6519
J. Phys. Chem. 1995, 99, 6519-6524
Formation and Reactivity of the Radical Cation of Bromobenzene in Aqueous Solution: A Pulse Radiolysis Study? Hari Mohan and Jai P. Mittal*9' Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India Received: April 8, 1994; In Final Form: February 6, 1995@
A transient optical absorption band (2- = 325 nm) is formed upon reaction of *OHradical with bromobenzene in neutral aqueous solution and is assigned to the OH adduct. In strongly acidic solution (HC104 > 3 mol dmw3),the OH adduct undergoes H+-catalyzed dehydroxylation to form the bromobenzene radical cation. This radical cation absorbs at 550 nm and at 270-310 nm. Even at this high H+ concentration, only a fraction of the OH adduct is converted to the radical cation. ,904.- reacts with bromobenzene to form the hydroxycyclohexadienyl radical (2- = 325 nm) and the phenoxy1radical (2- = 400 nm). The bromobenzene radical cation is also observed in irradiated 1,2-dichloroethane solutions. CsHsBr" is a strong one-electron oxidant and oxidizes Br-, SCN-, and organic sulfides with high rate constants. C&Br'+ undergoes electron transfer reaction with C1- to establish an equilibrium, from which a reduction potential value for C&Br'+/ C&Br was determined to be 2.31 & 0.15 V versus NHE.
Introduction
Experimental Section
Halogenated organic compounds are frequently employed as pecticides, refrigerants, fire retardants, and solvents in the chemical industry. It is important to know the identity and reactivity of transient species formed from halogenated organic compounds under different conditions. The identity of transients formed on y-radiolysis of halogenated organic compounds in hydrocarbon glasses at 77 K has been evaluated from the effect of known hole and electron scavenger^.'-^ The optical absorption bands are assigned to solute radical cations formed on charge transfer from solvent cation to solute molecules. In nonaqueous medium, experimental evidence for the formation of radical cations of halogenated organic compounds have also come from pulse radiolysis,8-ii flash photolysis,'2 ESR,13 and mass spe~trometric'~ investigations. Pulse radiolysis studies of bromoalkanes have shown the formation of bromine atom and its complex with the parent compound.15 In aqueous solutions, 'OH radicals are shown to react with alkyl bromides by H atom abstractioni6 and an electron transfer reaction mechanism17 in neutral and acidic solutions, respectively. The reaction of 'OH radicals with substituted halobenzenes is by addition to the benzene ring to form the hydroxycyclohexadienyl radical.18-2i Depending upon the electron donating power of substituents and the presence of H+, the OH adduct may be converted to solute radical cation in the presence of H+, as the electron withdrawing power of the 'OH radical is strongly increased by p r o t o n a t i ~ n . ~One-electron ~-~~ oxidation of benzene derivatives by specific one-electron oxidants such as S04*-, T12+,and Ag2+ have also been r e p ~ r t e d . ~ lThe - ~ ~oxidation of bromobenzene has not been reported so far. Our pulse radiolytic, 'OH radical induced reaction in acidic aqueous solutions of bromobenzene shows the formation of solute radical cation. The optical absorption spectrum matches well with that reported in glassy matrix of 3-methylpentane at 77 K5 and is supported by the photoelectron spectroscopic data.28
Bromobenzene (purity >99%), obtained from Aldrich Chemicals, was further purified by passing through a column of activated alumina. The solutions were prepared in deionized "nanopure" water, and fresh solutions were used for each experiment. The gases (N2, N20, and 0 2 ) used for purging the solutions were of Iolar-2 grade supplied by Indian Oxygen Ltd. The pulse radiolysis experiments were carried out with 7 MeV electrons (50 ns), which were generated from a linear accelerator.29 The dose delivered per pulse, determined by using an aerated neutral aqueous solution of KSCN (10 mmol dm-3), was 16 Gy (1 Gy = 1 J kg-'). The reaction of *OH radical in neutral aqueous solutions was studied in N20 saturated conditions where ea¶- are quantitatively converted to 'OH radicals (eaq- N20 *OH OHNz), with G('0H) = 6.0 (G denotes the number of species per 100 eV or micromolar concentration per 10 J of absorbed energy). In acidic solutions, the reaction of 'OH radical was studied in 0 2 saturated solutions to scavenge H and eaq- (eq- f H+ H; H 0 2 HOz). Steady state y-radiolysis experiments were carried out with a 6oCo y-source. The dose rate, as detennined by Fricke dosimeter, was 10.5 Gy min-'. The concentration of Br-, formed on y-radiolysis, was measured by colorometric analysis using the mercuric thiocyanate method.30 Since a high concentration of HC104 is employed in the present experiments, it is important to investigate the stability of bromobenzene in this medium. There was no change in the optical absorption spectrum of the acidic solution of bromobenzene with time, Br-/Brz products are expected to form on thermal/photochemical reaction of C&Br in acidic aqueous solution. Spectrophotometric analysis of an acidic aqueous solution containing C&Br did not show the formation of Br-I Br2.30
Preliminary results were presented at the Trombay Symposium on Radiation and Photochemistry, TSRP-94, Bombay, Jan 17-21, 1994, p 16. Also associated with the Jawahar Lal Nehru Centre for Advanced Scientific Research, Bangalore, India. Abstract published in Advance ACS Abstracts, April 1, 1995.
