Bromite ion catalysis of the disproportionation of chlorine dioxide with nucleophile assistance of electron-transfer reactions between ClO2 and BrO2 in basic solution

Article abstract of DOI:10.1021/ic010849w

The rate of ClO₂ conversion to ClO₂^- and ClO₃^- is accelerated by BrO₂^-, repressed by ClO₂^-, and greatly assisted by many nucleophiles (Br^- > PO₄^3- > HPO₄^2- > CO₃^2- > Cl^- ～ OH^- > CH₃COO^- ～ SO₄^2- C₅H₅N ? H₂O). The kinetics (at p[H⁺] = 9.3-12.9) show that the first step of the mechanism is an electron transfer between ClO₂ and BrO₂^- (k₁ = 36 M^-1 s_-1) to give ClO₂^- and BrO₂. This highly reversible reaction (k₁/k_-1 = 1 × 10^-6) accounts for the observed inhibition by ClO₂^-. The second step is an electron transfer between ClO₂ and BrO₂ to regenerate BrO₂^- and form ClO₃^-. A novel aspect of the second step is the large kinetic contribution from nucleophiles (k^Nu) that assist the electron transfer between ClO₂ and BrO₂. The k^Nu (M^-2 s^-1) values at 25.0 °C vary from 2.89 × 10⁸ for Br^- to 2.0 × 10⁴ for H₂O.

Full text of DOI:10.1021/ic010849w

Inorg. Chem. 2002, 41, 108−113
Bromite Ion Catalysis of the Disproportionation of Chlorine Dioxide with
Nucleophile Assistance of Electron-Transfer Reactions between ClO2
and BrO in Basic Solution
2
Lu Wang, Jeffrey S. Nicoson, Kara E. Huff Hartz, Joseph S. Francisco, and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
Received August 6, 2001
-
-
-
-
The rate of ClO
by many nucleophiles (Br > PO
2 2 3 2 2
conversion to ClO and ClO is accelerated by BrO , repressed by ClO , and greatly assisted
-
3-
2-
2-
-
-
-
2-
4
> HPO
4
> CO
3
> Cl ∼ OH > CH
3
COO ∼ SO
4
∼ C
5
H
5
2
N . H O).
+
The kinetics (at p[H ] ) 9.3−12.9) show that the first step of the mechanism is an electron transfer between ClO
2
-
-1 -1
-
-6
and BrO
2
(k
1
) 36 M s ) to give ClO
2
and BrO
2
. This highly reversible reaction (k
1
/k-1 ) 1 × 10 ) accounts
-
for the observed inhibition by ClO
2
. The second step is an electron transfer between ClO
2
and BrO
2
to regenerate
-
-
Nu
BrO
2 3
and form ClO . A novel aspect of the second step is the large kinetic contribution from nucleophiles (k )
that assist the electron transfer between ClO
Nu
-2 -1
8
2
2
and BrO . The k (M s ) values at 25.0 °C vary from 2.89 × 10
-
4
2
for Br to 2.0 × 10 for H O.
Introduction
2 2
transfer reactions between BrO and ClO are catalyzed by
many nucleophiles.
2
Bromine dioxide (BrO ) is much more reactive than the
corresponding chlorine species (ClO
2
) and less is known
Experimental Section
-
about its redox chemistry. Pulsed radiolysis of BrO
solutions has been used to generate trace levels of BrO
3
in
2 2 2
Preparation of NaBrO . Ba(BrO ) was prepared from the
disproportionation of Ba(OBr) as described by Noszticzius,
2
Noszticzius, and Schelly5 but with several modifications. The
reaction slurry was maintained at pH 10.0-10.5 (as opposed to
pH 11) to increase the rate of formation of Ba(BrO
for greater conversion of Ba(OBr) to Ba(BrO in a 2 h reaction
time. Also, the reaction slurry was kept at a lower temperature (-10
to -20 °C) in an ice-methanol bath. The lower temperature
2
neutral and basic solutions.1 This BrO
,2
undergoes very rapid
2
2
disproportionation with a rate proportional to [BrO
2
] and a
complex dependence in [OH ]. Micromolar concentrations
of BrO have been generated in sulfuric acid solution by
Ce(III) reduction of BrO
HBrO and BrO
-.4 For a brief period NaBrO
commercially, but this is no longer the case. We have
prepared NaBrO in a mixture of nonreactive salts on the
-
1
2 2
) . This allowed
2
2
2 2
)
3
-
3
and by the reaction between
was available
2
3
2
permitted increased precipitation and collection of Ba(BrO
collected barium salt, dissolved in a minimum amount of water,
contained mostly Ba(BrO
2 2
) . The
2
5
basis of the method of Noszticzius et al. with additional
steps to remove all NaOBr and most of the NaBr.
