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J. Phys. Chem. B 1998, 102, 10490-10497
Oxyhalogen-Sulfur Chemistry: Kinetics and Mechanism of the Formation Of Bromamines from the Reaction between Acidic Bromate and Aminomethanesulfonic Acid Cordelia R. Chinake† Department of Chemistry, UniVersity of Natal, P Bag X01, ScottsVille, 3209, South Africa
Reuben H. Simoyi* Department of Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506-6045 ReceiVed: August 19, 1998
The reaction between acidic bromate and aminomethanesulfonic acid (AMSA) has been studied in strongly acidic media. The reaction is very slow, and typically goes to completion after about 600 min at pH 0.0 and a [BrO3-]/[H2NCH2SO3H] ratio of 100. The reaction was followed by observing the absorbance of aqueous bromine at λ ) 390 nm and the formation of an intermediate bromamine species at λ ) 330 nm. The stoichiometry is 3H2NCH2SO3H + 2BrO3- + 4Br- + 6H+ f 3Br2NCH2SO3H + 6H2O. There is no cleavage of the C-S bond. Electrophilic substitution, instead, takes place at the nitrogen center to give bromamine derivatives as products. Dibromamine is the favored product over the monobromamine in high acid environments. Bromine, which is formed during the reaction, slowly reacts with AMSA to form a mixture of monobromoaminomethanesulfonic acid and dibromoaminomethanesulfonic acid. Initially, pure oxyhalogen chemistry produces bromine which then takes part in the electrophilic substitution reaction. An 11-step mechanism is proposed. The model derived from the mechanism gives relatively good agreement with experimental data.
Introduction Sulfur-containing compounds and oxyhalogens are very important species in chemistry, environmental chemistry, biochemistry, and physiology.1-3 Despite this, very little work has been done toward the elucidation of their basic chemistry and mechanisms of their reactions. Compounds containing a CdS double bond or a C-S single bond have exhibited some interesting chemistry4 and are also important biologically as amino acids, peptides, and proteins.5 In the presence of excess oxidant, it has been suggested that this C-S bond is oxidized by the successive addition of oxygen to the sulfur center up to the sulfonic acid followed by cleavage of the C-S bond to give sulfate and a carbonyl-type residue.6 The complexity of sulfur chemistry has long been recognized.7 Between 1940 and the late 1960s there was an increased interest in the study of mechanisms of reactions of sulfur compounds. There were many results which could not be explained which seemed to contradict each other.8 As the study of nonlinear kinetics and reaction dynamics comes of age it is becoming more and more apparent that there are many instances of “unexplained kinetic behavior”8 which can now be explained by invoking nonlinearity9 in the form of autocatalysis, autoinhibition, and-especially with sulfur chemistry-autoxidation.10 The effects of irregular or nonlinear kinetics are observed on a global level as nonlinear dynamical behavior in which chemical systems appear not to decay monotonically to positions of lowest free energy.11 The fact that sulfur chemistry can support * Author to whom correspondence should be addressed. † With great sadness we announce the death of Cordelia R. Chinake, a colleague and friend, on November 2, in Harare, Zimbabwe. She will be missed by all who had interest in her work. She is survived by her twoyear-old son, Anthony Chishamiso Chinake-Simoyi.
