Kinetics of one-electron oxidation by the ClO radical

Radiat. Phys. Chem. Vol. 32, No. I. pp. 85-88, 1988 Int..L Radlat. AppL lnstrum. Part C

0146-5724/88

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KINETICS OF ONE-ELECTRON OXIDATION BY THE C10 RADICAL Z~-v B. ALFASSl,* ROBERT E. HUIE, S. MO~ERI and P. NETA Chemical Kinetics Division, National Bureau of Standards, Gaithersburg, MD 20899, U.S.A. (Received 10 June 1987)

AIxtltract--Pulse radiolysis studies were carried out to determine the rate constants for reactions of CIO radicals in aqueous solution. These radicals were produced by the reaction of OH with hypochlorite ions in N20 saturated solutions. The rate constants for their reactions with several compounds were determined by following the build up of the product radical absorption and in several cases by competition kinetics. CIO was found to be a powerful oxidant which reacts very rapidly with phenoxide ions to form phenoxyl radicals and with dimethoxybenzenes to form the cation radicals (k = 7 x 108-2 x 109M-Is-I). CIO also oxidizes CIO~- and N~- ions rapidly (9.4 × 10s and 2.5 x l0 s M-is -I, respectively), but its reactions with formate and benzoate ions were too slow to measure. CIO does not oxidize carbonate but the CO~- radical reacts with CIO- slowly (k =5.1 x 10SM-Is-I).

INTRODUCTION

EXPERIMENTAL

Radiolytic studies with aqueous solutions of hypochlorite ions have shown that CIO- reacts with OH and O - radicals with rate constants of 9.0 x 109 and 2.4 x 10SM-~s-~, respectively, to produce the CIO radical. ")

The main experimental problem encountered in the study of the reactivity of CIO is that the precursors of this radical, CIO- or HCIO, are also powerful oxidants that react with many compounds rapidly. When this reaction takes place within several minutes or less, it becomes impossible to carry out the pulse radiolysis experiment. Therefore, we have examined the reactions of hypochlorite in alkaline solutions with all the reactants before attempting to measure the reactivities of these reactants with C]O radicals. We found, for example, that phenoxide ions, N H 3, Br-, and NO/- react with CIO- rapidly, but cyanophenoxide, methoxybenzenes, benzoate, formate, N~-, and CIO~ do not. In basic solution, cyanophenol did not react spontaneously, but did in neutral solution, presumably due to the greater reactivity of HCIO (pK, = 7.5). Because of these complications, the number of compounds for which the rate constant for reaction with CIO can be measured is much more limited than in the case of most other radicals. Nevertheless, several rate constants could be measured which give a clear indication of the reactivity of C10 in general. As the source for CIO radicals we used calcium hypochlorite (Fisher)t or sodium hypochlorite 5% (Aldrich). The concentration of CIO- was determined from its optical absorption at 292 nm taking = 4 2 0 M -~ cm-~/~) Sodium chlorite was obtained from Kodak and sodium azide from Sigma. The organic compounds tested were from Aldrich. Water was purified by a Millipore Milli-Q system. Fresh solutions were prepared before each experiment and were bubbled with N20. This gas reacts with the hydrated electrons to produce OH radicals. A certain portion of the e~ may react with CIO-, but this

OH + C I O - - , O H - + C I O .

(1)

This radical exhibits a relatively weak absorption with a maximum at 280 nm (~ ,,, 900 M - ' crn- l ) and decays by a very rapid radical-radical reaction, with 2k = 1.5 x 101°M-Is-J, to form the dimer C1202. The reactions of CIO with other compounds have not be studied, except for the case of C I O f . CIO + CIOf - , CIO- + CIO2.

(2)

Experiments at low concentrations showed complex kinetics and suggested that reaction (2) is slow. °) This suggestion is surprising in view of the higher electron affinity of CIO as compared with that of CIO2. c2) Since we have studied recently the reactivity of CIO2 radicals toward various organic and inorganic compounds °'4) it appeared worthwhile to study the reactivity of CIO in order to compare the oxidative behavior of these two radicals. Indeed, we find CIO to be a much stronger oxidant than C102 and we find reaction (2) to take place fairly rapidly.

* Visiting scientist from the Department of Nuclear Engineering, Ben-Gurion University of the Negev, Beer Sheva, Israel. ? The mention of commercial material or equipment does not imply recognition or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified are necessarily the best available for the purpose.