2
)
2
, BaBr
2
, and Ba(OH)
2
with small
-
amounts of Ba(OBr) and Ba(BrO ) . Since OBr may compete
2
3 2
-
This work pertains to the mechanisms of the reaction
with BrO
2
in many redox reactions, it is important that its
-
between ClO
2
and BrO
2
in basic solutions. We show that
via
concentration be minimized. The removal of Ba(OBr)
by stepwise addition of Na SO , which reduces OBr to Br . Sulfite
2
was achieved
-
-
-
BrO
BrO
2
catalyzes the overall disproportionation of ClO
as a steady-state intermediate. Furthermore, electron-
2
2
3
-
8
-1 -1 6
reacts much more rapidly with OBr (k ) 1 × 10 M
s
) than
2
-
-1 -1 7
with BrO
2
(k ) 3.7 M
s ) and permits preferential removal
-
*
Author to whom correspondence should be addressed. E-mail:
of OBr . The solution was monitored spectrophotometrically and
shown to be free of OBr (ꢀ ) 332 M cm at 329 nm). Bromide
margerum@purdue.edu.
-
-1
-1
6
(
(
1) Buxton, G. V.; Dainton, F. S. Proc. R. Soc. A 1968, 304, 427-439.
2) Field, R. J.; Raghavan, N. V.; Brummer, J. G. J. Phys. Chem. 1982,
ion was removed by ion exchange with solid AgOH (prepared fresh
by the addition of NaOH solution to either Ag
2
SO
The Ba2 ions were exchanged with Na ions by the addition of
Na SO to precipitate BaSO . The resulting solution was lyophilized
4 3
or AgNO ).
8
6, 2443-2449.
3) F o¨ rsterling, H. D.; Lamberz, H. J.; Schreiber, H. Z. Naturforsch. 1980,
5a, 329-331.
4) F o¨ rsterling, H. D.; Lamberz, H. J.; Schreiber, H. Z. Naturforsch. 1980,
5a, 1354-1360.
5) Noszticzius, Z.; Noszticzius, E.; Shelly, Z. A. J. Phys. Chem. 1983,
7, 510-524.
+
+
(
(
(
3
2
4
4
3
(6) Troy, R. C.; Margerum, D. W. Inorg. Chem. 1991, 30, 3538-3543.
(7) Lee, C. L.; Lister, M. W. Can. J. Chem. 1979, 57, 1524-1530.
8
108 Inorganic Chemistry, Vol. 41, No. 1, 2002
10.1021/ic010849w CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/08/2001

Disproportionation of Chlorine Dioxide
and stored at -10 °C as a dried salt mixture. Ion chromatographic
analysis determined that the solid product contained (by weight)
Table 1. Ion Chromatographic Determination of Products for the
Reaction between BrO2 and ClO2
-
a,b
29.9% NaBrO
2
, 3.2% NaBrO
3
, 0.3% NaBr, 4.2% NaNO
3
, and
p[H+]
-
-
c
-
- c
[BrO2 ]f/[BrO2 ]i
[ClO2 ]f/[ClO3 ]f
4
1.6% Na SO
2
4
. The remaining 20.7% is NaOH and waters of
1
1
0.17
0.43
1.00 ( 0.04
1.04 ( 0.04
1.04 ( 0.04
0.99 ( 0.04
1.00 ( 0.04
1.03 ( 0.04
crystallization. For our experiments, it was critical to remove all
OBr . Each of the other salts was tested to determine its effect on
-
11.35
the kinetic studies in basic solution.
a
-
Reactions and conditions: [BrO2 ]i ) 0.133 mM; [ClO2]i ) 0.271 mM;
[CO32 ]T ) 0.10 M; 25.0 °C; 300 s reaction time. b IC determinations
showed that the concentrations of small amounts of Br and BrO3 in the
reactants (from the NaBrO2 sample) were unchanged in the products.
-
Reagents. All solutions were made with distilled-deionized
water. Chlorine dioxide stock solution was prepared as described
previously8,9 and was protected from light and stored in a refrigera-
-
-
c
-
-
-
[
BrO2 ]f, [ClO2 ]f, and [ClO3 ]f are the product concentrations.
tor. This solution was standardized spectrophotometrically based
, ꢀ ) 1230 M cm at 359 nm.8
-
1
-1
on the molar absorptivity of ClO
Commercially available NaClO
2
2
was treated to remove sodium
carbonate and was recrystallized.8 Stock solutions NaClO
,10
were
2
-
1
standardized spectrophotometrically at 260 nm (ꢀ ) 154 M
-
1 8
cm ). Stock solutions of NaBrO
2
were prepared daily from the
-
stored solid, and the concentration of BrO
2
was determined
-
1
-1 11
spectrophotometrically at 295 nm (ꢀ ) 115 M cm
,
a value
that was confirmed iodometrically).12 Ionic strength (µ) was
controlled with recrystallized NaClO . All kinetic studies were
4
carried out at 25.0 ( 0.2 °C and µ ) 1.0 M.
pH Measurement. An Orion model 720A digital pH meter
equipped with a Corning combination electrode was used in all
measurements. The electrode was calibrated through titrations of
standard HClO
values to p[H ] and p[OH ] (pK
4
with standard NaOH to correct the measured pH
+
-
13
w
) 13.60) at 25.0 ( 0.1 °C and
4
µ ) 1.0 M (NaClO ).