oscillatory behavior12 and symmetry-breaking bifurcations13 provides examples of such exotic behavior. A complete understanding of the mechanism which supports nonlinear behavior requires, as basic information, the kinetics and mechanisms of the reactions involved. The established algorithm for the oxidation pathway for a sulfur center to sulfate in the oxidation of small organic sulfur compounds does not seem to apply to sulfur-based amino acids.14 Our recent work has shown that in a strongly oxidizing environment, 2-aminoethanesulfonic acid (taurine) is not oxidized to sulfate and the C-S in this molecule remains uncleaved.14 The only chemical reactivity observed is at the nitrogen center of the amino group. The elucidation of the mechanisms of taurine reactions, to us, is extremely important because taurine is the most abundant amino acid in the human body15 and has been implicated in several physiological roles.16 Some of these roles include osmoregulation,17 antioxidant activity,18 and the ability to kill Schistosomula mansoni19 which causes bilharzia as well as modification of proteins.20 Our recent studies have also shown that taurine is extremely inert and unreactive14stoo unreactive, in fact, to be considered as a possible viable antioxidant.21 Physiological studies, however, have shown that taurine is an efficient scavenger22 of the common tissue-damaging oxidantssHOCl and HOBr. Metabolic precursors of taurine such as cysteine23 and hypotaurine23 appear to be more efficient scavengers of the more dangerous reactive oxygen species HO‚ and O2‚-. Thus knowledge of the mechanism of taurine reactivity and regeneration will be helpful in defining its physiological roles. Some preliminary experiments conducted in our laboratories have shown that most of the simple amino acids such as cysteine
10.1021/jp9834154 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/26/1998
Oxyhalogen-Sulfur Chemistry Formation of Bromamines and glutathione, upon strong oxidation in vitro, are oxidized only to the sulfonic acid with no formation of sulfate.14,25 There are several inherited sulfur-based disorders in humans. One such disorder is homocystinuria which is a diminished capacity to metabolize methionine in the methionine-cystine pathway. If methionine is administered to homocystinuric patients about 70% of the sulfur appears as sulfate in the urine; a little of the sulfur appears as cystathionine, taurine, and cystine, but most of the remaining sulfur is unaccounted for.26 It has been suggested that the sulfur might be in the form of compounds such as methanesulfinic acid (CH3SO2H), methanesulfonic acid (CH3SO3H), and methylmethanethiolsulfonate (CH3SO2SCH3), all which arise from the oxidation of the CH3S group in methionine.21 These observations suggest that there are physiological mechanisms that can allow for the cleavage of a C-S bond for small thiol and sulfur-based amino acids. The oxidation products arising from the compounds involved in the methionine-cystine relationship are not fully known. As a first step toward understanding the oxidation of sulfur amino acids and hence of the sulfur-based metabolic disorders, we have embarked on a systematic study of the kinetics and mechanism of reactions of some of the postulated intermediates arising from the metabolism of sulfur-containing amino acids, peptides, and proteins.14 Aminomethane sulfonic acid, being a homologue of taurine, and also a postulated intermediate in the oxidation of sulfur-containing amino acids, is an appropriate selection for our initial studies. Its reactivity and response to oxidizing agents can be extrapolated to the more important amino acid, taurine. We report in this paper, on a comprehensive kinetics and reaction mechanism study of the oxidation of aminomethanesulfonic acid by acidified bromate and aqueous bromine.
J. Phys. Chem. B, Vol. 102, No. 51, 1998 10491 contribution from the dibromamine to the absorbance at 390 nm.28 Thus the amount of bromine formed could not be directly related to the stoichiometry of the reaction. The total oxidizing power was determined iodometrically by adding excess acidified iodide and titrating the liberated iodine against standard thiosulfate.29 Results Stoichiometry: The stoichiometry of the reaction was found to be
3H2NCH2SO3H + 2BrO3- + 4Br- + 6H+ f 3Br2NCH2SO3H + 6H2O (R1) Qualitative tests for sulfate using BaCl2 showed no sulfate formation and thus the C-S bond is not cleaved during this oxidation. The reaction of bromine with AMSA also confirmed the formation of bromamine and the dibromamine. Bromamine and dibromamine can be distinguished by their UV spectra. The Br2-AMSA reaction gave two stoichiometries depending upon the ratio of the initial [Br2]o/[AMSA]o ratio. In ratios around 1.00, the stoichiometry of the reaction was