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ZEEV B. ALFA.~Iet al. 9.4 x 108M-'s -m. This result indicates that CIO is a potent oxidant and, therefore, we have examined its reactivity toward other compounds. The rate constant for one-electron oxidation of 4-cyano- and 4-nitrophenoxide ions by CIO radicals were determined. Reactions of the more reactive phenoxide ions with CIO could not be investigated since these compounds are oxidized by CIO-. The reaction of 4-cyanophenoxide with CIO was found to take place with a rate constant of 1.4 x 109 M - i s -j to yield the cyanophenoxyl radical, which exhibits an absorption maximum at 410 nm.

reaction also leads to formation of OH. (~ The pulse radiolysis apparatus consisted of a Febetron 705 providing 50 ns pulses of 2 MeV electrons, a Varian 300 W xenon lamp, a Kratos high resolution monochromator, and the associated optical equipment. The signals were digitized with a Tektronix 7612 transient recorder and analyzed with a P D P - l l / 3 2 computer. All experiments were carried out at room temperature, 21 ± I°C. The rates of reaction of CIO radicals with the various compounds were determined in most cases by following the buildup of the radical produced from that compound, since CIO has only weak absorption at 280 nm where CIO- also absorbs. From a plot of the observed rate vs the concentration of the compounds we derived the second order rate constants. These values are generally accurate to + i 5 % . In certain cases to be discussed below, the rate constants were determined by competition kinetics and their accuracy is estimated to be +20%.

PhO- + CIO. --* PhO. + CIO-.

The reaction with 4-nitrophenoxide was monitored by following the bleaching of the nitrophenoxide absorption at 390-440 nm and the rate constant was determined to be 1.5 x 109M-'s -'. Because of the strong absorption of 4-nitrophenoxide it was difficult to observe the formation of the 4-nitrophenoxyl radical absorption. The rate constants determined from the reaction of CIO with these two phenoxide ions are higher than those determined for most other radicals, e.g. PO~(-, C O ; , Br~, and (SCN)~- by I-2 orders of magnitude. (5) More significantly, they are higher than those determined for ClO2(4)by more than four orders of magnitude, indicating again that C]O is a much stronger oxidant than CIO2. The N 3 radical reacts with 4-cyanophenoxide (~)more rapidly than CIO, but this may be due to the high self-exchange rate of N3/N; as discussed before. (7,s)

RESULTS AND DISCUSSION

The rate constant for the reaction of CIO radicals with chlorite ions, reaction (2), was monitored by following the buildup of CIO2 in solutions containing excess CIO- (,,- l x l0 -2 M) and varying CIO~- concentrations (3 x 10-5-2.5 x 10-4M) at pH 10.1. A sample kinetic trace, showing the buildup of the 360 nm absorption of CIO2, and a plot of kob= vs [CIO~] are shown in Fig. I. From this plot the rate constant for reaction (2) was determined to be

D

u')

o 18

m

~

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12--

~8-

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r~

:

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-2

0

(3)

0

4

8

12

16

I

t I

[C[Oz-]I 10-4 M Fig. 1

20

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24

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28

32

36

40

One-electron oxidation Table I. Rate constants for reactions o f C10 radicals Compound

pH

4-Cyanophenoxide ion 4-Nitropbenoxide ion l,~.Dimethoxybenmne 2,5-Dimethoxybenzoate ion 2,4,5-Trimethoxybenzoate ion 4.-Methoxybenzyl alcohol Benzoate ion F o r m a t e ion N~" CIO~

13 10 13 13 13 11 12 12 11 10

k ( M - i s - J) 1.4 1.5 2.1 7.0 I.I <1 <3 < I 2.5 9.4

x 10' x 10' x 109 x 10z x 109 x 10 7 x I0 6 x 10~ x 10g x I0s

Since CIO appears to be a strong oxidant we measured its reactivity with dimethoxybenzene and di- and trimethoxybenzoate ions. These compounds are known to undergo one-electron oxidation only by strong oxidants such as SO~-.tg) They yield the radical cations which exhibit characteristic absorption spectra with maxima at 430-460 nm. The reactions of these compounds with CIO were found to occur rapidly and to yield the same spectra as those reported previously for the radical cationsfl ) The rate constants were found to be between 7 x l0 a and 2 x 109M-ms -~ (Table 1), i.e. only 3-5 times lower than those measured for SO~- radicals, supporting the notion that CIO is a strong oxidant. CIO" + CrH4(OCH3):-~ CIO- + CrH4(OCH3)" +. (4) Further, we attempted to observe the reaction of CIO with other aromatic compounds that are more difficult to oxidize: anisole, 4-methoxybenzyl alcohol, benzoate, and toluene. These compounds are known to give strongly absorbing species upon addition of radicals to the aromatic ring. In none of these cases did we detect the formation of any absorption ascribable to a product radical. Therefore, we conclude that ClO does not add rapidly to the ring nor produces any other absorbing species from these compounds. To establish whether CIO reacts with these compounds to produce undetected products, we have carried out competition kinetic experiments employing 2,5-dimethoxybenzoate as a reference reactant. In these experiments, the reduction in yield of the dimethoxybenzoate radical cation absorption at 450 nm is monitored as a function of the concentration of an added compound. The solutions contained 0.5-1 mM dimethoxybenzoate and 20-80 mM CIOso that little of the OH should react with the dimethoxybenzoate. The reduction in yield of the 450 nm absorbance then can provide a measure of the relative rates of reaction of ClO toward dimethoxybenzoate and the added compound, provided that the added compound was not present at sufficiently high concentration to compete for the OH radicals. The effect of this possible complication can be assessed from the known rate constants for OH radicals or can be experimentally determined by changing the C10concentration. None of the above four compounds showed high reactivity toward CIO. With the more soluble