Products. The reaction products were identified and quantita-
tively determined via an EPA recommended method14 by use of a
Dionex DX-500 ion chromatograph. Samples were injected via an
autosampler (AS40) through a 25 µL injection loop into anion
exchange guard (AG9HC) and separation (AS9HC) columns. The
Figure 1. Dependence of the reciprocal of the second-order rate constant
kobs) for the disproportionation of chlorine dioxide on the chlorite
(
-
concentration. Conditions: 0.280 mM [ClO2]; 18.90 mM [BrO2 ]; 0.10 M
+
[
×
CO3]T; p[H ] 9.45. Slope ) (1.88 ( 0.07) s, and intercept ) (1.5 ( 3.0)
analytes were eluted with 9.0 mM Na
2
CO
3
at a flow rate of 1.0
-4
10 M s.
mL/min. Suppressed conductivity detection (ED40), with an ASRS-
Ultra suppressor in the gas-assisted self-regeneration mode (SRS)
Results and Discussion
and a current of 100 mA, was used to determine the analytes. The
Reaction Stoichiometry. Chlorine dioxide disproportion-
-
2 2
product distributions were obtained by mixing ClO and BrO
ates slowly in basic solution to form chlorite and chlorate
reactant solutions. The mixtures were capped, and the solutions
were reacted for 5 min (essentially to completion) and were
1
6
2 2
ions. The addition of NaBrO to ClO in base greatly
accelerates the disproportionation and gives 1:1 stoichiometry
that corresponds to eq 1. Ion chromatographic results (Table
quenched by purging any remaining ClO
2
with Ar.
A Perkin-Elmer Lambda-9 UV-vis-NIR spectrophotometer was
1
ClO
) show (within experimental error) that equal amounts of
used to acquire reaction spectra for slower reactions after ClO
2
-
-
-
-
solutions were T-mixed with BrO
2
solutions in carbonate buffer
2
and ClO
3
are formed and no BrO
2
is consumed.
+
p[H ] 10.30. Spectral scans were taken from 250 to 450 nm at
2 3
Since our added NaBrO has small amounts of NaBrO and
-
-
100 s intervals and after the reaction was complete.
NaBr present, we also analyzed for BrO3 and Br before
and after the reaction and found no change in their
Kinetics. Kinetic studies were performed on an Applied Photo-
Physics SX 18 MV stopped-flow spectrophotometer (APPSF,
optical path length ) 0.962 cm). The progress of the BrO
-
-
3 3
concentrations. Separate tests show that BrO , NO , and
ClO
-
-
2
/ClO
2
4 2
have no effect on the rate of ClO disproportionation.
reaction was followed by observing the disappearance of ClO
2
at
)
Spectrophotometric results from 250 to 450 nm show that,
-
(ꢀ359 ) 45.3 M cm-1
-1
3
59 nm in the presence of excess BrO
2
after all the ClO
exactly to the initial spectrum of the added BrO
spectrum of ClO
2
reacts, the remaining spectrum corresponds
-
(ꢀ359 ) 1.3 M cm-1). All rate constants were
-1
and excess ClO
2
-
2
plus the
1
5
determined from the average of five pushes. SigmaPlot 4.0 was
used for the regression analysis.
-
2
formed.
BrO2-
-
8 ClO2- + ClO3- + H O
2
ClO + 2OH
(1)
(
(
8) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-4327.
9) Masschelein, W. J. J. Am. Water Works Assoc. 1984, 76, 70-76.
2
2
(
(
(
(
10) Fabian, I.; Gordon, G. Inorg. Chem. 1992, 31, 3785-3787.
11) Lee, C. L.; Lister, M. W. Can. J. Chem. 1971, 49, 2822-2826.
12) Perrone, T. F. Ph.D. Thesis, Purdue University, 1999, pp 89-91.
13) Molina, M.; Melios, C.; Tognolli, J. O.; Luchiari, L. C.; Jafelicci, M.,
Jr. J. Electroanal. Chem. Interfacial Electrochem. 1979, 105, 237-
Kinetics. The rate of ClO
2
loss has a mixed order
dependence in ClO for which an integrated rate expression
2
cannot be determined. Although the data can be analyzed
by instantaneous rate methods, we found it more convenient
246.
(
(
14) EPA Method 300.1; U.S. EPA: Cincinnati, OH, 1997.