Br2 + H2NCH2SO3H f Br(H)NCH2SO3H + Br- + H+ (R2) With a ratio of 2.5 or greater the stoichiometry was
2Br2 + H2NCH2SO3H f Br2NCH2SO3H + 2Br- + 2H+ (R3)
Experimental Section Materials: The following reagent grade chemicals were used without further purification: perchloric acid, 70-72%, sodium perchlorate, sodium bromate, sodium bromide (Fisher), bromine, aminomethanesulfonic acid (Aldrich). A fresh batch of aminomethanesulfonic acid (AMSA) solution was prepared before each set of experiments. Methods: All experiments were carried out at 24 ( 2 °C and at an ionic strength of 1.0 M (NaClO4). This constant ionic strength was, however, not maintained in experiments run at [H+]o > 1.0 M. A secondary salt effect correction was applied. The reaction was monitored spectrophotometrically at λ ) 330 and λ ) 390 nm which, in our case, corresponded to the λmax’s of the monobromamine and bromine, respectively. The kinetic runs were carried out on a Shimadzu UV-2101PC and a PerkinElmer Lambda 2S spectrophotometers. Aqueous bromine has an absorptivity coefficient of 142 M-1 cm-1 at 390 nm. Generally, monobromamines, RNHBr, typically have a single peak in their UV spectrum centered at 290 ( 36 nm28 with an absorptivity coefficient of about 415-420 M-1 cm-1. On the other hand dibromamines (RNBr2), show two peaks in their spectra, one centered around 250 nm and another shifted about 40 nm from the bromamine peak. The absorptivity coefficients are in the range 370-430 M-1 cm-1, respectively.28 The use of λ ) 330 nm for monobromamine was to shift significantly away from the shoulder of the stronger 250 nm dibromamine peak while observing the concentrations of the monobromamine at a peak where its absorptivity is still strong. Attempts to determine the quantity of Br2(aq) produced under conditions of excess BrO3- were inaccurate because there was always some
The BrO3- - AMSA reaction was very slow and required high acid concentrations to obtain a reasonable reaction time. Figure 1a shows timed scans for the bromate-AMSA reaction at 5 min intervals for 100 min. Spectral activity is observed at 330 and 390 nm. Absorptivity at 250 nm is too high and offscale at the reagent concentrations used in Figure 1a. Figure 1b shows the bromine peak at 390 nm reaches a maximum value, while the peak at 330 nm reaches a maximum and then decays. The peak at 330 nm is due to the intermediate, Br(H)NCH2SO3H, the bromamine from AMSA. It reaches a maximum absorbance and is then depleted as the final organic product Br2NCH2SO3H is formed. There is a dibromamine absorption peak around 360 nm, which is overshadowed by the bromine peak at 390 nm and therefore cannot be used to monitor the progress of the dibromamine formation. There is a rapid formation of bromine in the early stages of the reaction which slows down as the intermediate (trace a, λ ) 330 nm) reaches its maximum absorbance (Figure 1b). Figure 1c shows the spectral scans of the direct reaction between bromine and AMSA. Initially, the strong bromine absorption peak (dashed line) at 390 nm is observed. The other scans (solid lines) were taken at 5 min intervals after a time lapse of 20 min. These spectra show that dibromamine is the final reaction product with the two peaks at λ ) 250 and 360 nm. The direct reaction of aqueous bromine with AMSA is strongly influenced by acid. With pH of 4-8, the first part of the reaction which forms the monobromamine is so fast that it is essentially diffusioncontrolled (see Figure 1d), but in conditions of pH 0.5 or less, the same reaction takes up to an hour for completion (Figure 1e).
10492 J. Phys. Chem. B, Vol. 102, No. 51, 1998
Chinake and Simoyi
Figure 1. (a) Spectrum of the bromate-AMSA reaction showing the formation and depletion of the bromamine at 330 nm and the formation of bromine at 390 nm. The scans are at every 5 min for a total of 100 min. [BrO3-]0 ) 0.12 M, [AMSA]0 ) 0.002 M, [H+]0 ) 2 M. (b) Time traces of the bromate-AMSA reaction showing the competition between the formation of bromine at 390 nm (trace a) and formation/depletion of the bromamine at 330 nm (trace b). [BrO3-]0 ) 0.08 M, [AMSA]0 ) 0.002 M, [H+]0 ) 3.5 M. (c) Reaction between AMSA and bromine. The first trace shown (---, maximum at 390 nm) is after 20 min. The subsequent traces are at 5 min intervals. [Br2]0 ) 0.019 M, [AMSA]0 ) 0.005 M. (d) Direct reaction of AMSA and bromine, monitored at λ ) 390 nm. No acid was added to this reaction mixture and the reaction is essentially complete in less than 1 s. [Br2]0 ) 0.003 M, [AMSA]0 ) 0.001 M. (e) Bromine-AMSA reaction with the same initial concentrations as in (d). This reaction was, however, run with [H+]0 ) 0.50 M.
