Kinetics and mechanisms of the reactions of hypochlorous acid, chlorine, and chlorine monoxide with bromite ion

Article abstract of DOI:10.1021/ic0301223

The reaction between BrO₂^- and excess HOCl (p[H ⁺] 6-7, 25.0 °C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO₂^- to form ClO₃^- and HOBr (85% of the initial 0.15 mM BrO ₂^-). Another pathway produces ClO₂ and HOBr (8%), and a third pathway produces BrO₃^- and Cl ^- (7%). With excess HOCl concentrations, Cl₂O also is a reactive species. In the proposed mechanism, HOCl and Cl₂O react with BrO₂^- to form steady-state species, HOClOBrO ^- and ClOClOBrO^-. Acid facilitates the conversion of HOClOBrO^- and ClOClOBrO^- to HOBrOClO^-. These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO^-) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O ^-). The HOBrOClO^- species dissociates into HOBr and ClO₂^- or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO·HOCl adduct in the presence of excess HOCl. The primary products, ClO ₃^- and HOBr, are formed from the hydrolysis of BrOClO·HOCl. A minor hydrolysis path for BrOClO·HOCl gives BrO₃^- and Cl^-. An induction period in the formation of ClO₂ is observed due to the buildup of ClO ₂^-, which reacts with BrOClO·HOCl to give 2 ClO₂ and Br^-. Second-order rate constants for the reactions of HOCl and Cl₂O with BrO₂^- are k₁^HOCl = 1.6 × 10² M^-1 s ^-1 and k₁^Cl2O = 1.8 × 10⁵ M^-1 s^-1. When Cl^- is added in large excess, a Cl₂ pathway exists in competition with the HOCl and Cl₂O pathways for the loss of BrO₂^-. The proposed Cl ₂ pathway proceeds by Cl⁺ transfer to form a steady-state ClOBrO species with a rate constant of k₁^Cl2 = 8.7 × 10⁵ M^-1 s^-1.

Full text of DOI:10.1021/ic0301223

Inorg. Chem. 2003, 42, 5818−5824
Kinetics and Mechanisms of the Reactions of Hypochlorous Acid,
Chlorine, and Chlorine Monoxide with Bromite Ion
Jeffrey S. Nicoson, Thomas F. Perrone, Kara E. Huff Hartz, Lu Wang, and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-2084
Received April 7, 2003
The reaction between BrO - and excess HOCl (p[H ] 6−7, 25.0 °C) proceeds through several pathways. The
+
2
primary path is a multistep oxidation of HOCl by BrO - to form ClO - and HOBr (85% of the initial 0.15 mM
BrO -). Another pathway produces ClO and HOBr (8%), and a third pathway produces BrO - and Cl- (7%). With
2
3
2
2
3
excess HOCl concentrations, Cl2O also is a reactive species. In the proposed mechanism, HOCl and Cl2O react
with BrO - to form steady-state species, HOClOBrO- and ClOClOBrO-. Acid facilitates the conversion of HOClOBrO-
2
-
and ClOClOBrO to HOBrOClO-. These reactions require a chainlike connectivity of the intermediates with alternating
halogen−oxygen bonding (i.e. HOBrOClO-) as opposed to Y-shaped intermediates with a direct halogen−halogen
bond (i.e. HOBrCl(O)O-). The HOBrOClO- species dissociates into HOBr and ClO - or reacts with general acids
2
to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO‚HOCl adduct in the presence
of excess HOCl. The primary products, ClO - and HOBr, are formed from the hydrolysis of BrOClO‚HOCl. A minor
3
hydrolysis path for BrOClO‚HOCl gives BrO - and Cl-. An induction period in the formation of ClO is observed
3
2
due to the buildup of ClO -, which reacts with BrOClO‚HOCl to give 2 ClO and Br-. Second-order rate constants
2
2
for the reactions of HOCl and Cl2O with BrO - are k1HOCl ) 1.6 × 102 M-1 s-1 and k1Cl O ) 1.8 × 105 M-1 s-1.
2
2
When Cl- is added in large excess, a Cl2 pathway exists in competition with the HOCl and Cl2O pathways for the
loss of BrO -. The proposed Cl2 pathway proceeds by Cl+ transfer to form a steady-state ClOBrO species with a
2
rate constant of k1Cl ) 8.7 × 10 M-1 s-1.
5
2
Introduction
acids to form BrOClO (or BrCl(O)O). The reaction of
BrOClO with ClO2- forms 2 ClO2 and Br-, which competes
The mechanisms of the redox reactions of ClO2- have been
studied much more than the corresponding BrO2- reactions.
The reaction of ClO2- with several nonmetal oxidizing agents
(HOCl, HOBr, Cl2) proceeds by X+ transfer (X ) Cl or Br)
to form a metastable XClO2 intermediate.1-3 Taube and
Dodgen4 used 38Cl-labeling experiments to show that the
connectivity of the Cl2O2 intermediate must be either ClCl-
(O)O (Y-shaped) or ClOClO (chainlike). Although many
authors imply the XCl(O)O structure for these intermediates,5-8
this work indicates a XOClO structure is preferred.
