Oxidation of naphthalene by radiolytically produced OH radicals

0146-5724/88 $3.00+0.00 Pergamon Press plc

Radiat. Phys. Chem. Vol. 32, No. 5, pp. 661-664, 1988 Int. J. Radiat. Appl. lnstrum. Part C

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OXIDATION OF N A P H T H A L E N E BY RADIOLYTICALLY P R O D U C E D "OH RADICALSt S. KANODIA, V. MADHAVEN and R. H. SCHULER Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, U.S.A. (Received 12 November 1987)

Abstraet--Radiolytically produced "OH radicals react with naphthalene in aqueous solution in the presence of ferricyanide mainly to yield 1- and 2-naphthol in a ratio of 2.1 : 1. Reaction takes place via addition of "OH to the naphthalene with a rate constant of 1.2 x 10~°M -~ S L. Because of the low solubility of naphthalene ( ~ 2 x 10-4 M) 3,-radiolysis studies were carried out at doses of ~ 103 tad. where the naphthols were produced at micromolar concentrations. Analysis was by high-performance liquid chromatography. The initial yields of naphthols from solutions saturated with N 20 and naphthalene total 4.7 molec/100 eV, indicating near to quantitative conversion of the "OH adducts to the naphthols by the ferricyanide. The observed product ratio is taken to reflect a two-fold preference for attack of "OH at the l-position as compared with that at the 2-position of naphthalene with only little ( < 10%) reaction at the 9-position.

were determined by Fricke dosimetry. (6) Solutions were prepared from water purified in a Millipore Milli-Q system and saturated with N 2 0 to convert e~q to "OH radicals. In the more recent studies products were examined with a Waters 990 + liquid chromatographic system employing a diode array detector

INTRODUCTION Previous studies of the oxidation of aromatic substrates by radiolytically generated "OH radicals have shown that the hydroxycyclohexadienyl radicals initially formed can be quantitatively oxidized to the corresponding phenols by ferricyanide, (''~) e.g.

.

•O H+ 0

(1) OHH + Fe(CN)] 3 In the present study we have used ferricyanide to oxidize the benzohydroxycyclohexadienyl radicals formed by "OH addition to naphthalene to examine the relative rates for "OH attack at its I- and 2positions. At low doses I- and 2-naphthol are virtually the only products observed and account for ~ 9 0 % of the "OH initially produced. The relative rate for addition at the I- and 2-positions given by the present study, 2. I, is considerably lower than indicated from the results of a previous study in which Fenton's reagent was used as the oxidant. (5) EXPERIMENTAL

Aqueous solutions saturated with naphthalene ( ~ 2 x 10-4M) containing 0.2 or l x 1 0 - 3 M ferricyanide were irradiated in a 6°Co source at a dose rate of 3.24 x l0 ~6or 1.24 x l017 eV g - ' min -~. Dose rates ~Document No. NDRL-3053 from the Notre Dame Radiation Laboratory.

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which allowed complete spectra to be recorded every second and stored for off-line display. Separation was on a 5 mm x 10cm N O V A - P A K C,s reverse-phase column using 40% acetonitrile in water as the eluent. The flow rate was 0.7 cm 3 rain '. In earlier studies the irradiated sample was extracted with CHCI 3 and the products concentrated for analysis by a factor of 10. Analysis was by normal-phase chromatography using a i m Pellosil silica column. In this case the eluent was 20% C H C l 3 in hexane and detection was at 270 nm using a Varian 635 M spectrophotometer as previously described. (2) The rate constants for reactions of "OH with naphthalene and the expected naphthol products were determined by examining the growth of the product benzohydroxycyclohexadienyl radicals using the L I N A C pulse radiolysis facilities of the Notre Dame Radiation Laboratory. (7) Naphthalene (sublimed before use) was from Eastman Organic Chemicals. The naphthols used for calibration and for the pulse radiolysis experiments were from Fluka. 661

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ELUTION TIME -min Fig. 1. Chromatogram of a 0.2 mM naphthalene solution containing 1 mM K3Fe (CN)6 irradiated to a dose of 1.2 × 10~TeVg-~ (2000 rad) and recorded at 220nm. The two major peaks are attributable to 2-naphthol (at 7.6 min) and l-naphthol (at 9.1 min). Concentrations determined from the peak areas are, respectively, 6.2 and 2.91tM.

