P e r g a m o n

Radiat. Phys. Chem. Vol. 49, No. 1, pp. 15-19, 1997 Copyright © 1997 Elsevier Science Ltd

Printed in Great Britain. All fights reserved PII : S0969-806X(96)00098-9 0969-806X/97 $17.00 + 0.00

PULSE RADIOLYSIS STUDIES ON "OH RADICAL INDUCED REACTIONS WITH SUBSTITUTED IODOBENZENES IN

AQUEOUS SOLUTIONS

HARI MOHAN Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India

Abstract--The transient optical absorption band formed on pulse radiolysis of N20 saturated neutral aqueous solution of substituted iodobenzenes (C6H4XI, X = H, Br, CH3, CF3) has been assigned to hydroxy cyclohexadienyl radical, which absorbs in the region of 320-325 rim. In acidic solutions, the hydroxyl radicals react to form two transient optical absorption bands (2~a~ = 310--320, 640--650 rim). The bands are assigned to solute radical cation formed on H ÷-catalyzed dehydration of OH-adduct. SO~-, specific one-electron oxidant, is found to react with substituted iodobenzenes and form transient optical absorption bands, similar to those formed on reaction of "OH radicals in acidic solutions and are assigned to solute radical cations. Copyright © 1997 Elsevier Science Ltd

INTRODUCTION

The hydroxyl radicals are known to react with aromatic compounds by addition reaction and form a transient optical absorption band with 2m~x in the region of 310-330 nm (Dorfman e t al . , 1962; Koster and Asmus, 1973; Steenken, 1987; Mohan e t al . , 1991; Merga et al . , 1994). The band is assigned to hydroxy cyclohexadienyl radical. Depending upon the nature of the substituents and the pH of the solution, the OH-adduct: may undergo acid-cata- lyzed dehydration to form the solute radical cation (Steenken, 1987; O'Neill e t a l . , 1975). The one-elec- tron oxidation of aromatic compounds is also possible by specific one-electron oxidants such as SO~-, Ag 2+, T12+ and the solute radical cation formed may tmdergo hydrolysis and yield hydroxy cyclohexadienyl radical (Sehested e t al . , 1975). The formation and stability of the solute radical cation of aromatic compounds is strongly influenced by electron donating power of the substituents (Mohan and Mittal, 1995a0 1995b, 1995c). The nature of the "OH radical induced reaction with substituted iodobenzenes have been investigated by using pulse radiolysis techniques, and the results are reported in this manuscript.

EXPERIMENTAL

Substituted iodobenzenes (pur i ty>99%) were obtained from Aldrich Chemicals and used without any further purification. The solutions were prepared in deionized nanopure water and freshly prepared solutions were used in each experiment.

The pulse radiolysis experiments were carried out with high energy electron pulses (7 MeV, 50 ns), which were generated from a linear electron accelerator (Priyadarsini e t al . , 1991; Guha e t al . ,

1987). The dose delivered per pulse was determined by using aerated aqueous solution of KSCN (10 m mol dm -3) (Fielden, 1984). and it was 15 Gy. (1 Gy = 1 J kg-1).

The reaction of "OH radicals in neutral aqueous solution was carried out in N20 saturated conditions where eft is quantitatively converted to "OH radicals (N20 q- e~ ~ ' O H + O H - + N2). In acidic sol- utions, the reaction of "OH radicals was carried out in aerated solutions to scavenge H atoms (eft + H + ~ H ' ; H" + O2~HO;- ) Pulse radiolysis experiments in highly acidic solutions had been carried out using HC104. The reaction with SO~- was carried out in N2 saturated solution of K2S2Os (2 x 10 -2 mol dm -3) containing 0.2 mol dm -3 t- butanol to scavenge "OH radicals (Neta e t al . , 1988).