-
with its hydrolysis to form ClO3 and Br-. The comple-
-
mentary reaction between HOCl and BrO2 has been
proposed to produce BrO3- and Cl-, presumably by a similar
mechanism.9 The products of the HOCl/BrO2- reaction were
not analytically determined by Lewin and Avrahami9 but
were assumed on the basis of previous knowledge of similar
reactions of halites with hypohalous acids. The present work
shows that this assumption is not valid. (The formation of
BrO2, analogous to the formation of ClO2 from the HOBr/
-
-
ClO2 reaction, is not observed from the HOCl/BrO2
Furman and Margerum2 have shown that the reaction of
HOBr with ClO2 forms a steady-state intermediate,
HOBrOClO- (or HOBrCl(O)O-), that reacts with general
reaction due to rapid BrO2 disproportionation.)10
-
In a gas-phase theoretical study of the reaction of HOCl
-
with BrO2 , Guha and Francisco11 found that a chainlike
* To whom correspondence should be addressed. E-mail:
margerum@purdue.edu.
(5) Aieta, E. M.; Roberts, P. V. EnViron. Sci. Technol. 1986, 20, 50-55.
(6) Peintler, G.; Nagypal, I.; Epstein, I. R. J. Phys. Chem. 1990, 94, 2954-
2958.
(1) Jia, Z.; Margerum, D. W.; Francisco, J. S. Inorg. Chem. 2000, 39,
2614-2620.
(2) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-4327.
(3) Nicoson, J. S.; Margerum, D. W. Inorg. Chem. 2002, 41, 342-347.
(4) Taube, H.; Dodgen, H. J. Am. Chem. Soc. 1949, 71, 3330-3336.
(7) Rabai, G.; Orban, M. J. Phys. Chem. 1993, 97, 5935-5939.
(8) Valdes-Aguilera, O.; Boyd, D. W.; Epstein, I. R.; Kustin, K. J. Phys.
Chem. 1986, 90, 6696-6702.
(9) Lewin, M.; Avrahami, M. J. Am. Chem. Soc. 1955, 77, 4491-4498.
5818 Inorganic Chemistry, Vol. 42, No. 19, 2003
10.1021/ic0301223 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/15/2003

-
Redox Reactions of BrO2
Br-,20 and ClO2 to ClO2- 21 and prevents chromatographic column
degradation by these species. An aliquot of the quenched reaction
was removed and mixed with a solution containing excess OH- to
prevent further reactions between NO2- and ClO2-. Samples were
HOClOBrO- adduct is preferred over the Y-shaped HOCl-
Br(O)O- structure. They also state that the interconversion
from HOClOBrO- to HOBrOClO- is not allowed (in the
gas phase), due to a large kinetic barrier. However, the
present work examines the HOCl/BrO2- reaction in aqueous
solution where this interconversion is possible due to the
presence of proton donors. The rate constants and mecha-
nisms for the reactions of BrO2- with HOCl, Cl2O, and Cl2
are determined. The connectivity of HOClOBrO-, HOBrO-
ClO-, and BrOClO intermediates is discussed on the basis
of the proposed mechanisms.
-
then injected on the ion chromatograph to determine BrO3 and
-
Br-. The concentration of BrO3 and Br- was corrected for the
initial concentration of these ions in the BrO2- stock solution. Due
to the NO2- quench reaction, the concentration of Br- determined
in the product study was the sum of [HOBr] and [Br-]. The yield
-
of ClO3 was determined by subtracting the yield of ClO2 from
the yield of Br-. The yield of ClO3- was also determined by using
the ion chromatograph and gave similar results ((5%).
Results and Discussion
Experimental Section
Products of the HOCl/BrO2- Reaction. The reaction of
Reagents. Stock solutions of NaClO4 were prepared from the
recrystallized salt. The preparation of NaBrO2 was reported
previously.12 The NaBrO2 salt (63.4 wt %) contained H2O (12.2%),
NaOH (14.1%), Na2SO4 (1.9%), NaBrO3 (7.0%), and NaBr (1.3%)
-
-
0.15 mM BrO2 with excess HOCl produces ClO3 , ClO2,
-
Cl-, BrO3 , and HOBr. Any Br- produced from the HOCl/
BrO2- reaction reacts rapidly with HOCl to form HOBr and
-
Cl-.22 Variation of [HOCl]T (6.53-11.4 mM), [H2PO4 ]T
-
impurities. Solutions of BrO2 were standardized spectrophoto-
metrically at 295 nm (ꢀ ) 115 M-1 cm-1).13 Stock solutions of
OCl- ([OCl-] ) [Cl-]) were prepared by dispersing Cl2 gas through
0.3 M NaOH. “Chloride-free” OCl- solutions were made as
reported previously.1 Solutions of OCl- were standardized spec-
trophotometrically at 292 nm (ꢀ ) 362 M-1 cm-1).2
(25-75 mM), and p[H+] (6.16-6.98) has little effect on the
product distribution (Supporting Information Table S1). The
primary path (85 ( 1%, based on the initial concentration
-
-
of BrO2 ) is a multistep oxidation of HOCl by BrO2 to
-
form ClO3 , HOBr, and Cl- (eq 1). Other pathways produce
BrO3 and Cl- (eq 2) or ClO2, HOBr, and Cl- (eq 3) in
Methodology and Instrumentation. All pH measurements were
corrected to p[H+] on the basis of electrode calibration (µ ) 0.50
M). Kinetic data were obtained with the ionic strength controlled
at 0.50 M (NaClO4). The pKa values for HOCl and H2PO4- at µ )
0.50 M are 7.5014 and 6.46,15 respectively. The concentration of
HBrO2 (pKa 3.59(5)16) is negligible from p[H+] 6-7. The dispro-
-
yields of 7 ( 2% and 8 ( 1%, respectively. These yields
-
apply to an initial BrO2 concentration of 0.15 mM.