RESULTS AND DISCUSSION Irradiations were carried out in acidic solutions (pH 3.6, 5.0 or 6.2) because naphthols are moderately rapidly oxidized by ferricyanide in neutral and alkaline solutions. At the high pHs the solutions were buffered with 2.5 m M phosphate. The chromatogram given in

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Fig. 1 shows that in the presence of ferricyanide only two major products, 1- and 2- naphthol, are formed on irradiation of a 2 × 10 a M naphthalene solution to a dose of 1.2 × 1017 eV g ~(2000 rad). The contour plot given in Fig. 2 indicates that at a five-fold greater dose, where ~ 20% of the naphthalene is converted into products, a number of additional products which elute in front of the naphthols (indicated at A - E in Fig. 2) are formed as the result of secondary processes. However, these products absorb only weakly. Since all aromatic compounds absorb strongly at < 250 nm these products appear to be produced only in low yield. No products were observed to elute after the 1-naphthol. The data of Fig. 3 show that in acidic solutions the yields of the naphthols are, to first order, independent of pH and ferricyanide concentration but fall off appreciably with dose. The ratio of 1-naphthol to 2-naphthol in all experiments is quite constant (2.15 + 0.05) even up to the highest doses used. Because of the low solubility of naphthalene ( ~ 2 × 10 4 M) the fall offin yield with dose noted in Fig. 3 is, of course, not surprising. As the irradiation progresses reaction of "OH with the organic products and with the ferrocyanide produced will compete with addition to naphthalene. It is important, therefore, to know the rate constants for these competing processes. Measurements of the rates of reaction of "OH with naphthalene and the naphthols were made

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Fig. 2. Contour plot of chromatographic signals from a 0.2 mM naphthalene solution irradiated to a dose of 6 x 1017 eVg -~ (104 rad). Contours represent absorbances from 0 to 0.004 in steps of 0.004. Absorbances of 1- and 2-naphthol reach 0.11 and 0.14 at 220 nm and 0.011 and 0.01 at 275 nm. Five minor (presumably secondary) products which elute in front of the dominant naphthols are noted at A-E.

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Oxidation of naphthalene by "OH radicals by following absorption of the intermediate benzohydroxycyclohexadienyl radicals using pulse radiolytic methods. Typical kinetic traces are shown in Fig. 4. Since naphthalene is readily lost from solution as the result of purging its concentration and that of the naphthois were measured spectrophotometrically before and after each kinetic trace was taken. The traces of Fig. 4 exemplify simple exponenetial growth. At pH 5 decay of "OH adducts is appreciable only on the 100/~s time scale and does not interfere with interpretations of the growth kinetics on the time scale of these experiments. From the growth period and known solute concentration the rate constant for reaction o f ' O H with naphthalene is 1.23 x 10-10 M-1 s -1, with 1-naphthol is 1.31 x 101°M -1 s -~ and with 2-naphthol is 1.23 x 101° M -I s -1. Possible errors are largely limited by systematics in interpretation of the growth curves and are estimated to correspond to a standard deviation of ~ 5%. The value for reaction with naphthalene is in agreement with that reported by Zevos and Sehested (1.2 x 1 0 1 ° M - i s - l ) (8) but considerably higher than that given by Evers, et al. ( 5 × 1 0 9 M -1 8 - 1 ) . (9)

The similarity between the "OH addition rate constants shows that substitution by OH at the 1- or 2-position has no significant effect on the addition rate. The values are only slightly higher than the value of 1.05 x 10 ~°M -1 s -~ for reaction o f ' O H with ferrocyanide.(1°) It is clear, therefore, that as the irradiation progresses reaction of "OH with the naphthols and with the ferrocyanide produced, including that resulting from reduction by the H-atom adducts, will compete with reaction with the naphthalene. In fact the effect of reaction with the naphthols will be magnified by their loss as the result of subsequent oxidation. One therefore expects substantial curvature to the yield-dose plots at the low concentrations of naphthalene involved. At a dose of 1017 eV g-i the

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Fig. 4. Growth of signals of the benzohydroxycyclohexadieny] radicals produced in the pulse irradiation of 1.47 x

I0-4M naphthalene (0), 4.77 × 10-+M -u naphthol (A) and 4.20 x 10-+M 2-naphthol (A). Measurements were made, respectively, at 320, 330 and 340 nm. The half periods for exponential growth indicated by the superimposed lines are 385, 111 and 138 ns. Solutions were buffered at pH 5 with phosphate (2.5 mM) and saturated with N20. Initial "OH concentration was ~4 x 10 -6 M.