RESULTS AND DISCUSSION

Figure l(a) shows the transient optical absorption spectrum obtained on pulse radiolysis of N20 saturated neutral aqueous solution of 2-iodo toluene (2IT), which exhibits absorption band with 2ma~ = 325 rim. The band was not observed in the presence of t-butanol (0.2 mol dm-3), an efficient "OH radical scavenger, suggesting that the band was due to the reaction of "OH radicals with the solute. The rate constant for the reaction of "OH radicals with the solute was determined by for- mation kinetic studies on monitoring the growth of the band as a function of solute concentration [(0.8-4.0) x 1 0 - 3 m o l d m -3] and the value was 1.7 × 109 dm 3 mol- l s - 1 The absorbance of the band (2m~ = 325 nm) remained independent of solute concentration [(0.8--4.0) x 10 -3 mol dm-3], showing that all the "OH radicals had reacted with the solute and the concentration of the "OH radicals could be

15

16 Hari Mohan

0.03 --

6

0.02

0.01

e e

o 250 350 450 550 650 750

k (rim)

Fig. 1. Transient optical absorption spectra obtained on pulse radiolysis of aqueous solution of 2IT (3,8 × 10 -3 mol dm -3) (a) N20, pH--- 6; (b) N2, pH = 1; (c) N2, pH = 1, t-butanol 0.2 mol dm 3;

(d) aerated, pH --- 1 and (e) aerated, HC104 = 6 mol dm-3;

taken equal to the concentration of the transient species [Fig. l(a)]. The extinction coefficient of the transient was determined to be equal to 2.1 × 103 dm 3 mol - l cm -1 (2 = 325 nm). The transi- ent was observed to decay by second order kinetics with 2k = 1.1 x 109 dm 3 mol - l s -l .

Similar studies had been carried out with a number of other substituted iodobenzenes and the kinetic and spectral parameters of the transient species formed on reaction of "OH radicals are given in Table 1. The decay kinetics and position of the transient optical absorption bands formed on reaction of "OH radicals with these substituted iodobenzenes were similar to those reported for benzene and substituted benzenes and assigned to hydroxycyclohexadienyl radicals (Dorfman et al., 1962 ; Steenken, 1987, O'Neill et al., 1975). Therefore, the transient bands formed with substituted iodobenzenes [Fig. l(a), Table 1] are also assigned to hydroxycyclohexadienyl radicals (2, Scheme 1).

The OH-adduct of halogenated organic com- pounds is known to undergo acid,catalyzed dehy- dra t ion and form solute radical cation (Steenken, 1987). Therefore, the formation and characterization of solute radical cation of substituted iodobenzenes

could be studied at lower pH. In acidic solutions, the yield o f ' H atoms is quite high due to the reaction of e~ with H ÷. Therefore, while studying the reaction of 'OH radicals with the solute at lower pH, either H atoms should be scavenged or contribution of its reaction with the solute should be determined separately.

Figures l(b) and (c) show the transient optical absorption spectrum on pulse radiolysis of N2 saturated acidic (pH = 1) aqueous solution of 2IT in absence and presence of t-butanol (0.2 mol din-3) respectively. Figure l(c) would mainly be due to the transient species formed on reaction of H atoms with the solute; It shows that the transient species has very low extinction coefficient. The reaction of OH radicals in acidic solutions could be investigated in aerated solutions as H atoms would be scavenged by Ov Figure 1 (d) shows the transient optical absorption spectrum on pulse radiolysis of aerated acidic (pH = 1) aqueous solution of 2IT, which was similar to that observed in N2 saturated solutions [Fig. l(b)]. From these results, it appeared that the contribution of H atom reaction with 2IT was very small and the reaction of "OH radicals could be studied in aerated solutions.