Increasing the concentration of BrO2- to 0.62 mM increases
the yield of ClO2 to 19 ( 1%.
portionation of BrO2 is very slow17 and under the conditions of
-
2HOCl + BrO2- f ClO3- + HOBr + Cl- + H+ (1)
-
this study does not compete with the HOCl/BrO2 reaction.
Spectrophotometric measurements were performed on a Perkin-
HOCl + BrO2- f BrO3- + Cl- + H+
(2)
Elmer Lambda 9 UV-vis-NIR spectrophotometer. Stopped-flow
measurements of the loss of BrO2- at 250 nm (ꢀ ) 345 M-1 cm-1
)
or the formation of ClO2 at 360 nm (ꢀ ) 1220 M-1 cm-1) were
performed on an Applied Photophysics stopped-flow spectrometer.
Ion chromatographic separations were obtained as described previ-
ously.18
3HOCl + 2BrO2- f 2ClO2 + 2HOBr + Cl- + OH- (3)
The distribution of products from the reaction of HOCl
-
with BrO2 is quite surprising. First, the assumption that
The products of the reaction were determined by mixing solutions
-
-
HOCl oxidizes BrO2 only to BrO3 is not correct,9 since
this path (eq 2) represents only 7% of the reaction. In fact,
the oxidation of HOCl by BrO2- to form ClO3- (eq 1) is the
major stoichiometric path for this reaction. Another unex-
pected result is the formation of ClO2 as a product of the
-
of excess HOCl (also containing H2PO4- and HPO42-) with BrO2
.
Within 10 s, a portion of the HOCl/BrO2- mixture was transferred
to a cuvette and inserted into a Spectronic 20-D spectrophotometer.
The increase in absorbance due to ClO2 at 359 nm was followed
until the absorbance reached a maximum. The maximum absorbance
was used to calculate the yield of ClO2 from the reaction. Aliquots
were removed and mixed with a solution of excess NO2- to quench
-
reaction. Previous studies of the HOBr/ClO2 reaction2
-
showed that the reaction of ClO2 with BrOClO is respon-
-
the reaction. The NO2 rapidly reduces HOCl to Cl-,19 HOBr to
sible for the formation of ClO2 in that system. Since there is
no added ClO2- in the HOCl/BrO2- reaction, it was not clear
initially how ClO2 could be formed in the reaction. We will
show in the following sections that ClO2- plays a vital role
(10) (a) Buxton, G. V.; Dainton, F. S. Proc. R. Soc. A 1968, 304, 427-
439. (b) Nicoson, J. S.; Wang, L.; Becker, R. H.; Huff Hartz, K. E.;
Muller, C. E.; Margerum, D. W. Inorg. Chem. 2002, 41, 2975-2980.
(11) Guha, S.; Francisco, J. S. Chem. Phys. 2001, 269, 179-187.
(12) Wang, L.; Nicoson, J. S.; Huff Hartz, K. E.; Francisco, J. S.; Margerum,
D. W. Inorg. Chem. 2002, 41, 108-113.
(13) Perrone, T. F. Ph.D. Thesis, Purdue University, 1998.
(14) Gerritsen, C. M.; Margerum, D. W. Inorg. Chem. 1990, 29, 2757-
2762.
-
as an intermediate in the HOCl/BrO2 reaction.
Kinetics of ClO2 Formation. The increase in absorbance
due to ClO2 at 360 nm from the reaction of excess HOCl
with BrO2- is typical of an autocatalytic reaction (Figure 1a
(15) Beckwith, R. C.; Margerum, D. W. Inorg. Chem. 1997, 36, 3754-
3760.
(19) Johnson, D. W.; Margerum, D. W. Inorg. Chem. 1991, 30, 4845-
4851.
(16) Huff Hartz, K. E.; Nicoson, J. S.; Wang, L.; Margerum, D. W. Inorg.
Chem. 2003, 42, 78-87.
(20) (a) Lister, M. W.; McLeod, P. E. Can. J. Chem. 1971, 49, 1987-
1992. (b) Huff Hartz, K. E. Ph.D. Thesis, Purdue University, 2002.