The peak at 330 nm, (trace b in Figure 1b) shows an initial increase in the bromamine concentration, which reaches a maximum value at the end of what can be called an initial stage of the reaction in the λ ) 330 nm trace followed by a slow decay. This is typical of reaction intermediates. Reactions which are run in 1:1 mole ratios of aqueous bromine with AMSA in low acid concentrations show a monotonic increase in the monobromamine peak at 330 nm with a negligible subsequent decay in the monobromamine concentration. Reaction Kinetics: The reaction was generally very slow, and can be conveniently separated into two distinct stages: (a) production of monobromoaminomethanesulfonic acid (which coincides with a rapid bromine production), and (b) the depletion of monobromoaminomethanesulfonic acid (which coincides with a slower bromine production). Both traces at λ ) 330 and 390 nm (see Figure 1b) show a hint of autocatalysis with an ever-increasing rate of production of bromine and bromamine all the way to the maximum concentrations. Bromate oxidations are generally characterized by an induction period followed by a rapidly increasing reaction rate. The initial stages of the reaction are dominated by the autocatalytic formation of the reactive species HBrO2, BrO2, and HOBr.30 The accumulation of the reactive species encourages a rapid rate of reaction. There was, however, no marked induction period for the formation of both the bromamine intermediate and bromine. Effect of Acid. Acid has a very strong catalytic effect on the reaction. We have examined its effect on formation of bromamine (Figure 2a) and on formation of bromine (Figure 2b). The interesting features of Figure 2a include the observa-
tion that bromamine attains its maximum concentration in a much shorter time in higher acid concentrations. The acid effect is so strong that at high enough concentrations the formation of the bromamine (λ ) 330 nm) becomes too fast to follow by conventional kinetics methods (Figure 2a, trace d). The other feature to note from Figure 2a is that there is no monotonic linear dependence with respect to acid on the maximum transient bromamine concentrations formed. There seems to be some optimum acid concentration that can lead to the maximum bromamine concentration. The reason for this is evident from the observation from Figure 2a which shows that acid also catalyzes the consumption of bromamine. Such an observation has also been made on a similar reaction involving taurine.14 Thus the functional dependencies of the formation and consumption of bromamine with respect to acid concentrations will determine the set of initial conditions which can deliver the maximum transient bromamine concentrations. The effect of acid on the bromine formation is also nonlinear (Figure 2b). The rate of formation of bromine is catalyzed by acid. The first stage of the reaction becomes shorter with acid. As the acid is increased, the inflection point (trace d) which signals the end of the first stage and the beginning of the second stage changes to become a transient peak (trace b) and even to the point of not having a discernible induction period (trace a). The trajectories for the absorbance-time data (bromamine formation vs bromine formation) are quite dissimilar in high acid concentrations (Figure 1b); however, at low acid concentrations, these trajectories become very similar (Figure 3a,b). The only conclusion to draw is that one becomes a slave of the other,
Oxyhalogen-Sulfur Chemistry Formation of Bromamines
(a)
J. Phys. Chem. B, Vol. 102, No. 51, 1998 10493
(a)
(b)
(b) Figure 2. (a) Effect of high acid concentration on the formation of the bromamine at 330 nm. [BrO3-]0 ) 0.08 M, [AMSA]0 ) 0.002 M, [H+]0: traces a-d represent 3.0, 4.0, 4.5, and 5.0 M, respectively. (b) Effect of high acid concentration on the formation of bromine at 390 nm. [BrO3-]0 ) 0.08 M, [AMSA]0 ) 0.002 M, [H+]0: traces a-d represent 5.0, 4.5, 3.5, and 3.0 M, respectively.