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Results and Discussion Reaction of 'OH Radical with Bromobenzene at Neutral pH. Pulse radiolysis of NzO saturated neutral aqueous solution of bromobenzene (2 x mol dm-3) showed the formation of a transient band with Amax = 325 nm (Figure 1). It was not observed in the presence of 'OH radical scavenger, tert-butyl 0 1995 American Chemical Society
6520 J. Phys. Chem., Vol. 99, No. 17, 1995
Mohan and Mittal 0.0 2 *
U 4
0.02-
250
a 250
Ti me (us 1
350 L50 W A V E L E N G T H (nm)
1
550
Figure 1. Transient optical absorption spectrum obtained on pulse radiolysis of a NzO saturated neutral aqueous solution of bromobenzene mol drrW3).Inset show the absorption time signal at 325 (2 x nm in NzO saturated solutions. Dose per pulse = 1 x 1017eV ~ m - ~ .
alcohol (0.5 mol dm-3), suggesting a reaction of 'OH radicals with bromobenzene. There was no change in the conductivity of the solution before and immediately after the pulse, thus suggesting the neutral nature of the transient species. The 325 nm band was observed to grow by pseudo-first-order kinetics with kObsincreasing linearly with bromobenzene concentration (1.5-4.6) x mol dm-3. The bimolecular rate constant for the reaction of 'OH radicals with bromobenzene in neutral N20 saturated solutions, determined from the slope of linear plot of k b s versus solute concentration, was 5.2 x 109 dm3mol-' s-'. The OH adduct was observed to decay by second-order kinetics with 2Wd = 4.8 x 105 s-l (inset of Figure 1). Timeresolved studies do not show the formation of any new band with the decay of the 325 nm band. The intensity of this band remained independent of solute concentration (8 x -8 x mol dm-3). Therefore, all the *OH radicals can be considered to have reacted with bromobenzene, and the concentration of *OH radicals may be taken equal to the concentration of the transient species absorbing at 325 nm. Under these conditions, the molar absorptivity at 325 nm was evaluated to be 4.52 x lo3 dm3 mol-' cm-'. It was observed to decay by second-order kinetics with 2k = 2.2 x lo9 dm3 mol-' s-l. The rate constant for the reaction of *OH radicals with bromobenzene, determined by competition kinetics using KSCN as the reference (k-OH+SCN- = 1.1 x 1O1Odm3 mol-' s-', A = 500 nm), was 7.4 x lo9 dm3 mol-' s-l, close to that determined by formation kinetics studies. These results suggest that the overall reaction of *OHradicals with bromobenzene is by addition to the benzene ring forming a hydroxycyclohexadienyl radical (Am, = 325 nm). y-Radiolysis of a neutral aqueous solution of bromobenzene (1 x mol dm-3) showed the formation of Br- with a yield of G(Br-) = 0.43. Addition of 'OH radical at Br site is expected to yield Br- and phenoxy1 radical which absorbs in the region of 400 nm. The yield of Br- is lower than the expected value of 1 (one-sixth the yield of *OH radicals if the 'OH radical addition is equally probable at different sites). The lower Bryield may be due to the fact that *OH radical addition is not equally probable at all the positions; this may be due to the high electron affinity of bromine. These results are similar to those observed for the reaction of 'OH radicals with benzene.'* The OH adduct of benzene is reported to decay by a disproportionation mechanism, giving a complex mixture of products such as phenol, biphenyl, and hydroxycy~lohexadiene.~
10 [HCIOA
/mol
dm-3
Figure 2. Variation of absorbance of transient band formed on pulse radiolysis of an 0 2 saturated aqueous solution of bromobenzene (2 x mol dn~-~) with [HC104] at (a) 325 nm and (b) 550 nm. Dose per pulse = 0.9 x 1017eV ~ m - ~ . 0.c
a
4.