(21) Stanbury, D. M.; Martinez, R.; Tseng, E.; Miller, C. E. Inorg. Chem.
1988, 27, 4277-4280.
(17) Faria, R. B.; Epstein, I. R.; Kustin, K. J. Phys. Chem. 1994, 98, 1363-
1367.
(18) Margerum, D. W.; Huff Hartz, K. E. J. EnViron. Monit. 2002, 4, 20-
26.
(22) Kumar, K.; Margerum, D. W. Inorg. Chem. 1987, 26, 2706-2711.
Inorganic Chemistry, Vol. 42, No. 19, 2003 5819

Nicoson et al.
Figure 2. Dependence of the observed rate constant on the concentration
-
-
of HOCl for the HOCl/BrO2 reaction. [BrO2-] ) 0.158 mM, [H2PO4
]
T
) 80 mM, p[H+] 6.38, µ ) 0.50 M (NaClO4), λ ) 250 nm, and T ) 25.0
°C. The line is a curve fit of eq 6 to the data. The error bars represent the
standard deviation of an average of three runs (each run consists of an
average of five kinetic traces).
that ClO2- reacts rapidly with another reactive intermediate
(BrOClO) to form ClO2.
Kinetics of BrO2- Decay. The loss of absorbance due to
BrO2- at 250 nm (Figure 1b) in the presence of excess HOCl
is a first-order decay with no evidence of an induction period
(eq 4). The observed rate constants determined for the loss
Figure 1. (a) Absorbance at 360 nm versus time following the formation
-
of ClO2 from the HOCl/BrO2 reaction with variable concentrations of
-
ClO2-. [BrO2-] ) 1.52 mM, [HOCl]T ) 16.1 mM, [H2PO4-]T ) 0.100 M,
p[H+] 6.32, µ ) 0.50 M, λ ) 360 nm, and T ) 25.0 °C. For each condition,
three replicate traces were obtained and are included in this graph. (b)
of BrO2 are similar to those determined for the formation
of ClO2 after the induction period. Since the kinetic traces
at 250 nm are not preceded by an induction period, these
data are used to determine the rate parameters for the
reaction.
-
Absorbance at 250 nm versus time following the loss of BrO2 from the
-
HOCl/BrO2 reaction. [BrO2-] ) 0.185 mM, [HOCl]T ) 16.5 mM,
-
[H2PO4
]
T
) 0.080 M, p[H+] 5.99, µ ) 0.50 M, λ ) 250 nm, T ) 25.0
-
°C, and no added ClO2
.
-
-
-d[BrO2 ]/dt ) kobsd[BrO2 ]
(4)
and Supporting Information Figure S1). The formation of
ClO2 is preceded by an induction period of 0.3-10 s that
decreases with increasing concentrations of HOCl. After the
induction period, the kinetic traces fit a pseudo-first-order
formation of ClO2. Over long time periods, ClO2 decays
according to its disproportionation reaction that is catalyzed
by HOCl.23,24 However, this has less than a 1% effect on
A plot of the observed rate constant versus the concentra-
tion of HOCl shows a first- and second-order dependence
in [HOCl] (Figure 2). HOCl is in equilibrium with small
amounts of chlorine monoxide (Cl2O) in aqueous solution
(eq 5).25
2HOCl h Cl2O + H2O KfCl O ) 0.0115 M-1
(5)
-
2
the yield of ClO2 from the relatively rapid HOCl/BrO2
reaction.
-
Chlorine monoxide, despite its relatively small equilibrium
concentration, has been shown25 to compete with HOCl in
the oxidation of NiII(CN)42-. The reaction of Cl2O with
The addition of initial concentrations of ClO2 to the
HOCl/BrO2 reaction reduces the induction time and gives
-
a marked increase in the yield of ClO2 (Figure 1a). When
-
BrO2 , in addition to a direct HOCl/BrO2- pathway, accounts
-
-
[ClO2 ]i ) [BrO2 ]i, there is no evidence of an induction
for the observed kinetic dependence in [HOCl] (eq 6).
period and the kinetic trace follows a first-order formation
-
of ClO2. The reaction of HOCl with ClO2 is much slower
kobsd ) ka[HOCl] + kb[HOCl]2
(6)
than the observed reaction and cannot explain these observa-
tions.1 When the induction period is excluded from the first-
order fit, the observed rate constants (kobsd) are identical for
each of the kinetic traces in Figure 1a. This shows that the
observed rate constant for the formation of ClO2 is inde-
The conditional rate constants, ka and kb, are dependent upon
the concentration of H+. The values of ka and kb were
determined at several acidities (Figure 3 and Supporting
Information Table S2). These data indicate the presence of
acid-catalyzed pathways for the reactions of both HOCl and
-
pendent of the concentration of ClO2 . On the basis of these
-
data, we conclude that ClO2 is an intermediate species in
-
Cl2O with BrO2 and also show a kinetic saturation effect
-
the HOCl/BrO2 reaction that builds up to an appreciable
at large [H+]. This type of kinetic dependence is consistent
with the expressions for ka and kb in eqs 7 and 8 (where w,
x, y, and z are parameters defined by the mechanism). The
concentration in the induction period. Earlier work2 shows
(23) Csordas, V.; Bubins, B.; Fabian, I.; Gordon, G. Inorg. Chem. 2001,
40, 1833-1836.