15) SigmaPlot 4.0 for Windows; SPSS Inc.: Chicago, IL, 1997.
(16) Gordon, G. Pure Appl. Chem. 1989, 61, 873-878.
Inorganic Chemistry, Vol. 41, No. 1, 2002 109

Wang et al.
also shown in Figure 3, where the intercept permits a rate
constant for H
of kobs to concentrations of OH , PO
2
O to be evaluated. Although the sensitivity
-
3-
2-
2-
4
3 4
, CO , and HPO
is large, there is almost no dependence on the Brønsted base
strength of these species. However, this acceleration does
not extend to every anion as shown by the lack of effect of
-
-
-
4 3 3
ClO , NO , or BrO . A fit of the rate dependence on
+
p[H ] from 9.5 to 10.53 in carbonate buffer indicates the
-
absence of a HCO
3
path. Additional studies showed
significant acceleration by pyridine and sulfate, as well as
acetate, chloride, and bromide ions. This suggests that it is
the nucleophilic properties of the species rather than their
proton abstraction properties that govern their reactivity.
Mechanism. The mechanism in eqs 2-4 is proposed for
-
the reaction between ClO
2
and BrO
2
, where BrO
2
is a
steady-state intermediate:
Figure 2. Bromite dependence of kobs for its reactions with ClO2.
k1
-
+
-
-
Conditions: 0.322 mM [ClO2]; 5.48 mM [ClO2 ]; 0.10 M [CO3]T; p[H ]
ClO + BrO { } ClO + BrO2
(2)
(3)
2
2
2
3
-
k-1
9
1
.50. Regression fit: kobs ) (2.7 ( 0.4) × 10 [BrO2 ] + (1.0 ( 0.1) ×
5 - 2
0 [BrO2 ] .
kNu
+
-
ClO + BrO + Nu 8 [NuClO ] + BrO
2
2
2
2
fast
+
-
-
[
NuClO ] + 2OH
8 Nu + ClO3 + 2H O (4)
2
2
The net result is that ClO
eq 1 and the reaction rate is catalyzed by BrO
2
disproportionates in accord with
-
2
and by
nucleophiles (Nu). The first step (eq 2) is a thermodynami-
-
cally uphill electron-transfer reaction to give ClO
2
and BrO
2
.
Bromine dioxide is a highly reactive compound that is
-
- 1-4
capable of disproportionating to form BrO
2 3
and BrO .
However, this does not take place because the concentration
of BrO
2
is very low compared to ClO
2
so that the reactions
-
-
in eqs 3 and 4 generate ClO
reaction between BrO
intermediate step in the reduction of bromate ion by chlorous
3
and regenerate BrO
2
. The
2
2
and ClO has been proposed as an
17,18
acid.
The rate constant for the BrO
2
/ClO
2
reaction at 1.0-
+
5
-1 -1
Figure 3. Dependence of kobs on the concentration of nucleophiles where
2
.0 M [H ] has been estimated to be 1.2 × 10 M s .
-
-
Nu
-
kobs[ClO2 ]/2[BrO2 ] ) (k1/k-1)(∑k [Nu]). For [OH ] the intercept is
H O
BrO
-
-
ClO
-
-
SO 2-
2-
Thermodynamically, eqs 3 and 4 are very favorable and
overcome the unfavorable step in eq 2. Nucleophiles (Nu)
(
k1/k-1)(k
2
+ k
2
[BrO2 ] + k
2
[ClO2 ] + k
4
[SO4 ]). For other
OH-
-
nucleophiles the intercept includes contributions k [OH ] as well.
assist the electron transfer in eq 3 because the formation of
to add excess chlorite ion ([ClO -
]
i
> 6[ClO
]
) to the BrO
-
ClO
+
is not favorable for this electron transfer. Equation 3
2
2
i
2
2
reactant solution to give excellent second-order fits where
includes more than one step, but it is written to indicate a
transition state that consists of (ClO BrO Nu) . The resulting
2 2
2
q
-
d[ClO
2
]/dt equals kobs[ClO
2
] .
rate expression given in eq 5 corresponds to the observed
kinetics:
Chlorite ion suppresses the rate, and Figure 1 shows that
-
reciprocal values of kobs are directly proportional to [ClO
2
],
-
-
when ClO
2
is added in excess. Added BrO
2
greatly
Nu
-
2
-
d[ClO2]
2k (∑k [Nu])[BrO ][ClO ]
1 2 2
accelerates the rate, and kobs has both a first-order and a
)
(5)
-
-
Nu
second-order contribution in [BrO
2
] as seen in Figure 2.
dt
k [ClO ] + (∑k [Nu])[ClO ]
-
1
2
2
Effect of Nucleophiles on the Kinetics. Buffers were
present in the kinetic studies to ensure that constant pH was
maintained despite the consumption of base (eq 1). The large
Typical concentrations of ClO
2
are low and k-1 is very
1
9
-
large, so that the addition of excess ClO
2
leads to the
-
Nu
molar absorptivity of ClO
2
permitted use of low concentra-
inequality k-1[ClO
2
2
] . (∑k [Nu])[ClO ]. This gives therate
-
4
tions in the kinetic studies, typically 3 × 10 M or less.