T a b l e 1. K i n e t i c a n d s p e c t r a l p a r a m e t e r s o f t he O H - a d d u c t f o r m e d on r eac t i on "OH r ad i ca l s w i t h subs t i t u t ed i o d o b e n z e n e s

So lu t e 2m.~ (nm) e ( d m 3 m o l - L c m - i) kr (d in 3 m o l - ~ s - t) 2 k ( d m 3 m o l - ~ s - t)

2 - 1 o d o t o l u e n e 325 2.1 x 10 ~ 1.7x 109 1. I x 10 ~ 2 - B r o m o - l - i o d o benzene 3 2 0 2,9x103 0.9x109 1.6x109 2 - 1 o d o b e n z o t r i f l uo r ide 325 2.4x103 0 .8x10 ~ 1.3x10 ~ l o d o b e n z e n e 325 4 .5x 103 2 .9x10 ~ 3 .4x 10 ~

(1)

X

@ I+ "OH

Pulse radiolysis studies on •OH radical induced reactions

(2) (3)

~]S I + H+ ' _ H 2 0 ,

- H + , + H20

OH

Scheme 1

+

17

The band at 640 nm [Fig. l(b)] was observed to decay by first order kinetics with k = 2.7 x 105 s - (pH = 1.0). The decay of the band in the region of 290-350 nm was different f rom that of 640 nm at p H = 1.0. The decay did not follow either first or second order kinetics; and suggests the presence of more than one species absorbing in this region (290-350 nm). It is possible that only a fraction of "OH radicals are able to react with H ÷ to form the solute radical cation• The absorption in the region of 290-350 nm could be due to the OH-adduct and the solute radical cation• In acidic solutions, the format ion of 640 nm band did not follow the decay of OH-adduct . The decay of OH-adduct of 2IT (325 nm) was observed to remain independent of H in the pH range of 1.0-6.0. The reaction of H ÷ with the initially formed OH-adduct is very fast. The solute radical cation would be formed on removal of an electron by "OH radicals from I atom. [t would the be delocalized over the benzene ring (Scheme 1).

Figure 2 shows the variat ion of the absorbance (2 = 640 nm) as a function o f p H 0.5-10.0. Since this band appeared in acidic solutions, it could be due to solute radical cation (3. Scheme 1). It is possible that the solute radical cation has another absorption band in the region of 290-330 nm.

The absorbance at 640 nm was observed to increase further in highly acidic solutions (HC104 > 0.2 mol dm-3). In highly acidic solutions, the entire radiation energy would not be absorbed by water alone and a part o f it would be absorbed by HC104. The radiolysis of HC104 does not yield "OH

x

6 o

5

4

3 - -

2 -

1 -

0 0

\ \,

I I 1 2

\ \

3 4 5 10

pH

Fig. 2• Variation of absorbance (640 nm) as a function of pH in N2 saturated solutions of 2IT (3.8 x 10 - 3 mol dm- 3).

k¢

d .o ,el

radicals (Domae et al•, 1996). Therefore, the effective yield of "OH radicals would decrease with increasing concentration of HCIO4.

Figure 3 shows the variation o f the absorbance at 640 and 315 nm as a function of HC104 concen- tration. Based on electron density variation between H20 and HC104 for various concentrations o f HC104, the absorbance values have been corrected for "OH radical yields. The absorbance values increased with HC104 and appear to reach saturation at 10 mol d m - 3 concentration. Although the ab- sorbance of the transient band with 2m~ = 640 and 315 nm appeared to reach saturation when the concentration of HC104 was equal to 10 mol d m - 3 lower concentration of HC104 (7,8 mol dm-3) was used for the determination o f kinetic and spectral parameters of the transient species• The transient optical absorption spectrum was recorded at HC104 = 6.0 mol dm-3. Under these conditions, interference from the transient species produced from

15

10

- - 640 nm

-

0 I I 0 5 10

HC104 (mol dm -3)

Fig. 3. Variation of absorbance at 315 and 640 nm as a function of HC104 concentration in aerated solution of 2IT

(3.8 x 10 -~ mol din-3).