(24) Wang, L.; Margerum, D. W. Inorg. Chem. 2002, 41, 6099-6105.
(25) Beach, M. W.; Margerum, D. W. Inorg. Chem. 1990, 29, 1225-1232.
5820 Inorganic Chemistry, Vol. 42, No. 19, 2003

-
Redox Reactions of BrO2
k2
HOClOBrO- + H+ 98 HOBrOClO- + H+
(10)
(11)
Cl2O
Cl2O + BrO2- yk1 z ClOClOBr-
Cl
O
2
k-1
k3
ClOClOBrO- + H+ + H2O
98
HOBrOClO- + HOCl + H+ (12)
On the basis of the mechanism in eqs 9-12, the constants
in the rate expressions in eqs 7 and 8 are w ) k1HOCl, x )
k2/k-1HOCl, y ) KfCl Ok1Cl O, and z ) k3/k-1Cl O. A nonlinear
2
2
2
fit of the data in Figure 3 according to these rate expressions
determines k1HOCl ) 1.6(3) × 102 M-1 s-1 and k1Cl O ) 1.8-
2
(2) × 105 M-1 s-1. Therefore, Cl2O is 1100 times more
-
reactive toward BrO2 than HOCl. The rate constants and
ratios of rate constants are compiled in Table 1.
The HOBrOClO- and HOClOBrO- species are structural
isomers, and the kinetic evidence shows that the reverse
reaction in eq 10 is not appreciable. This indicates that the
HOBrOClO- isomer is preferred over the HOClOBrO-
isomer, in agreement with gas-phase theoretical calculations11
that show a greater stability (9.2 kcal/mol) of HOBrOClO-
over HOClOBrO-. Further evidence of the lack of revers-
ibility of eq 10 is seen in previous studies on the HOBr/
Figure 3. Dependence of ka and kb on the concentration of H+ for the
-
ClO2 reaction,2 where the conversion of HOBrOClO- to
HOCl/BrO2- reaction, where ka represents the HOCl path and kb represents
HOClOBrO- (the reverse reaction in eq 10) is not observed.
The decomposition of HOBrOClO- does not contribute
-
the Cl2O path. [BrO2-] ) 0.173-0.185 mM, [H2PO4
] ) 80 mM, µ )
T
0.50 M (NaClO4), λ ) 250 nm, and T ) 25.0 °C. The lines are the curve
fits of eqs 7 and 8 to the data. The error bars represent the standard error
from the fit of eq 6 to kobsd vs [HOCl] data at each [H+].
-
to the rate of the HOCl/BrO2 reaction, but these reactions
are crucial in the determination of the final products. The
reactions of HOBrOClO- have been described in a previous
values of ka increase slightly (<10%) with increasing
concentrations of phosphate buffer (Supporting Information
Figure S2 and Table S3), while the values of kb increase by
a factor of 2. However, this effect is small compared to the
effect of increasing [H+] and the rate parameters for the
phosphate-assisted pathway were not resolved.
-
-
study of the HOBr/ClO2 reaction, where ClO2 was in
excess of HOBr.2 The dissociation of HOBrOClO- to HOBr
-
and ClO2 (eq 13) competes with its general-acid-assisted
decomposition to give BrOClO (eq 14). The metastable
-
BrOClO species either hydrolyzes to form ClO3 and Br-
(eq 15) or reacts with ClO2 to form 2 ClO2 and Br- (eq
-
w(x[H+])
1 + x[H+]
16).
ka )
kb )
(7)
k4
HOBrOClO- y z HOBr + ClO2
(13)
-
k-4
y(z[H+])
1 + z[H+]
(8)
HA
k5
HOBrOClO- + HA
BrOClO + H2O
8 BrOClO + H2O + A- (14)
-
Mechanisms of the HOCl/Cl2O Reaction with BrO2 .
The lack of variation of the product distribution with
changing HOCl concentration indicates that HOCl and Cl2O
react with BrO2- by similar pathways. We propose that HOCl
k6
8 ClO3- + Br- + 2H+
(15)
rapid
k7
8 2ClO2 + Br-
(16)
-
BrOClO + ClO2
-
and Cl2O bond with BrO2 to form steady-state species,
rapid
HOClOBrO- and ClOClOBrO- (eqs 9 and 11). These
adducts react with H+ to form HOBrOClO- (eqs 10 and 12).
Equations 9-12 are the rate-determining steps for the
reaction and account for the experimentally determined
kinetics when the steady-state approximation is applied to
HOClOBrO- and ClOClOBrO-. The subsequent reactions
of HOBrOClO- are rapid and do not contribute to the
observed kinetics of the reaction.
