contributed to
the transient absorption observed in O3/Fe2+ experi-
ments containing these substrates. The higher differ-
ence absorbance at ␭ > 320 nm observed for experi-
ments in the presence of the nitrocompound were due
to substrate depletion.
New experimental information on the reactivity and
reaction mechanisms of FeO2+ ions toward organic
substrates in aqueous solutions is of importance for
evaluating the role of iron in the chemistry of the
aqueous atmosphere, and as a comparative source for
elucidating the nature of the active intermediates in
Fenton-type reactions. In this paper, the in situ genera-
tion of FeO2+ ions by reaction (1), using the stopped-
flow technique was used for the study of the reactions
of FeO2+ ions with substrates of environmental im-
portance [3,6]. Phenol, nitrobenzene, and nitrophenols
were chosen as model substrates since their degrada-
tion to less harmful materials by Fenton-type reactions
is effective [7] despite the fact that most of them are
difficult to degrade by natural processes.
EXPERIMENTAL
Distilled water was passed through a Millipore system.
All chemicals were purchased from Merck (p.a.) and
used without further purification. A homemade ozona-
tor was used in order to obtain gaseous ozone through
an electrical discharge in dry oxygen [8]. Stock so-
lutions of ozone were prepared by bubbling ozone in
1 M HClO4 solutions. The ozone concentration was
Product Analysis
measured spectrophotometrically at 260 nm (ε = 3300
1
M
cm 1 [9]). Small amount of H2O2 is reported to be
LC-MS analysis was performed with an Agilent 1100
LC-MSD system with diode array and quadrupolar
mass selective detectors. The LC was equipped with
a Hypersil AA-ODS 200 mm 2.1 mm 5 ␮m col-
umn. The solvent was an isocratic methanol/water 1:1
mixture. The analysis was performed at 25 C with a
solvent flow rate of 1 ml/min. Injection volumes were
20 ␮l. The MS detector was coupled through an API
interface (electrospray). LC-MS runs were performed
in the positive and negative modes. The mass range was
from 90 to 300 amu.
formed as a result of O3 degradation in acidic solutions
(pH 0–4) initiated by the reversible surface-catalyzed
decomposition of ozone to atomic oxygen and O2 and
propagated by HO and O2 /HO2 radicals [10]. The
concentration of H2O2 formed during our preparation
of ozone/molecular oxygen saturated acidic solutions
was measured by the method reported by Løgager et al.
[3]. In all cases [H2O2] was
FeO2+ ions showing a characteristic absorption
band in the 290–350 nm wavelength range with ε320
2
10 4 M.
=
1
1
420 M cm [3,6] were generated in situ from the
reaction of ozone with Fe2+ in 1M HClO4, reaction
(1). The buildup and decay of FeO2+ was conveniently
monitored in the 300–360 nm range, where the con-
tribution of the absorption of Fe2+, Fe3+, and O3 is
negligible. FeO2+ ions were thus prepared by mix-
ing equimolar amounts of (NH4)2Fe(SO4)2 6H2O and
ozone in HClO4 medium in a Hi-Tech Scientific SFA-
20 Rapid Kinetics stopped- flow accessory. The ab-
sorbance was measured with a sensitive detection sys-
tem conventionally used for flash-photolysis experi-
Separate solutions containing O3, Fe2+, and o- or
m-nitrophenol were mixed and then allowed to stand
for 1 min before being bubbled with nitrogen. The final
concentrations of O3, Fe2+, and the organic compound
were 6 10 5 M. Two series of blank experiments un-
der similar conditions were performed: one with solu-
tions without Fe2+ and another with solutions without
O3.
Blank samples showed no chromatographic peaks
except for those corresponding to the nitrophenol re-
actant. For samples where FeO2+ ions were involved,
the MS negative mode detected mainly nitrophenol and
the positive mode provided evidence for the presence
of the oxidation products, whose molecular formulas
ments [11]. The concentrations of ozone and Fe2+ used
4
before mixing were of the order of 1.7
10 M.
Blankexperimentsperformedwith[H2O2]=(0.3–2)

´
MARTIRE ET AL.