(a)
(b) Figure 4. (a) Effect of [BrO3-]0 on the formation and depletion of bromamine at 330 nm. [AMSA]0 ) 0.002 M, [H+]0 ) 3.0 M, [BrO3-]0:--- 0.10, ‚‚‚ 0.12, -‚- 0.14 M. Inset shows effect of [BrO3-]0 on the maximum absorbance at 330 nm. [AMSA]0 ) 0.002 M, [H+]0 ) 3.0 M. (b) Effect of [BrO3]0 on the peak at 390 nm. [AMSA]0 ) 0.002 M, [H+]0 ) 3.0 M, [BrO3-]0: --- 0.10, ‚‚‚ 0.12 -‚- 0.14, -‚‚0.16, s 0.18 M.
Figure 3. (a) Effect of low acid concentration on the formation of the bromamine at 330 nm. [BrO3-]0 ) 0.27 M, [AMSA]0 ) 0.011 M, [H+]0: traces a-d represent 0.5, 0.4, 0.3, and 0.2 M, respectively. (b) Effect of low acid concentration on the formation of bromine at 390 nm. [BrO3-]0 ) 0.27 M, [AMSA]0 ) 0.011 M, [H+]0: traces a-d represent 0.5, 0.4, 0.3, and 0.2 M, respectively.
with the rate of formation of a key intermediate being controlled by one of the reactions. Effect of Bromate. The effect of the initial bromate concentrations on the rates of formation of bromamine and bromine was also investigated. Bromate has a minimal catalytic effect
on the rate of production of bromamine (Figure 4a). It has a negligible effect on the consumption of bromamine. Bromate, however, does affect the time taken for the maximum bromamine concentrations to be attained and also controls the amount of transient bromamine formed (Figure 4a, inset). Higher bromate concentrations reduced the amount of bromamine formed. Bromate also strongly catalyzed the rate of formation of bromine (Figure 4b). The first stage of the reaction becomes much shorter (while attaining higher bromamine concentrations) at high initial bromate concentrations. Higher bromate concentrations represent excess oxidant (e.g., after the AMSA has been completely oxidized), and this excess bromate can be used to produce bromine by reacting with the bromide
10494 J. Phys. Chem. B, Vol. 102, No. 51, 1998
(a)
Chinake and Simoyi aged the formation of bromine from bromate. Thus both the rates of formation of bromine and bromamine as well as maximum bromamine concentrations were strongly elevated with high initial bromide concentrations. Mechanism In excess oxidant, the reaction sequence takes the AMSA to bromamine and then to dibromamine:
H2NCH2SO3H f BrNCH2SO3H f Br2NCH2SO3H The reaction requires high acid concentrations for initiation. The reactive intermediates derived from bromate require high acid concentrations for reaction initiation:31
BrO3- + Br- + 2H+ h HBrO2 + HOBr
(R4)
The amino acid, at low pH environments, exists predominantly as the protonated form rather than the zwitterionic form:31
H2NCH2SO3H + H+ h [H3NCH2SO3H]+
(b)
(R5)
All reactivity occurs on the nitrogen center of AMSA. Stoichiometric studies have confirmed that no reactivity occurs on the sulfonic acid moiety. Analytical techniques have shown that the C-S bond is not cleaved as no sulfate is observed in the product mixture, even with the strongest oxidants. The initiation of the reaction can be through the formation of the reactive intermediates as in reaction R4, or the direct reaction between AMSA and BrO3-:
BrO3- + H2NCH2SO3H + H+ f (HO)HNCH2SO3H + HBrO2 (R6) The oxime, (HO)HNCH2SO3H, can then be converted to the bromamine:
(HO)HNCH2SO3H + Br- + H+ f BrHNCH2SO3H + H2O (R7)
Figure 5. (a) Effect of [AMSA]0 on the formation and depletion of the bromamine at 330 nm. [BrO3-]0 ) 0.2 M, [H+]0 ) 3.0 M, [AMSA]0: --- 0.002, ‚‚‚ 0.004, -‚- 0.006, -‚‚- 0.008, s 0.01 M. Inset shows effect of [AMSA]0 on the maximum amount of absorbance at 330 nm. [BrO3-]0 ) 0.2 M, [H+]0 ) 3.0 M. (b) Effect of [AMSA]0 on the peak at 390 nm. [BrO3-]0 ) 0.2 M, [H+]0 ) 3.0 M, [AMSA]0: --- 0.002, ‚‚‚ 0.004, -‚- 0.006, -‚‚- 0.008, s 0.01 M.