H+
Furman and Margerum2 determined the ratios k5 /k4 )
-
H2PO4
3.1 × 105 M-1, k5
/k4 ) 8.3 M-1, and k6/k7 ) 1.02 ×
-
10-2 M for the reaction of HOBr with excess ClO2 .
However, the distribution of products (i.e. the k6/k7 value)
-
is quite different when HOBr is in excess of ClO2 . Data
obtained previously27 showed that the reaction of 12.6 mM
HOBr with 0.5033 mM ClO2- achieved a 21% yield of ClO2.
(26) Adam, L. C.; Fabian, I.; Suzuki, K.; Gordon, G. Inorg. Chem. 1992,
31, 3534-3541.
(27) Furman, C. S. Ph.D. Thesis, Purdue University, 1998.
HOCl
HOCl + BrO2- yk1 z HOClOBrO-
(9)
HOCl
k-1
Inorganic Chemistry, Vol. 42, No. 19, 2003 5821

Nicoson et al.
Table 1. Rate Constants for the HOCl, Cl2O, and Cl2 Reactions with
BrO2
the BrOClO‚HOCl intermediate hydrolyzes to form pre-
- a
-
dominantly ClO3- and Br-. As [ClO2 ] becomes larger, the
rate const
value
k7′ pathway begins to compete with the k6′ pathway. These
competing pathways coupled with the build up of ClO2
provide the mechanistic explanation for the delayed forma-
tion of small amounts of ClO2 observed experimentally.
-
k1HOCl, M-1 s-1
1.6(3) × 102
1.8(2) × 105
8.7(6) × 105
2.8(8) × 105
1.9(5) × 106
3.5 × 10-4
12(3)
k1Cl O, M-1 s-1
2
Cl2
k1 , M-1 s-1
k2/k-1HOCl, M-1
-
-
k3/k-1Cl O, M-1
2
From the relative yields of BrO3 , ClO3 , and ClO2, the
-
k6′/k7′, M
k6′/k8′
ratios k6′/k8′ ) 12(3) and k6′/k7′[ClO2 ] ) 11(1) are
determined. Of the initial BrO2 (0.15 mM), only 43% of
its loss leads to the formation of ClO2- due to the competing
-
k-1 /k6′, M-1
5(2)
Cl2
k-1 /k8′, M-1
6(1) × 101
Cl2
k4 and k5 steps. This gives a maximum yield of 64.5 µM
a Conditions: 25.0 °C, µ ) 0.50 M (NaClO4).
-
-
ClO2 . Assuming the average concentration of ClO2 over
the course of the reaction is half the maximum concentration,
the ratio k6′/k7′ ) 3.5 × 10-4 M can be estimated (compared
to k6/k7 ) 1.02 × 10-2 M in the absence of HOCl). Therefore,
Scheme 1. Structure of the BrOClO‚HOCl Adduct
-
the formation of ClO2 from the reaction of ClO2 with
BrOClO is approximately 30 times more favorable in the
presence of excess HOCl. A proposed electron-transfer
reaction between ClO2 and a BrOClO‚HOCl adduct is
-
-
The average concentration of ClO2 present in the course
of the reaction is approximately 0.25 mM. On the basis of
the k6/k7 ratio determined in excess ClO2 , a yield of less
than 5% is predicted. Therefore, the presence of excess HOBr
facilitates the formation ClO2. In this study of the HOCl/
BrO2- reaction, a similar trend is observed due to the excess
HOCl.
Reactions of BrOClO in the Presence of Excess HOCl.
Under the conditions of the current study (p[H+] 6.5,
[H2PO4 ] ) 0.04 M), approximately 43% of HOBrOClO-
dissociates into HOBr and ClO2 and the other 57% reacts
with general acids to form BrOClO. Using the k6/k7 ratio
shown in Scheme 3. The cyclic intermediate helps to promote
-
-
an electron-transfer reaction with ClO2 to give 2 ClO2,
HOBr, and Cl-.
-
Interconversion of HOCl and BrO2 to HOBr and
-
ClO2 . The sum of eqs 9, 10, and 13 provides an intercon-
version pathway for the formation of HOBr and ClO2- from
HOCl and BrO2- (eq 17). On the basis of standard aqueous
potentials (Table 2) and the pKaHOCl value, an equilibrium
constant calculated for the reaction in eq 17 at p[H+] 6.5 is
K ) 6.30 × 10-2. Accordingly, when [HOCl] ) 10 mM,
-
-
-
83.4% of the BrO2- would be converted to HOBr and ClO2 .