490
FeO2+ + HO2 (+H+) → Fe3+ + O2 + H2O
k9 = 2 106 M 1 s
Table I Detected Oxidation Products for o-, m-, and
p-Nitrophenols
1
(9)
(10)
(11)
(12)
(13)
Molecular Weight,
Fe2+ + HO2 (+H+) → Fe3+ H2O2
Formula and/or Name
o-Nitrophenol m-Nitrophenol
1
k10 = 1.2 106 M 1s
(I) 153, Nitro
X
–
p-benzoquinone
Fe3+ + HO2 ( H+) → Fe2+ + O2
(II) 169, C6H3O3(NO2)
(III) 183, C6HO4(NO2)
(IV) 185, C6H3O4(NO2)
(V) 124, C6H4O3
X
X
X
X
X
X
X
–
X
X
1
1
1
k11 = 2 105 M 1s
HO + H2O2 → H2O + HO2
(VI) 138, C6H2O4
k12 = 2.7 107 M 1s
FeO2+ + FeO2+ → [FeOOFe]4+
are listed in Table I. The detection of oxidation prod-
ucts only in the positive mode is due to the ease of
protonation of the products with multiple carbonyl
groups for which the method is very sensitive. Neg-
ative ionization by hydrogen abstraction is not favored
by the mobile phase.
k13 < 50 M 1 s
HO2 radicals may also decay by bimolecular recom-
bination [12].
Figure 1 shows the difference absorption spectrum
at time t = tmax = 0.1 s when the reactants are almost
consumed and FeO2+ ion reaches its maximum con-
centration, as shown in Fig. 2 (inset). The difference
spectrum may be estimated taking the reported absorp-
RESULTS AND DISCUSSION
The first-order decay rates observed for FeO2+ ions in
the absence of organic substrates are faster than those
reported in the literature [3]. Since pure chemicals were
used, the faster decay is not due to the presence of im-
purities but due to the formation of H2O2 during ozone
generation ( 2 10 4 M). In fact, our observed first-
order apparent decay rate constant of FeO2+ ions in 1 M
HClO4 solutions containing H2O2 is of the same order
as that reported in Ref. [3] for the measured [H2O2] in
each set of experiments. As a consequence, the reac-
tion mechanism should include not only reactions (4)–
(7), but also reactions (8)–(12) [3]. The contribution
of a second-order rate law to the decay rate of FeO2+
ions is negligible under our experimental conditions, in
agreement with k < 50 M 1 s 1 for the recombination
of FeO2+ ions [3,12], reaction (13).
3+
tion spectra of FeO2+ and Fe(H2O)6 ions [3,10]. A
goodagreement isfound, withinthe experimental error,
for a 1:1 molar ratio of FeO2+ consumed to Fe3+ pro-
duced, 1[FeO2+]/1[Fe3+], thus indicating that mainly
reactions (4), (5), (8) and (9) yielding Fe3+ contribute
FeO2+ + H2O (+H+) → Fe3+ + H2O + HO
1
k4 = 0.013 s
(4)
(5)
(6)
(7)
(8)
FeO2+ + HO (+H+) → Fe3+ + H2O2
k5 = 1.0 107 M 1 s
FeO2+ + Fe2+ (+2H+) → 2Fe3+ + H2O
k6 = 1.4 105 M 1 s
1
1
1
Figure
1 Transient spectrum for experiments with
0.08 mM O3 and 0.08 mM Fe2+ solutions at tmax = 0.1 s
5
HO + Fe2+ → FeOH2+
( ). Dashed lines correspond to the spectrum of 3.6 10
N
M (a) FeO2+ ion [3] and (b) Fe(H2O)63+, ε240 = 3850 M
1
k7 = 3.2 108 M 1 s
cm 1[10], multiplied by ( 1). The solid line is the expected
transient absorption, 1A(t) = εFeO
l
[FeO2+]t + εFe
FeO2+ + H2O2 (+H+) → Fe3+ + H2O + HO2
l
{[Fe3+]t-[Fe3+
]
final} for 1[FeO2+] = 1 [Fe3+] = 3.6
k8 = 1 104 M 1 s
1
5
10 M and l = 1 cm.

KINETIC STUDY OF THE REACTIONS OF OXOIRON(IV)
491
In order to further prove the proposed reaction mecha-
nism and the validity of the simplified kinetic analysis,
a computer program based on the numerical resolution
of the differential equations system by the third-order
Runge Kutta method [11,13] was used to simulate the
experimental absorbance profiles at 320 nm for exper-
iments in the absence of organic substrates and in the
presence of phenol and m-nitrophenol.
The initial ozone and Fe2+ concentrations in the
reaction mixture were taken as input parameters. The
program incorporates a set of well-established reac-
tions involving the species present in the system, reac-
tions (1), (4)–(14), the reaction of HO radicals with
the organic substrates, recombination of HO radi-
cals to yield H2O2, further reactions involving H2O2,
HO , and HO2 radicals [14] and the decomposition
reactions of ozone in acid medium [10]. The values
of k14 used for the simulations are those depicted in
Table II. Both Fe2+ or Fe3+ could be formed as prod-
ucts of reaction (14). However, the simulated FeO2+
concentration profiles are not sensitive to the oxida-
tion state of iron formed as a product of this reaction.