18 Hari Mohan

Table 2. Kinetic and spectral parameters of the solute radical cation formed on reaction "OH radicals with substituted iodobenzenes in acidic solutions (HCIO4 = 7.8 mol dm-3) Solute 2 ~ (nm) kr (dm 3 mol - l s - i) kd (s - 1)

2-Iodotoluene 310, 640 1.0xl09 2.3x104 2-Bromo-l-iodo benzene 320, 650 2.4x109 3.0xl& 2-Iodobenzo trifluoride 320, 650 3.3x109 3.1xl& lodobenzene 310, 650 4.7x109 2.8x104

radiolysis of HC104 would be negligible (Domae et al., 1996) and the kinetic and spectral parameters would be main ly due to the t rans ient species formed f rom the solute and not f rom HC104.

Figure l(e) shows the t ransient optical absorp t ion spect rum on pulse radiolysis of aerated aqueous solut ion of 2IT (3.8 x 10 -3 mol dm -3) in 6.0 mol d m -3 HC104. The absorp t ion bands with 2max = 310 and 640 n m were observed. The shoulder at 340 n m observed at p H = 1 [Fig. l(b)] was not seen in this figure. It suggests tha t the fract ion of OH-adduc t reacting with H ÷ to form the solute radical ca t ion had increased. The absorp t ion due to OH-adduc t would be very small and the absorp t ion in 290-350 n m region would mainly be due to the solute radical cation. In highly acidic solut ions ( H C 1 0 4 = 10 .0mol dm-3) , the decay of b o t h the bands was nearly same and suppor t the observat ion that the entire spectrum is due to one species only in highly acidic solutions. The bimolecular rate constant for the react ion of "OH radicals with 2IT, as de termined by fo rmat ion kinetic studies at 310 and 640 nm was same and the value was equal to 1 × 10 9 d m 3 mol -~ s - l . The t ransient band (2m,~ = 640 rim) formed in acidic solutions, should be due to the radical cat ion formed on H - - catalyzed dehydra t ion of OH-adduc t (3, Scheme 1). Similar studies had been carried out with a n u m b e r of o ther subst i tuted iodobenzenes and the kinetic parameters for the solute radical cat ion are given in Table 2.

The yield and the life-time of the solute radical ca t ion was observed to increase with H ÷ concen- t ra t ion as more of OH-adduc t would react with H ÷ to form the solute radical cation. The value of decay cons tan t of the solute radical cat ion of different subst i tu ted iodobenzenes was nearly same (Table 2), suggesting tha t the subst i tuents have very small effect on the stability of the solute radical cations.

The SO~- is a s t rong one-electron oxidant with oxidat ion potent ia l ( E ° = 2.5-3.1 V vs N H E ) (Neta et al., 1988). The absorp t ion signal at 460 nm decayed faster in the presence of low concent ra t ions of the solute (1 x 10 .3 tool dm-3) , suggesting the

Table 3. Kinetic and spectral parameters of the transient species formed on reaction of SO~ with substituted iodobenzenes

Solute )~,x (nm) kf (dm mol -~ s- ~)

2-1odotoluene 315, 640 0.7x10 s 2-Bromo-l-iodo benzene 320, 650 0.8X10 s 2-1odobenzo trifluoride 320, 650 1.3x10 s lodobenzene 315, 645 0.9x10 s

electron t ransfer f rom the solute to SO~-. The bimolecular rate cons tan t for the react ion of SO~- with subst i tuted iodobenzenes, determined f rom the decay of 460 n m band , is shown in Table 3. Time-resolved studies showed the fo rmat ion of t ransient optical absorp t ion bands whose •max matched with tha t of the solute radical cat ion formed on react ion of "OH radicals in acidic solutions and thus suppor t the ass ignment of the t ransient bands to solute radical cation. However the difference was observed in the decay kinetics. This could be due to the fact tha t the bands of solute radical cat ion formed on react ion of SO~- are at neut ra l pH whereas those formed on react ion with "OH radicals were in acidic solutions. The yield and the life-time of the solute radical ca t ion formed on react ion of "OH radicals was observed to increase with concen t ra t ion of the acid.