Hence, buffer concentrations of 0.05 M or less were sufficient
for pH control during the reaction. However, buffer con-
centrations have a large effect on the kobs values as shown
expression in eq 6 that allows use of second-order integrated
rate constants where kobs is expressed by eq 7.
(
17) Thompson, R. Inorg. Chem. 1973, 12, 1905-1909.
(18) Schmitz, G.; Rooze, H. Can. J. Chem. 1988, 66, 231-235.
-
in Figure 3. The effect of [OH ] in the absence of buffers is
(19) Huie, R. E.; Neta, P. J. Phys. Chem. 1986, 90, 1193-1198.
110 Inorganic Chemistry, Vol. 41, No. 1, 2002

Disproportionation of Chlorine Dioxide
-
Table 2. Summary of Rate Constants for the Reaction between ClO2
dependence on [BrO
] for kobs, which is consistent with our
] dependence study.
2
-
a
and BrO2
nucleophiles pKaHNu
-
results from the [BrO
2
nb
10-2(k1/k-1)kNu, M-2 s-1 kNu, M-2 s-1 e
-
H2O
Py
-1.74
0
0.00020(2)d
0.350(1)
0.36(2)
0.41(1)
1.25(4)
1.28(5)
1.49(6)
1.64(7)
2.2(6)
2.0 × 10
3.5 × 10
3.6 × 10
4.4 × 10
4
2k (∑k [Nu] + k
Nu
BrO2
[BrO ])[BrO ]
2 2
-
-
1
7
7
7
5.19 3.6
1.12 2.5
4.61 2.72
15.39 4.2
3.04
kobs
)
)
2
-
-
SO4
k [ClO ]
-1 2
-
OAc
-
8
8
8
8
-
- 2
OH
1.25 × 10
1.28 × 10
1.49 × 10
1.64 × 10
a[BrO ] + b[BrO ] (8)
2
2
-
Cl
2
-
CO3
9.46
8.21 3.8
1.72
11.08
3.43
c
2
-
-. The ClO
-
HPO4
Nucleophilic Catalysis by ClO
tions used in most of our kinetic studies were at least 25-
fold smaller than [buffer]
Therefore, the effect of ClO
be overlooked. If ClO
not be able to observe an acceleration of the rate by
] because k-1[ClO
suppress the rate. However, catalytic enhancement by ClO
2
2
2
concentra-
-
8
ClO2
c
2.2 × 10
3
-
8
PO4
c
2.3(3)
2.3 × 10
-
-
8
and 5-fold smaller than [BrO
].
BrO2
c
2.3(3)
2.89(6)
2.3 × 10
T
2
-
8
-
Br
3.89
2.89 × 10
) for CH3X
2
as a nucleophilic catalyst could
-
a
Conditions: 25.0 °C; µ ) 1.0 M. b n ) log(kNu/k
H O
acts as a nucleophile, we would
2
2
c
substitution; the values are obtained from ref 22. The n values are not
d
H O
-2 -1
e
Nu
available. (k1/k-1)(k
calculated from the k1 value (36 M
k-1 value (3.6 × 10 M
2
/55.5) to give M
s
.
The k values were
s ) obtained in this work, and the
-
-
increasing [ClO
2
2
] would simultaneously
-
1
-1
-
7
-1 -1
s
) is from ref 19.
in eq 3 can compensate for the inhibition. Thus, at high
Nu
-
2
-
-
d[ClO2] 2k (∑k [Nu])[BrO ][ClO ]
[ClO
[ClO
], the reaction rate would no longer depend on
1
2
2
2
2
)
) kobs[ClO2]
-
-
-
2
]. When ClO
has much higher concentrations than
dt
2
2
k [ClO ]
-1
-
all other nucleophiles, the ClO
2
path becomes dominant in
(
6)
(7)
kNu/k-1 Values. As the concentration
1
the nucleophilic route and the following inequalities are valid:
ClO -
-
Nu
-
Nu
Nu
-
2
k
[ClO
2
] . ∑(k [Nu]) and k-1[ClO ] . (∑k [Nu])-
2
2
k (∑k [Nu])[BrO ]
1 2
2
kobs
)
[ClO
] . Equation 9 gives the corresponding rate expression
where the rate is independent of the concentration of ClO
2
-
2
k [ClO ]
-
-1
2
.