(reduction product of BrO3-) in solution. Thus we expect a higher rate of formation of bromine which translates to a much faster depletion of AMSA. Effect of AMSA. AMSA catalyzed both stages of the reaction. For example, in Figure 5a, AMSA catalyzed the rate of formation of bromamine, its rate of consumption, as well as the amount of bromamine formed (Figure 5a, inset). AMSA also strongly catalyzed the formation of bromine (Figure 5b), but lengthened the first stage of the reaction. This suggests that the total consumption of AMSA is a prerequisite for the consumption of bromamine. Effect of Bromide. The effect of bromide was similar to the effect of AMSA. High initial bromide concentrations encour-
Reaction R6 is very slow and is only relevant at the beginning of the reaction. Standard bromate solutions contain 10-6 M Br-, and this should be sufficient to initiate reaction R4.31 The reactive intermediate, HOBr, is responsible for the bulk of the oxidation:
HOBr + H2NCH2SO3H f (HO)HNCH2SO3H + H+ + Br- (R8) A nonlinear buildup of HOBr is expected from the decomposition of bromous acid:
HBrO2 + Br- + H+ h 2HOBr
(R9)
If we assume oxidations by HOBr are relatively fast, then reaction R6 cannot be rate-determining. Instead, the rate of formation of HOBr will be the rate-determining step, which explains the observed nonlinearity in the formation of bromine and the formation of bromamine. The bromide formed in reaction R8 can form bromamine as in reaction R7, or it can react with HOBr to form Br2:
HOBr + Br- + H+ h Br2(aq) + H2O
(R10)
Oxyhalogen-Sulfur Chemistry Formation of Bromamines
J. Phys. Chem. B, Vol. 102, No. 51, 1998 10495
TABLE 1: Mechanism of the Oxidation of Aminomethanesulfonic Acid by Acidic Bromate reaction no.
reaction
rate constants
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11
BrO3- + Br- + 2H+ h HBrO2 + HOBr HBrO2 + Br- + H+ h 2HOBr HOBr + Br- + H+ h Br2 + H2O H2NCH2SO3H + H+ h [H3NCH2SO3H]+ BrO3- + H2NHC2SO3H + H+ 6 H(OH)NCH2SO3H + HBrO2 HBrO2 + H2NCH2SO3H 6 H(OH)NCH2SO3H + HOBr H(OH)NCH2SO3H + Br- + H+ 6 Br(H)NCH2SO3H + H2O H2NCH2SO3H + HOBr 6 H(OH)NCH2SO3H + H+ + Br2BrNCH2SO3H + H+ h Br2NCH2SO3H + [H3NCH3SO3H]+ Br2 + H2NCH2SO3H 6 Br(H)NCH2SO3H + H+ + BrBr2 + Br(H)NCH2SO3H 6 Br2NCH2SO3H + H+ + Br-
2.1 (M-3 s-1); 1.0 × 104 (M-1 s-1) 2.5 × 106 (M-2 s-1); 2.0 × 10-5 (M-1 s-1) 8.9 × 108 (M-2 s-1); 1.10 × 102 (s-1) 1.0 × 108 (M-1 s-1); 1.0 × 106 (s-1) 1.1 × 107 (M-2 s-1) 1.0 × 107 (M-1 s-1) 1.0 × 106 (M-2 s-1) 1.0 × 104 (M-1 s-1) 1.0 × 105 (M-2 s-1); 1.0 × 10-3 (M-1 s-1) 5.6 × 107 (M-1 s-1) 1.5 × 102 (M-1 s-1)
Reaction R10 was studied by Eigen and Kustin32 by temperature-jump spectrophotometry and was found to be extremely rapid, with a favored rate constant of 8.9 × 108 M-2 s-1. The bromine formed in reaction R7 can also react with AMSA to form monobromamine:
Br2(aq) + H2NCH2SO3H f BrHNCH2SO3H + Br- + H+ (R11) The bromide obtained in reaction R11 is very important in determining the reaction dynamics observed in Figure 2b. Bromide can enhance formation of the reactive intermediates HBrO2, HOBr (reaction R4), and Br2 (reaction R10). The observed rate of formation of Br2 is a result of reaction R10 (formation of Br2) and reaction R11 (consumption of Br2). In high excess of BrO3- (i.e., higher HOBr concentrations enhance reaction R10) and in high acid concentrations (retarding reaction R11), the accumulation of Br2 before the total consumption of AMSA can be justified. Inflection Point. The total consumption of AMSA shuts down reaction R11 which rapidly produces (in the first stage of the reaction) Br- which was needed for the formation of Br2. This results in the observed lower rate of production of Br2. Dibromamine can be formed in two ways:
2BrHNCH2SO3H + H+ f Br2NCH2SO3H + [H3NCH2SO3H]+ (R12) and