87

compounds, methoxybenzyl alcohol and benzoate, a decrease in the yield of the dimethoxybenzoate radical cation became apparent at high concentrations, but the experiments indicated that this decrease is ascribable almost fully to scavenging of OH (in competition with CIO- ) rather than to scavenging of CIO. Therefore, we report in Table I only the upper limits for these rate constants. Similarly, we carried out competition kinetic experiments to examine whether CIO can abstract hydrogen from formate ions and again only an upper limit was derived (Table 1). Competition experiments gave very good results for azide ions. This ion turned out to be quite reactive toward CIO so that it was added to the solutions at sufficiently low concentrations that its competition with CIO- for OH radicals did not have a major effect. An absorbance of 0.024 was measured upon pulse radiolysis of N20 saturated 0.01 M CIOsolution containing 0.89 mM dimethoxybenzoate, in the absence of azide. The addition of 0.56, 1.06, and 1.75 mM azide reduced the absorbance to 0.019, 0.014, and 0.011, respectively After correction for the slight competition for OH radicals we calculate a rate constant for C I O + N f of 2.6 x 10aM-Is -~ at pH 11.3. Similar experiments were carried out with 0.07 M CIO- and 1.0-2.2 mM N f , where the competition of azide for OH is negligible, and a rate constant of 2.4 x 10SM-ts -] was calculated. Therefore, the rate constant for reaction (5) is (2.5+0.5) x 10aM-Is -l. CIO" 4- N~ ~ CIO- ÷ N~.

(5)

This rate constant is higher than that measured for HPO4- radical reaction with azide but lower than the values for Cl~- and SO~-.(s) It is somewhat similar to the rate constant reported for Br~- with azide. The fact the CIO oxidizes N~- fairly rapidly indicates that the redox potential for CIO/CIO- is higher than that for N3/N ~- (1.3 V). (7,s) The finding that the rate constant for reaction of CIO with benzoate is < 3 x l06 while the value for oxidation of benzoate with SO4- is 1.2 x 109M-is -Its) indicates that CIO is a much weaker oxidant than S O i (E(SO4-/SO~4=) was estimated to be between 2.5 and 3.1 V). °°) Since Cl~- oxidizes 4-methoxybenzoate with k = 2 x l0 s M -) s -~(6) while CIO does not appear to oxidize 4-methoxybenzyl alcohol, and in other oxidation reaction CIO reacts more slowly than Cl~-, we conclude that E(CIO/CIO- ) must be below the value of E(CI~-/2CI-)= 2.1 V vs NHE. m) Furthermore, if reaction (l) involves direct oxidation of CIO- by OH rather than an addition-elimination mechanism, we may conclude that E(CIO/CIO-) is lower than E ( O H / O H - ) = i.89 V. (u) It appears, therefore, that the redox potential for CIO/CIO- is in the same range as that for Br~-/2Br- (E ffi 1.63 V). (") Unfortunately, it is not possible to measure the equilibrium constant between these two systems because of the rapid reaction of C10- with Br-. Instead, we attempted to

B. ALv~sl et al.

88

obtain equilibrium with COl/CO~3-, whose redox potential was estimated to be in the range of 1.51.8 V. ~4~ Experiments with excess hYlx)chlorite and low concentrations of carbonate indicated no reaction of CIO radicals with carbonate ions. On the other hand, experiments with 0.5 M CO~3- and 2-15 mM C10- at p H I 1.6, monitoring the rate of decay of the 600 nm absorption of C O l , gave a rate constant of (5.1 + 0.5)x 105M-is -I for reaction(6). CO/-. + CIO- --, CO~3- + CIO..

(6)

We saw no indication that this reaction was going to equilibrium. In fact because the low rate constant for reaction (6) and the very rapid decay of CIO radicals it does not appear possible to obtain such an equilibrium. Nevertheless, the rate of reaction (6) suggests that the redox potential of CIO is only slightly below that of CO3. In summary, the C10 radical is found to react as a powerful one-electron oxidant, with a redox potential estimated to be between 1.5 and 1.8 V vs NHE. It accepts an electron very rapidly from phenoxide ions and dimethoxybenzenes but not from monomethoxybenzenes or from benzoate. On the other hand, CIO does not appear to be very reactive in hydrogen abstraction or addition reactions. Low

reactivity in hydrogen abstraction or addition reactions was demonstrated also in the gas phase. °2~ Acknowledgement--The research described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. RF.FF.iiENClgS

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