Determination of k
-
ClO2
-
-
2
d[ClO2]
2k (k
1
[ClO ])[BrO ][ClO ]
2
2
2
of the nucleophiles increase, there is a directly proportional
-
)
)
-
increase in kobs in accord with eq 7. The effects are large;
dt
k [ClO ]
-
1
2
2
-
for example, there is a 4-fold increase in kobs with [CO
3
]
-
ClO2
-
2
2
k k
[BrO ][ClO ]
(
Figure 3). Variation of hydroxide ion concentration in the
1
2
2
(9)
-
-
absence of buffers gives values for kobs[ClO
where the slope equals (k
2
]/2[BrO
2
],
k
-1
OH-
1
/k-1)k
1
and the intercept is (k /
H O
ClO -
-] + kBrO2-[BrO
-
SO 2-
2-
k
(
-1)(k
2
+ k
The last term is included because Na
NaBrO preparation). The rate constant for the water path
2
[ClO
2
2
] + k
4
[SO
4
]).
To test this proposal, the rates were measured using
] ) 1.193 M and [ClO
spectrophotometer under the conditions p[H ] ) 9.34, [CO
] ) 0.0259 M, and [ClO
M. Excellent second-order reactions in [ClO
-
-
SO is present in the
4
[ClO
2
2
] ) 1.550 M on the UV
2
+
3
-4
]
T
2
H O
-1 -1
-
(k
1
k
2
/k-1 ) 1.1 ( 0.1 M s ) is determined by subtracting
) 0.10 M, [BrO
2
2
]
i
) 2.8 × 10
-
-
2-
the contributions by ClO
intercept value. Similarly, variation of [CO
p[H ] in Figure 3 gives a slope equal to (k
an intercept equal to (k
2
, BrO
2
, and SO
4
-
from the
2
] were observed
2
-1 -1
-
3
] at constant
with kobs ) 11.7 ( 0.1 M
s
for 1.193 M [ClO
2
] and
+
CO 2-
-1 -1
for 1.550 M [ClO
2
-]. These two
/k-1)k
+ k [OH ] +
3
and
k
obs ) 11.8 ( 0.2 M
s
1
H O
OH-
-
1
/k-1)(k
2
k
obs values agree well, and the result supports a substantial
nucleophilic role for the ClO
ClO -
-
BrO -
-
SO 2-
2-
-
ClO2
-
k
2
[ClO
2
] + k
2
[BrO
2
] + k
4
[SO
4
Nu
]). In this
2
. The (k
1
/k-1)k
value is
2
-2 -1
manner, corresponding values of the (k
for PO
1
/k-1)k rate constants
calculated to be (2.2 ( 0.6) × 10 M s .
3-
2-
-
2-
4
, HPO
4
, CH
3
COO , SO
4
, and pyridine are
Effect of Higher [ClO ] on the Reaction Order. The
2
determined and given in Table 2. Initially, we expected a
general rate expression in eq 5 predicts that an increase in
-
3-
2-
-
correlation between the base strength of OH , PO
4
, CO
and their k values, but these ions have similar
rate constants despite large differences in their basicity. The
3
,
the initial ClO
reduce the reaction order in [ClO
order dependence in [ClO ] is analyzed by an initial rate
2
concentration at lower levels of [ClO
2
] will
2-
Nu
and HPO
4
2
]. The resulting mixed-
2
Nu
20,21
k
values decrease somewhat for pyridine and acetate ion,
but these species are still effective as nucleophiles that assist
method with a five-point smoothing derivative program.
The initial rate in terms of the absorbance (A) change can
-
the reaction between ClO
Second Catalytic Role of BrO
2
and BrO
2
.
be expressed by eq 10, where a ) 2lꢀk
1
[BrO
2
], b ) k-1
-
-. According to the
2
[ClO ]/(∑k [Nu]), l is the cell path length (0.962 cm), and
-
Nu
2
proposed mechanism, the reaction is expected to be first-
ꢀ is the molar absorptivity of ClO
2
. Figure 4 shows the fit
-
-
order in bromite. However, Figure 2 shows that the rate
of eq 11 when [BrO
2
] ) 16.68 mM, [ClO
2
] ) 1.82 mM,
-
2-
+
constant also has a squared dependence in [BrO
2
]. This
[CO
) 36 ( 2 M
[Nu] ) (k
3
]
T
) 0.10 M, and p[H ] ) 9.44. This slope gives k
1
-
-1 -1
Nu
Nu
indicates that BrO
2
may also act as a nucleophile in eq 3.
s
and k-1/(∑k [Nu]) ) 2.41, where ∑k -
CO 2-
[CO32-] + k
BrO -
-
ClO -
-
3
2
[BrO ] + k
2
2
[ClO ]).