competitive reactions will reduce the differential yield by ~ 9% and additionally ~ 4% of the naphthol8 will be lost in secondary processes. The yields will, therefore, be low by ~ 6 % at a dose of only 1017 eVg -1 and ~ 25% at a dose of 5 x 1017eV g-I. Taking these differences into account the initial slopes correspond to radiation yields of 3.19 for production of 1naphthol and 1.52 for production of 2-naphthol. The ratio, 2.1, is in agreement with the average from f / 2o i,/.//t the individual experiments. The total yield, 4.73, is, / -/however, only ~ 9 0 % of the 5.33 expected for "OH +// • reaction at 0.2 mM naphthalene (1~)[see equation (10) I in Ref. 11]. Either the correction for side reactions is somewhat underestimated or there is significant IO' (,,~ 10%) attack of "OH at the 9-position of naphthalene. If the latter takes place there are no products 5 apparent in the chromatographic analysis (see Fig. 2). It was found in a study of the oxidation of naphthalene by Fenton's reagent iS) that I- and 2-naphthol f I I I I cI ~ I 2 3 4 $ 6 were produced in the ratio of 3.8:1. Because the DOSE x I0-rr eVg-u radiation-chemical approach used here allows studies Fig. 3. Production of l-naphthol (solid symbols) and of "OH reactions under well-defined conditions the 2-naphthol (open symbols) in the irradiation of a solution ratio observed here, 2.1, provides a more direct saturated with N20 and naphthalene: O 0 , pH 3.6, 1 mM measure of the relative rates for attack at the two K3Fe(CN)+; D II, pH 3.6, 0.2 mM K3Fe(CN)6; /X &, pH positions. Conversely, the higher ratio observed in 6.2, 0.2 mM K 3Fe(CN)6; all at a dose rate of 4.2 x 1016eV g-i min-i; ~ # , pH 5.0, I mM K~Fe(CM)6 at a dose rate the Fenton experiment manifests either that in that of 1.24 x 1017eVg-amin -t. case oxidation involves intermediates other than the

664

S. KANODIA et al.

"OH radical or t h a t the overall study is substantially complicated by secondary processes. T h e present results clearly show a n i m p o r t a n t effect of the second ring system in directing a d d i t i o n of "OH at the I-position o f n a p h t h a l e n e . The effect observed here parallels the p r e d o m i n a n c e o f nitration ~2) a n d p h e n y l a t i o n ~3) at this same position a n d manifests the electrophilic n a t u r e of the "OH radical, a property t h a t has been pointed out before in the literature. (j4) Acknowledgement--This research was supported by the Office of Basic Energy Sciences of the Department of Energy. REFERENCES

1. K. Bhatia and R. H. Schuler, J. Phys. Chem. 1974, 78, 2335. 2. G. W. Klein, K. Bhatia, V. Madhavan and R. H. Schuler, ibid. 1975, 79, 1767.

3. K. Bhatia, ibid. 1975, 79, 1032. 4. G. W. Klein and R. H. Schuler, Radiat. Phys. Chem. 1978, 11, 167. 5. E. Boyland and P. Sims, J. Chem. Soc. 1953, 2966. 6. J. Weiss, A. O. Allen and H. A. Schwarz, Proc. Int. Conf. Peaceful Uses of Atomic Energy, Vol. 14, p. 179. United Nations, New York (1956). 7. E. Janata and R. H. Schuler, J. Phys. Chem. 1982, 86, 2078. 8. N. Zevos and K. Sehested, ibid. 1978, 82, 138. 9. E. L. Evers, G. G. Jayson, 1. D. Robb and A. J. Swallow, J. Chem. Soc. Faraday Trans, 1 1980, 76, 528. 10. G. W. Buxton, G. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref Data. In press. 11. R. H. Schuler, A. L. Hartzell and B. Behar, J. Phys. Chem 1981, 85, 192. 12. J. D. Roberts and M. C. Caserio, Basic Principles o[ Organic Chemistry, p. 812. Benjamin, New York, 1965. 13. D. H. Hey, in Advance in Free Radical Chemisto', Vol. II (Edited by G. H. Williams) p. 47. Logos Press Academic Press, New York. 1967. 14. E.J. Fendler and J. H. Fendler, Prog. Phys. Org. Chem. 1970, 7, 229.