CONCLUSIONS

The hydroxyl radicals react with subst i tuted iodobenzenes by addi t ion to the benzene ring and by electron t ransfer react ion mechanism in neutral and acidic solutions respectively.

Acknowledgements Sincere thanks are due to Dr C. Gopinathan, Head, Chemistry Division and J. P. Mittal, Director, Chemistry Group, BARC, for their keen interest and support to this work.

R E F E R E N C E S

Domae M., Katsmura Y., Jiang P. Y., Nagaishi R., Ishigure K., Kozawa T. and Yoshida Y. (1996)Radiolysis of concentrated perchloric acid solutions studied by both pulse and 7-radiolysis. J. Chem. Soc. Faraday Trans. 92, 2245.

Dorfman L. M., Buhler R. E. and Taub I. A. (1962) Absolute rate constants for the reaction of hydroxyl radicals with benzene in water. J. Chem. Phys. 36, 549.

Fielden E. M. (1984) The Study of Fast Process and Transient Species by Electron Pulse Radiolysis (Edited by Baxendale J. H. and Busi P.), p. 49. D. Reidel, Boston.

Guha S. N., Moorthy P. N., Kishore K., Naik D. B. and Rao K. N. (1987) One-electron reduction in thionine study by pulse radiolysis. Proc. Indian Acad. Sci., (Chem. Sci.) 99, 261.

Koster R. and Asmus K.-D. (1973) Reactions of fluorinated benzenes with hydrated electrons and hydroxyl radicals in aqueous solutions. J. Phys. Chem, 77, 749.

Merga G., Rao B. S. M., Mohan H. and Mittal J. P. (1994) Reaction of "OH and SO2 with some halobenzenes and halotoulenes: a pulse radiolysis study. J. Phys. Chem. 98, 9158.

Mohan H., Mittal J. P., Mudalaiar M., Aravindakumar

Pulse radiolysis studies on "OH radical induced reactions 19

C. T. and Rao B. S. M. (1991) Studies on structure- reactivity in the reaction of OH radicals with substituted halobenzene in aqueous solution. J. Chem. Soc., Perkin Trans. 2, 1387.

Mohan H. and Mittal J. P. (1995a) Formation and reactivity of the radical cations of bromobenzene in aqueous solution: a pulse radiolysis study. J. Phys. Chem. 99, 6519.

Mohan H. and Mittal J. P. (1995b) Formation of radical cation of chlorobenzene in aqueous solution: a pulse radiolysis study. Chem. Phys. Lett. 235, 444.

Mohan H. and Mittal J. P. (1995c) Direct evidence for H + -catalysed dehydration offluoro hydroxy cyclohexadi- enyl radical: a pulse radiolysis study. J. Chem. Soc., Faraday Trans. 91, 2121.

Neta P., Huie R. E. and Ross A. B. (1988) Rate constants for reactions of inorganic radicals in aqueous solution. J. Phys. Chem. Ref. Data. 17, 1027.

O'Neill P., Stcenken S. and Schutle-Frohlinde D. (1975) Formation of radical cations of methoxylated benzene by reactions with OH radicals, TI 2 +, Ag ~÷ , SO4 in aqueous solution. An optical and conductometric pulse radiolysis and in situ radiolysis electron spin resonance study. J. Phys. Chem. 79, 2773.

Priyadarsini K., Naik D. B., Moorthy P. N. and Mittal J. P. (1991) Energy transfer study of coumarin dyes using electron pulse radiolysis. Proc. 7th Tihany Symp. Radiat. Chem., Hungarian Chem. Soc., Budapest, p. 105.

Sehested K., Corfitzem K., Christensen H. C. and Hart E. J. (1975) Rates of reaction of O- OH and H with methylated benzenes in aqueous solution. Optical absorption spectra of radicals. J. Phys. Chem. 79, 310.

Steenken S. (1987) Addition-elimination paths in electron transfer reactions between radicals and molecules. J. Chem. Soc., Faraday Trans. 83, 113.