If this is the case, the observed second-order rate constant
2
Nu
-
1 2
can be expressed by eq 8, where a ) 2k ∑k [Nu]/k-1[ClO ]
BrO -
-
Nu
(
20) The program, provided by H. L. Pardue, Department of Chemistry,
and b ) 2k
1
k
2
2
/k-1[ClO ] and ∑k [Nu] is the sum of
Purdue University, is based on ref 21.
the other nucleophilic paths. Equation 8 gives a quadratic
(21) Savitzky, A.; Golay, M. Anal. Chem. 1964, 36, 1627-1639.
Inorganic Chemistry, Vol. 41, No. 1, 2002 111

Wang et al.
Figure 5. Nucleophilicity correlation.
Figure 4. Initial instantaneous rate date (dA/dt) at higher concentrations
a NuClO
2
species that is a more favorable electron donor.
-
1
-
-1
of ClO2 used to resolve values for k1 which equals (slope) (2lꢀ[BrO2 ])
-
-
When Nu is OH , even a weakly associated HOClO
species could transfer an electron to BrO
2
-
1
-1
)
36 ( 2 M
s .
2
to give HOClO
) is a stable molecule and
2
Contributions from kOH-[OH ] and k
-
H O
2
are negligible under
-
and BrO
2
. Chloric acid (HOClO
2
is a very strong acid that would react very rapidly with any
of the bases present to give chlorate ion (ClO
product. With other nucleophiles, intermediate products such
the conditions used.
-
3
) as a final
2
dA
dt
a[ClO2]
-
)
(10)
(11)
2-
as (O
3
3-
POClO
and ClO
2
) , for example, may rapidly hydrolyze to
b + [ClO2]
-
PO
4
3
. Since this occurs after the rate-determining
2
electron-transfer reaction, these hydrolysis reactions do not
appear in the rate expressions, nor do any of the stoichiom-
etry studies show evidence of NuClO
[ClO2]
[ClO2]
b
a
-
)
+
dA/dt
a
+
2
species as products.
On the other hand there would be ample time for hydrolysis
reactions (eq 4) to occur with any of the nucleophiles.
Another detailed mechanism to consider is the initial
1
9
Resolution of Rate Constants. Huie and Neta generated
BrO by pulsed radiolysis and reported a rate constant of
2
7
-1 -1
3
.6 × 10 M
s
(at pH 9.2 and an unspecified room
-
formation of an electron pair bond between ClO
2 2
and BrO ,
temperature) for its reaction with ClO
BrO
2
2
to give ClO and
-
. With this k-1 value, it is possible to assign kNu values
which are both free radicals. The disproportionation reaction
2
1
,2
of BrO
the BrO
tion in eq 2 is thermodynamically unfavorable. However,
the ClO concentration is much greater and reactant inter-
mediates such as O ClOBrO or OClOBrO may be possible.
2
is believed to proceed via Br
2
O
4
.
In our studies,
for each of the nucleophiles (Table 2). These third-order rate
-
2
-1
8
-
2
concentration is extremely low because its forma-
constants (M s ) vary from 2.89 × 10 for Br to 2.0 ×
4
Nu
1
0 for H
2
O. The relative magnitude of k is the following:
-
-
3-
-
2-
2-
-
2
Br > BrO
2
∼ PO
4
∼ ClO
2
> HPO
4
> CO
O. The values do not
correlate with the base strength of these species. For example,
hydroxide ion is a stronger Brønsted base than ClO
3
> Cl
-
-
2-
2
2
∼
OH > OAc > SO
4
∼ py . H
2
The role of the nucleophile would then be to associate with
these intermediates via electron-pair donation that would
-
2
by a
+
1
3.7
-
permit heteronuclear bond cleavage to give NuClO
2
and
factor of 10 , but ClO
by a factor of 1.6 in assisting electron transfer from ClO
BrO
Nucleophilicity scales (n) have been established on the
2
is more reactive as a nucleophile
-
BrO
2
as initial products.
2
to
2
3
We have used ab initio calculations to determine
2
.
-
-
structures for proposed adducts between ClO
2
and OH , Br ,
-
-
-
and Cl to give intermediates HOClO
2
, BrClO
2
, and
basis of the substitution with CH X (X ) Br or I) that include
3
-
some of the species in this study.22 Figure 5 shows the
ClClO that could transfer an electron to BrO to give
2
2
-
BrO
atomic charges are shown in Figure 6. In each case there is
a weak interaction between the nucleophile and ClO with
bond distances and ∆H values of 2.487 Å and -34.2 kcal
2
. These structures along with the bond distances and
correlation between nucleophilicities of eight species and
2 2
their catalysis of ClO oxidation by BrO .