However, as discussed later, the proposed reaction
Figure 2 Log 1A vs. t for the signals obtained at ␭
=
obs
320 nm for experiments with 1 10 4 M O3 and 9 10 5 M
4
Fe2+ solutions containing (a) 1
10 M and (b) 5
4
10 M p-nitrophenol. The solid lines represent first-order
fittings. Inset: Difference absorbance profiles obtained at ␭ =
320 nm for a solution containing 5.5 10 5 M ozone, 6.3
10 M Fe2+, and 4 10 M H2O2 in the absence (lower
5
5
5
trace) and in the presence of 1.5 10 M m-nitrophenol
(upper trace). The solid lines show the computer simulations.
to FeO2+ ion decay. Consequently, reaction (6) yield-
ing 2 moles of Fe3+ per FeO2+ consumed [3] is not
important for t > tmax. Computer simulations [11] of
the experiments, vide infra, indicate that under our ex-
perimental conditions reaction (6) is relevant only for
t < tmax. Its effect is that of depleting the maximum con-
centrationofFeO2+ comparedtothatexpectedfromthe
initial amounts of ozone and Fe2+
.
The traces obtained in the wavelength range from
320 to 360 nm for experiments in the presence of the
substituted benzenes (S) decay following a pseudo-
first-order kinetics as shown by the solid lines in
Fig. 2 for p-nitrophenol. The apparent rate constant,
kapp, linearly increases with increasing organic sub-
strate concentration, as expected from reaction (14)
assuming [S] constant throughout the experiment.
In fact, the absorption spectra taken a few minutes
after mixing the reactants indicate that the consump-
tion of organic substrates in each experiment is 15%.
Figure 3 shows the linear plots of kapp vs. [S] for all
the substrates. The slopes of the lines in Fig. 3 are the
absolute bimolecular rate constants for the reaction of
FeO2+ ions with the organic substrates, k14. Table II
depicts the values of k14 obtained for the five
substrates.
Figure 3 Dependence of the apparent decay rate of FeO2+
ions, kapp, on substrate concentration for (a): ( ) phenol, ( )
N
p-nitrophenol, and ( ) nitrobenzene and (b): (4) m-
nitrophenol and ( ) o-nitrophenol. Each reported value of
kapp is the average of no less than five experiments. The error
in kapp is <15% for a 95% confidence level. The error bars
are smaller than or of the order of the symbol size.
FeO2+ + S → Products
(14)

´
MARTIRE ET AL.
492
Table II Absolute Rate Constant for the Reaction of FeO2+ with the Organic Substrates
H O Bond Dissociation
1
1
1 a
Substrate
k14 (M
s
)
Energy (kJ mol
)
(NO2)b
E/Vd
Phenol
(1.5 0.2) 104
(5.8 0.4) 103
(13.6 0.9) 103
(12.3 0.3) 103
(1.05 0.3) 103
361.9
374
390
396
–
0
0.97
o-Nitrophenol
m-Nitrophenol
p-Nitrophenol
Nitrobenzene
0.95c
0.71
0.81
–
1.13
1.23
aFrom Ref. [27].
bPhenol was taken as the parent compound for which (NO2) = 0. Values taken from Ref. [28], otherwise indicated.
cFrom Ref. [29].
d One electron reduction potential for the reaction S + + e → S, taken from Ref. [30].
mechanism may only be explained if Fe2+ is formed
after reaction (14).
assigned to the fast reaction of FeO2+ ions with the
secondary organic products when the surplus of phe-
nol overFeO2+ isinsufficient. The verygoodfirst-order
plots found here for all the substrates over a wide con-
centration range indicate that further reactions of the
secondary organic products with FeO2+ ions are of no
importance under our experimental conditions.
An H-abstraction mechanism [6] seems not to apply
for the phenols studied here, since the absolute values
for k14 increase with increasing O H bond dissociation
energy, BDE, as depicted in Table II. A reasonable cor-
relation with a negative slope is found between log k14
FeO2+, Fe3+, and m-nitrophenol are the main
species contributing to the absorption measured at
320 nm. In a first approximation, the organic reaction
products were assumed not to significantly absorb at
320 nm, vide infra. Simulated concentration profiles
for FeO2+, Fe3+, and m-nitrophenol were converted
into the corresponding difference absorbance (1A)
1
profiles at 320 nm [14] taking ε(FeO2+) = 420 M
cm [3], ε(Fe3+) = 14 M cm [15a], and ε(m-
nitrophenol) = 1950 M 1 cm 1. A good agreement be-
tween experimental and simulated traces was found for
all the simulated experiments, as shown in the insets of
Figs. 2 and 4.