-
-
determined in the presence of excess ClO2 , we calculate
However, in the reaction of HOCl with BrO2 , this equilib-
that 0.44 mM ClO2- is necessary to produce the experimen-
tally determined 8% yield of ClO2. However, the initial
concentration of BrO2- is only 0.15 mM and a maximum of
rium is not reached due to the competition between the
general-acid-assisted decomposition of HOBrOClO- to BrO-
-
ClO (eq 14) and its dissociation to HOBr and ClO2 (eq
-
43% of this can be converted to ClO2 . Therefore, an
13).
alternative explanation must be considered to account for
the larger than expected yield of ClO2 as well as for the
formation of 7% BrO3- as a minor product. The excess HOCl
must react with BrOClO to form an alternate intermediate
that reacts differently than BrOClO. We propose that
BrOClO associates with HOCl to form a cyclic BrOClO‚
HOCl adduct. A slight rearrangement of the bonding in
BrOClO‚HOCl leads to a ClOBrO‚HOCl adduct (Scheme
1). These structures are similar to those proposed for the
adducts of Cl2O26 and BrOCl18 with OCl-. The cyclic
intermediates in Scheme 1 constitute a species that can react
similarly to either BrOClO or ClOBrO. Therefore, nucleo-
philic attack by H2O at the internal Cl atom (k6′) leads to
-
HOCl + BrO2- h HOBr + ClO2
(17)
Guha and Francisco11 indicate that the HOClOBrO- to
HOBrOClO- conversion in eq 11 is not allowed due to a
kinetic barrier of 30.0 kcal/mol in the gas phase. In aqueous
solution, however, water and/or general acids are able to
facilitate the interconversion process. On the basis of the
kinetic evidence of acid catalysis for this reaction, we propose
that water and protons mediate the transfer of H+ to convert
HOClOBrO- to HOBrOClO- (Scheme 4).
Another possible pathway exists for the formation of
-
-
BrO3 and ClO3 that does not require the interconversion
pathway in Scheme 4. A competing pathway to the inter-
conversion of HOClOBrO- to HOBrOClO- in eq 11 could
be the acid-catalyzed decomposition of HOClOBrO- to
ClOBrO (eq 18). In the presence of excess HOCl, the
ClOBrO intermediate could form the cyclic intermediate in
Scheme 1 (ClOBrO‚HOCl) and hydrolyze to form BrO3- or
-
the formation of ClO3 , HOBr, and Cl- or attack by H2O at
-
the Br atom (k8′) forms BrO3 , HOCl, and Cl-. The
formation of 2 ClO2 and Br- (k7′) takes place by the reaction
-
of ClO2 with BrOClO‚HOCl. The detailed mechanism of
-
the HOCl reaction with BrO2 is shown in Scheme 2. This
scheme also includes an alternative pathway (k9) that is
discussed later.
-
ClO3 as represented in Scheme 2. The pathway in eq 18
-
On the basis of the mechanism in Scheme 2, the auto-
catalytic profile of ClO2 formation can be rationalized. Before
cannot be distinguished from the formation of BrO3 and
ClO3- through the interconversion pathway. Regardless, the
interconversion pathway is necessary to form appreciable
-
the concentration of ClO2 builds up to appreciable levels,
5822 Inorganic Chemistry, Vol. 42, No. 19, 2003

-
Redox Reactions of BrO2
-
Scheme 2. Proposed Mechanism of the HOCl Reaction with BrO2
Scheme 3. Proposed Mechanism for the Electron Transfer of
Scheme 4. Acid-Catalyzed Interconversion of HOClOBrO- to
-
BrOClO‚HOCl with ClO2 To Give 2 ClO2, HOBr, and Cl-
HOBrOClO-
oxygen bonding, HOClOBrO-. The Y-shaped structure with
halogen-halogen bonding, HOClBr(O)O-, does not permit
interconversion to HOBrCl(O)O- without breaking and
reforming many bonds. The acid-catalyzed decomposition
of HOClBr(O)O- would form ClBr(O)O. Hydrolysis of
ClBr(O)O can only form BrO3- and Cl- as products, which
is in disagreement with the experimental data. It follows that
HOBrOClO- and BrOClO also possess a chainlike arrange-
ment because they originate directly from HOClOBrO-.
The formation of the chainlike adduct is logical when
nucleophilic/electrophilic interactions are considered. The Cl
atom on HOCl is electrophilic and subject to attack by a
nucleophile. The O atoms on BrO2- are more electronegative
than the Br atom and are the most nucleophilic atoms of the
Table 2. Reduction Potentials of Chlorine and Bromine Species in
Aqueous Solution
half-reacn
E°, V
ClO2- + H2O + 2e- f OCl- + OH-
0.681a
1.289b
1.16c
-
BrO2 + e- f BrO2
BrO3- + 2H+ + e- f BrO2 + H2O
BrO3- + 5H+ + 4e- f HOBr + 2H2O
1.447a
a Bard, A. J., Parsons, S. R., Jordan, J., Eds. Standard Potentials in
Aqueous Solution; Marcell Dekker, Inc.: New York, 1985; pp 74-75, 82.
b Reference 12. c Stanbury, D. M. AdVances in Inorganic Chemistry;
Academic Press, Inc.: San Diego, CA, 1989; Vol. 33, p 125.
-
concentrations of the ClO2 , which must build up prior to
the formation of ClO2. Therefore, the value of k9 must be
small compared to k2 to obtain the products that are observed.