Nucleophile Assistance of Electron Transfer. The ex-
perimental rate laws indicate a transition state composition
2
r
-
1
-
-1
-
q
mol for OH , 2.988 Å and -16.4 kcal mol for Cl , and
of [ClO
ClO to BrO
process would require a large reorganizational energy to form
2
BrO
2
Nu] for the electron-transfer reaction from
-1
-
-
3
.197 Å and -14.9 kcal mol for Br . The HOClO
2
adduct
2
2
. Without the nucleophile the electron-transfer
prefers an orientation for OH that suggests some H-bonding
may contribute to its ∆H value. These anionic adducts should
be suitable to transfer an electron to BrO .
2
+
-
ClO
2 2
and BrO with very unfavorable initial products. It
seems reasonable that the role of the nucleophile is to form
(
22) Hine, J. Physical Organic Chemistry; McGraw-Hill: New York, 1962;
(23) Frisch, M. J.; et al. Gaussian 98, ReVision A.7; Gaussian, Inc.:
p 161.
Pittsburgh, PA, 1998.
112 Inorganic Chemistry, Vol. 41, No. 1, 2002

Disproportionation of Chlorine Dioxide
mol ) for binding at the Cl site to give [OBrOCl(O)
2
OH]-
-
1
-
1
(
Figure 7b). ∆H for its formation is -62.6 kcal mol . The
r
calculations indicate that a significant amount of negative
-
charge from OH is transferred to the O-Br-O portion of
the molecular ion. Heteronuclear cleavage of the elongated
-
O-Cl bond (2.361 Å) in the [OBrOCl(O)
2
OH] adduct
-
would give BrO
2
and HOClO
2
as the initial products.
, and BrO must come
together in the transition state. The ab initio calculations
support NuClO , ClO BrO , and NuClO BrO as possible
The kinetics show that Nu, ClO
2
2
2
2
2
2
2
intermediates. Although solvation of these species is not
included, the calculations are helpful in visualizing possible
mechanisms. The electron-transfer step between BrO and
2
the adducts in Figure 6 could be “outer sphere”, but if a
bonding pathway occurs, then a structure such as Figure 7b
is likely.
Electrode Potentials for Bromine Dioxide. Our experi-
1
9
mental value for k
give an equilibrium constant K
which corresponds to an E° value of -0.355 V for eq 2. E°
1
and the pulsed radiolysis value for k-1
-
6
1
) k
1
/k-1 ) 1.0 × 10 ,
Figure 6. Equilibrium geometries (bond distances in Å) and atomic charges
-
-
-
-
24,25
(
italics) for possible intermediate adducts of ClO2 with OH , Cl , and Br :
for the ClO
2
/ClO
2
couple is 0.934 V,
so this gives E°
-
-
-
(a) HOClO2 ; (b) ClClO2 ; (c) BrClO2 .
-
)
1.289 V for the BrO
2
/BrO
2
couple. This value falls
25
between E° values of 1.21 V estimated by Stanbury and
2
6
-
1
.33 V given by Field et al. for the BrO
Conclusions
Electron-transfer reactions between ClO
2
/BrO
2
couple.
2
2
and BrO are
catalyzed by many nucleophiles. The magnitude of the effect
is large and does not depend on the Brønsted base strength
of the nucleophiles but rather on the extent of their interaction
2 2 2
with ClO (or a ClO BrO intermediate). The results suggest
that this process might be an overlooked phenomenon in
other electron-transfer reactions where one of the reactants
needs to expand its coordination in order to permit favorable
electron transfer. The kinetic consequence in the present
system is that bromite ion and other nucleophiles cause a
manyfold increase in the rate of disproportionation of
chlorine dioxide.
Acknowledgment. This work was supported by National
Science Foundation Grant CHE-98-18214 and by a research
grant to L.W. from the Purdue Research Foundation.
Figure 7. Equilibrium geometries (Å) and atomic charges (italics) for (a)
OBrOCl(O)O and (b) its OH- adduct [OBrOCl(O)2OH] .
-
Supporting Information Available: Tables and figures with
supplemental data. This material is available free of charge via the
Internet at http://pub.acs.org.
Figure 7a shows a structure for an alternative mechanism
where the two free radicals (ClO and BrO ) first form a
2 2
weak electron pair bond to give OBrOCl(O)O, where ∆H
r
is -1.43 kcal mol-1 and the connecting O-Cl bond is 2.275
IC010849W
Å. An alternate OClOBr(O)O structure has the same energy.
Addition of OH to either OBrOCl(O)O or OClOBr(O)O
(
24) Klaning, U. K.; Sehested, K.; Holcman, J. J. Phys. Chem. 1985, 89,
-
760-763.
(
25) Stanbury, D. M. AdV. Inorg. Chem. 1989, 33, 69-138.
26) Field, R. J.; K o¨ r o¨ s, E.; Noyes, R. M. J. Am. Chem. Soc. 1972, 94,
8649-8664.
can occur at the Cl or Br sites. Optimization of the structures
(
-
for the OH adduct show a consistent preference (ca. 20 kcal
Inorganic Chemistry, Vol. 41, No. 1, 2002 113