BrHNCH2SO3H + Br2 f Br2NCH2SO3H + Br- + H+ (R13) Reaction R13 is much slower than reaction R11 (which is strongly inhibited by acid). The disproportionation in reaction R12, however, is catalyzed by acid as the dibromamine is more stable in high acid environments. Reaction R12, however, does not produce Br-. The Br- produced in reaction R13 will further produce Br2 as in reaction R10. The composite reaction which accounts for Br2 formation is33
BrO3- + 5Br- + 6H+ f 3Br2(aq) + 3H2O
(R14)
Reaction R14 is faster than R13 at high acid concentrations, and thus Br2 accumulates much more slowly. Reactions R12 and R13 are so much slower than R11 that reaction R11 essentially goes to completion before the consumption of the bromamine can be observed. In high acid concentrations, reaction R11 and R13 are so strongly inhibited by the protonation of the amino group that Br2 formed from reaction R10 will accumulate. Note that addition of reactions
R10 and R11 gives the following stoichiometry:
H2NCH2SO3H + HOBr f BrHNCH2SO3H + H2O
(R15)
Thus, if R11 is faster than R10, then no accumulation of Br2 will be observed. Acid Effect. Bromate oxidations cannot occur in the absence of high acid concentrations, as reaction R4 initiates all BrO3oxidations and has a square acid dependence term in its rate law. Reactions R4, R6, R9, and R10 form the reactive intermediates HOBr, HBrO2, and Br2 which are responsible for the bulk of the oxidations. Acid, however, inhibits the oxidation of the amino acid by deactivating the nitrogen center to electrophilic attack. The acid catalysis observed in Figure 2a is due to the rapid rate of formation of Br2 which can overcompensate for the slower reactions R11 and R13 through the basic mass-action kinetics. (All reactions were run in high excess of BrO3-.) Computer Simulation Our proposed mechanism was distilled to the 11 reactions shown in Table 1. The ordinary differential equations derived from the rate laws of the 11 reactions were numerically simulated using semi-implicit fourth-order Runge Kutta techniques.34 The first 3 reactions (M1-M3) are standard oxyhalogen reactions, and the kinetics parameters used for the simulations were derived from literature values.32,35-36 We assumed that the protonated amino acid was inert and thus reaction M4 was important in determining acid retardation of the reaction. Being a protolytic reaction, M4 is expected to be very rapid in either direction. An equilibrium constant of 100 was used and any variations in the kinetics parameters of this reaction were made without violating the value of the equilibrium constant. Above a forward rate constant of 1.0 × 108 M-1 s-1, the simulations were no longer sensitive to any further increase in the kinetics parameters of M4. Reaction M5 is a slow initiation reaction which is unimportant once the reaction proceeds. The simulations were extremely sensitive to the rate constant of this reaction in environments where no Br- was added at the beginning of the reaction. In reactions spiked with Br- (10-4 to 10-3 M), reaction M5 became insignificant in the reaction scheme as reaction M1 took over as the initiator reaction. Reaction M6 was not very significant in this reaction scheme. The higher rates of reaction with HOBr (reaction M8) coupled with rapid reversible reaction M2 ensured that reaction M6 remained insignificant. It was retained in this reaction scheme for stoichiometric consistency. Reaction M7 is a fast reaction, but it competes for Br- ions with the even faster reaction M3. Its role is to partition the bromide between the formation of Br2 and formation of the bromamine. Its effectiveness is limited by the low concentrations of the oxime present in the reaction solutions (≈10-7 M according to simulations).