-
ion. Thus, nucleophilic attack of BrO2 on HOCl leads to
k9
HOClOBrO- + H+ 98 ClOBrO + H2O
(18)
the chainlike adduct. This rationale should extend to other
XOXO-type intermediates in aqueous solution (X ) Br, Cl).
However, our kinetics evidence requires only the HOClOBrO-,
HOBrOClO-, and BrOClO species to have the chainlike
configuration.
Connectivity of HOClOBrO-, HOBrOClO-, and BrO-
ClO Intermediates. The formation of ClO2 from the reaction
-
of HOCl with BrO2 provides excellent evidence for the
-
existence of the interconversion pathway in Scheme 4. One
requirement of this process is the formation of a chainlike
adduct between HOCl and BrO2- with alternating halogen-
Mechanism and Rate Expression for the Cl2/BrO2
Reaction. Small amounts of Cl2 are in equilibrium with
HOCl when Cl- is added. The addition of 81.7 mM Cl- to
Inorganic Chemistry, Vol. 42, No. 19, 2003 5823

Nicoson et al.
amounts of Cl2 are in equilibrium with HOCl in the presence
of Cl- (eq 19, KfCl ) 960 M ).28 The transfer of Cl+ from
-2
2
-
Cl2 to BrO2 forms ClOBrO and releases Cl- (eq 20).
KfCl
2
HOCl + Cl- + H+ y z Cl2 + H2O
(19)
(20)
Cl
k1
2
Cl2 + BrO2- + H+ y z ClOBrO + Cl-
Cl
2
k-1
In the presence of excess HOCl, ClOBrO can form a cyclic
intermediate, ClOBrO‚HOCl (Scheme 1), that reacts as
discussed earlier (Scheme 2). Since this mechanism does not
-
contain an interconversion pathway to form ClO2 , the yield
of ClO2 decreases as the concentration of Cl- increases.
Allowing eq 20 to be reversible and applying the steady-
state approximation to the ClOBrO‚HOCl intermediate
produces the expression in eq 21 for the Cl2/BrO2- pathway.
Figure 4. Dependence of the observed rate constant on the concentration
-
of Cl- for the HOCl/BrO2 reaction. [BrO2-] ) 0.225 mM, [HOCl]T
)
-
6.53 mM, p[H+] 6.28, [H2PO4
]
T ) 0.080 M, µ ) 0.50 M (NaClO4), λ )
250 nm, and T ) 25.0 °C. The line is a curve fit of eq 21 to the data.
Cl2
+
-
KfCl k1 [HOCl][H ][Cl ]
2
-
-
the HOCl/BrO2 reaction system (with 0.15 mM BrO2 )
decreases the yield of ClO2 from 7.7% to 4.9% and increases
the BrO3- yield from 6.9% to 9.1% (Supporting Information
kobsd
)
(21)
Cl2
k-1
1 +
[Cl-]
k6′ + k8′
-
Table S1). Once again, the major oxidation product is ClO3
(∼86%). Stopped-flow studies of the formation of ClO2 with
A curve fit of eq 21 to the data in Figure 4 is used to
-
obtain k1 ) 8.7(6) × 105 M-1 s-1 and k-1 /(k6′ + k8′) )
Cl2
Cl2
higher initial concentrations of BrO2 (0.21 mM) show an
even larger drop in ClO2 yield with increasing Cl- concen-
tration. The yield of ClO2 decreases from 14% to 4% as the
concentration of Cl- increases from 0 to 0.25 M (Supporting
Information Table S4). Therefore, Cl- must provide a
Cl2
4.8(7) M-1. The k1 value shows that Cl2 is greater than
5000 times more reactive than HOCl and almost 5 times
-
Cl2
more reactive than Cl2O toward BrO2 . The values k-1
/
Cl2
k6′ ) 5(2) M-1 and k-1 /k6′ ) 6(2) × 101 M-1 are also
determined from the previous k6′/k8′ ratio.
-
catalytic pathway for the formation of ClO3 (eq 1) and
-
BrO3 (eq 2) but not for the formation of ClO2 (eq 3).
Acknowledgment. This work was supported by National
Science Foundation Grant CHE-01-39876. Thanks to Robert
H. Becker of Purdue University for helpful discussions.
A plot of the observed rate constant versus the concentra-
tion of Cl- (Figure 4) shows an increase in the rate of the
reaction with increasing [Cl-]. The intercept in Figure 4
corresponds to the competing HOCl and Cl2O pathways for
Supporting Information Available: Tables and graphs of
kinetic data. This material is available free of charge via the Internet
at http://pubs.acs.org.
-
the loss of BrO2 . The saturation of the observed rate
constant at high [Cl-] indicates that there is a reversible path
that releases Cl-. On the basis of the kinetic data and the
product distribution with variable [Cl-], we propose that Cl2
contributes to the rate of the reaction at large [Cl-]. Small
IC0301223
(28) Wang, T. X.; Margerum, D. W. Inorg. Chem. 1994, 33, 1050-1055.
5824 Inorganic Chemistry, Vol. 42, No. 19, 2003