1
1
1
The final absorption spectra were recorded a few
minutes after mixing the reactants with a conventional
spectrophotometer. The upper solid line in Fig. 4 corre-
sponds to the final spectrum taken for the stopped-flow
experiments depicted in the traces shown in the inset
([O3] = [Fe2+] = 9 10 5 M, [H2O2] = 2 10 4 M,
and [Phenol] = 5 10 4 M). Subtraction of the phenol
and Fe3+ absorbance at the final concentrations val-
ues obtained from the simulation of the traces, 4.5
10 4 and 6.8 10 5 M, respectively, yields a spectrum
(dashed line) very similar to that of benzoquinone.
A similar analytical procedure for the experiments
5
initially containing 5.5
10
M ozone, 6.3
10 M Fe2+, 4 10 M H2O2, and 1.5 10
M
5
5
5
m-nitrophenol (Fig. 2 inset) yields a remaining spec-
trum (not shown) with an absorption maximum around
215 nm and negligible absorption at ␭ > 300 nm. The
latter observations seem to support the negligible ab-
sorption of the organic reaction products at 320 nm for
phenol and m-nitrophenol.
Our results show a linear dependence of kapp with
[S] for a broad concentration range. Jacobsen et al. [6]
investigated the reaction of FeO2+ ions with phenol
and found a nonlinear dependence of kapp with the con-
centration of phenol. The curvature of the plots were
Figure 4 Absorption spectra taken a few minutes after mix-
ing the reactants ([O3] = [Fe2+] = 9 10 5 M, [H2O2] = 2
10 4 M, and [Phenol] = 5 10 4 M) (shown by the upper
solid line: final spectrum). Subtraction of the absorbance of
4
a 4.5 10 M solution of phenol and the final absorbance
5
of a 6.8 10 M Fe3+ solution from the final spectrum
yields the dashed line. Inset: Difference absorbance profiles
obtained at ␭ = 320 nm for the experiments of the main
figure in the presence and absence of phenol (lower and up-
per traces, respectively). The solid lines show the computer
simulations.

KINETIC STUDY OF THE REACTIONS OF OXOIRON(IV)
493
and the Hammett parameter measuring the electron
withdrawing ability of the NO2 group on the phenol
ring, (NO2) in Table II. Moreover, a good correla-
tion is also observed between log k14 and the reported
to hydroquinone formation (reaction (16)), which may
be reversibly oxidized by Fe3+ (reaction (17)) and/or
HO2 radicals to hydroxyphenoxyl radicals [12]. Oxy-
gen transfer and charge transfer assisted by Fe3+ may
also lead to hydroxyphenoxyl radical formation, reac-
tion (18). The hydroxyphenoxyl radical disproportion-
ates to quinone and hydroquinone [16] (reaction (19)),
otherwise it may be reversibly oxidized to quinone by
Fe3+ [17], reaction (20). Formation of products more
oxidized than benzoquinones may be due to secondary
thermal reactions, such as that of semiquinone and/or
quinone with HO2 radicals [17] or with molecular oxy-
gen. The observed products may be rationalized by nu-
cleophilic reactions showing similar patterns to that
of ␣, ␤ unsaturated ketones towards molecular oxygen
and acid catalyzed hydrolysis. Fe2+ ions may be finally
oxidized to Fe3+ by HO2 and/or quinone [12,17].
An electron transfer from the aromatic ring to
FeO2+ would yield a radical cation of the organic
substrate, which in aqueous solutions reversibly hy-
drates to hydroxycyclo-hexadienyl radicals (HCHD).
In particular, HCHD radicals of phenols eliminate wa-
ter within less than 400 ␮s to yield phenoxyl radicals
[18]. Recombination of these radicals yield dimeriza-
tion products [19], which were not observed in our
experiments, thus indicating that the contribution of an
electron transfer mechanism is negligible.
Despite that there are only a few studies involving
FeO2+ ions in aqueous solutions [3,6], oxoiron(IV)
formation in biological systems has been more in-
tensively explored [20]. Oxoiron(IV) complexes with
porphyrins have been shown to react with alkenes
by either direct oxygen transfer, free radical addition,
electrophilic addition and/or oxetane formation [4].
Oxoiron(IV) complexes seem to react by diverse
reaction channels depending on the nature of the
substrate. Reaction Scheme 1 involving electrophilic
addition and oxygen transfer from FeO2+ to car-
bon combines already known reaction channels of
oxoiron(IV) complexes.
one-electron reduction potential for the reaction S.+
+
e
→ S, E in Table II. The observed trends may be
explained by two different attacks of FeO2+ ions to the
aromatic ring: (i) electrophilic addition and (ii) electron
transfer.