10496 J. Phys. Chem. B, Vol. 102, No. 51, 1998
Chinake and Simoyi AMSA ([RNH2] + [RNH3+]) and Keq is the equilibrium constant of reaction M4. The value of kR10 was evaluated as 5.6 ( 1.0 × 107 M-1 s-1. Kinetics parameters for reaction M11 could not be determined in this study, but kR11 was pegged at 1.5 × 102 M-1 s-1. The rate equation was derived on the assumption that the protonated AMSA is inert to electrophilic attack. This simple mechanism gave very good qualitative and quantitative agreement with experimental results. Figure 6a is a set of simulations results which show the experimentally observed initial rapid formation and subsequent depletion of bromamine. As soon as bromine is formed it reacts with the AMSA to give the bromamine. The bromine also reacts with the bromamine, but not with dibromamine. As the bromamine starts to fall from its maximum value the formation of the dibromamine commences. Our simulations allow us to examine other species in the reaction medium which we could not experimentally observe (e.g., Figure 6a data for the AMSA). The simulations show the expected initial rapid increase in the monobromamine concentrations. As the bromamine reaches a peak, the bromine is essentially used up. The very same mechanism can also be used to simulate the Br2-AMSA reaction. This is done by initially setting bromate concentrations at zero (or a very small value, e.g., 10-12 M to avoid diVision by zero errors) and setting the desired initial bromine concentrations. Figure 6b shows one such simulation. Our experimental data consist of bromine formation and consumption rates only. The agreement with experiments is satisfactory. Acknowledgment. This work was supported by an NSF grant CHE-9632532 to R.H.S. and the University of Natal Research Fund (C.R.C.). References and Notes
Figure 6. (a) Computer simulations showing the relatively slow formation and depletion of the bromamine (s), the formation of dibromamine (- - -), and depletion of AMSA (---). [BrO3-]0 ) 0.08, [AMSA]0 ) 0.002, [H+]0 ) 3.5 M. (b) Computer simulations for the direct Br2-AMSA reaction for conditions used in Figure 1d. There is excellent agreement between the model and the experimental data.
Reaction M8 could have been utilized as the composite reaction R15 in which Br- is not formed and instead the bromamine is formed directly from HOBr. When using R15 the simulations could not reproduce Br2 formation. Br2 would only be produced at the start of the second stage of the reaction after all the bromamine has been formed. R15 also elevated the amounts of bromamine formed. Reaction M9 represents an ongoing equilibrium in highly acidic bromamine solutions with a very slow to negligible back reaction. This reaction is extremely important in acid concentrations higher than 1.0 M. The kinetics parameters for reaction M10 were evaluated in this study. The rate of reaction is given by
rate )
kR10[RNH2]TOT[Br2] 1 + Keq[H+]
where [RNH2]TOT is the sum of the unprotonated and protonated
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