Electrophilic addition of FeO2+ ions to the aromatic
substrates yields a positively charged intermediate
adduct. The HO group being a strong activating sub-
stituent in electrophilic additions and substitutions di-
rects the attack to the ortho and para positions, in agree-
ment with the formation of nitro p-benzoquinone as a
product of the reaction of o-nitrophenol with FeO2+
.
The effect of the NO2 substituent is that of diminishing
the charge density available for reaction.
Unfortunately, little information is obtained from
our experiments on the reaction channels following the
initial attack of FeO2+ to the aromatic ring. The fi-
nal products detected in the reaction mixture several
minutes (spectroscopic detection) to hours (chromato-
graphic analysis) after mixing support formation of
quinones and unsaturated ketones.
Reaction (15) in Scheme 1 shows the electrophilic
addition of FeO2+ to the para position of the HO group
in phenol, o-, and m-nitrophenol. Further oxygen atom
transfer from iron to C atom and Fe2+ elimination leads
CONCLUSION
According to the proposed mechanism, the main dif-
ference between FeO2+ and HO radical reactions with
phenols is given by the electrophilic addition of FeO2+
to the aromatic ring leading to intermediates other than
the hydroxycyclohexadienyl radical (HCHD) formed
in HO radical reactions [12]. Thus, HCHD recombi-
nation products usually observed in HO reactions with
aromatic substrates are not expected to be formed in
FeO2+ oxidation reactions.
Scheme 1 Proposed reaction mechanism for the reaction of
FeO2+ with phenol, o- and m-nitrophenol (see text).

´
MARTIRE ET AL.
494
Studies on the effect of Fenton reactions on the
degradation of aromatic substrates show two times
faster decay rates for 4-nitrophenol than for 2-
nitrophenol [7]. The latter observation, the favorable
formation of quinone and hydroxylated products in
Fenton reactions [4,21–23], and the electrophilic char-
acter of Fenton intermediates towards phenolic rings
[24] are in agreement with the observed behavior for
Kinetics Database Version 3.0. Notre Dame Radiation
Laboratory: Notre Dame, IN, and National Institute of
Standards and Technology: Gaithersburg, M.D., 1998
and references cited therein.
13. Gonzalez, M. C.; Braun, A. M. Res Chem Intermed
1995, 21, 837.
14. Ma´rtire, D. O.; Gonzalez, M. C. Int J Chem Kinet 1997,
29, 589.
15. (a) Benkelberg, H. J.; Warneck, P. J Phys Chem 1995, 99,
5214; (b) Zuo, Y. G. Geochim Cosmochim Acta 1995,
59, 3123; (c) Sedlak, D. L.; Hoigne´, J. Atmos Environ
1993, 27A, 2173.
16. Land, E. J. J Chem Soc, Faraday Trans 1993, 89, 803.
17. (a) Isaacs, N. S. Reaction Intermediates in Organic
Chemistry; Wiley: London, 1974; (b) Schuchmann, M.
N.; Bothe, E.; von Sonntag, J.; von Sonntag, C. J Chem
Soc, Perkin Trans 2 1998, 791.
FeO2+
.
Degradation of p-nitrophenol by Fenton reagents
([H2O2] = 5 mM, [Fe2+
]
0.18 mM) was reported
to fit to a phenol pseudo-first-order decay with a rate
constant of
9
10 4 s 1 [25]. Taking our value of k14
= 1.2 104 M 1 s 1 and assuming a low FeO2+ steady
7
concentration of 10 M in the reported experiments
[25,26], the estimated pseudo-first-order decay rate of
p-nitrophenol due to its reaction with FeO2+ is 1.2
10 3 s 1. Consequently, despitethemoderatereactivity
shown by FeO2+ ions, their reactions with the organic
substrates are not expected to be the rate-limiting step
in Fenton reactions if oxoiron(IV) were the reactive
intermediate involved.
18. Ma´rtire, D. O.; Gonzalez, M. C. Prog React Kin Mech
2001, 26, 201.
19. Howard, J. A.; Scaiano, J. C. In Rate and Equilibrium
Constants for Reactions of Polyatomic Free Radicals,
Biradicals and Radical Ions in Liquids; Fischer, H. (Ed.)
(New Series II/19); Landolt-Bo¨rnstein: Berlin, 1991;
Ch. 8, p. 142.
20. Fleet, G. W. In Organic Reaction Mechanisms; Knipe,
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