0.0
0
350
LS 0
5 50
650
W A V E L E N G T H (nm)
Figure 3. Transient optical absorption spectrum obtained on pulse radiolysis of an 0 2 saturated aqueous solution of bromobenzene (2 x mol dm-3) in 3 mol dm-3 HC104: (a) immediately and (b) 5 ,us after the pulse. Inset shows the absorption-time signal at 325 nm in (c) [HClOh] = 3 mol dm-3) and (d) neutral solutions. Dose per pulse = 1.1 x ioL7eV ~ m - ~ .
Reaction of 'OH Radical with Bromobenzene in Acidic Solution. In acidic solutions, eaq- would be converted to a H atom, owing to its reaction with H+. In order to avoid interference of H atom while studying the reaction of *OH radicals in acidic solution, pulse radiolysis studies were carried out in 0 2 saturated conditions to convert H to HO2 radicals. Under the present experimental conditions, the concentration of 0 2 (-1 x mol dm-3) is sufficient to react with H (5 x mol dm-3). Therefore, H would not be reacting with CsHsBr. The HO2 radical is a mild oxidizing agent and was not found to react with C&Br (see text). In 0 2 saturated solutions, the nature of the spectrum and absorbance at 325 nm remained independent of pH in the region of 1- 10 (Figure 2). However, when [HC104] was increased from lo-' mol dm-3, the absorbance at 325 nm was observed to increase with [HC104] and the plateau value was observed when [HC104] was '3.0 mol dm-3 (Figure 2). Simultaneously, another band with Am, = 550 nm was observed to grow when [HC104] > 2.0 mol dm-3. In Figure 3, a and b show the transient optical absorption spectra obtained on pulse radiolysis of 0 2 saturated aqueous solution of bromobenzene (2 x mol dm-3) in 3 mol dm-3 HC104 immediately and 5 ,us after the pulse. It exhibits an absorption band with A, = 325 nm. The rate constant for the reaction of *OHradicals with bromobenzene in 3.0 mol dm-3 HClO4, determined from buildup kinetics ([C&Br] = (1.5 mol dm-3), was 1 x 1O'O dm3 mol-' S-'. 4.3) x
Radical Cation of Bromobenzene in Aqueous Solution
The initial portion of the signal at 325 nm decayed faster (Figure 3c) and by first-order kinetics. The latter portion of the signal at 325 nm decayed slowly (Figure 3c) and by secondorder kinetics with 2Wd = 5.5 x lo5 s-l. Based on the absorbance value at 0 and 5 ps, about 40% of the transient signal at 325 nm was observed to decay during the initial 5 ps whereas, in neutral solution, the decrease during the same time was less than 10% (figure 3d). It has been reported that the electron withdrawing power of the 'OH radical is strongly increased in acidic solutions, and H+-catalyzed dehydration of hydroxycyclohexadienyl radical result in the formation of a solute radical c a t i ~ n . ~ ' The -~~ ultimate fate of the OH adduct would also depend on the nature of substituents.21 In the case of benzene, H+-catalyzed dehydration of the OH adduct is inefficient, as the solute radical cation can undergo hydrolysis to form the cyclohexadienyl radical.21 Therefore, it appears that only a fraction of the OH adduct is undergoing Hf-catalyzed dehydration to form the solute radical cation (2), Scheme 1. The nature of transient species can be identified by its reaction with 0 2 . Solute radical cations are known to have very low reactivity with 0 2 , whereas carbon centered neutral radicals have high reactivity. In order to distinguish between these two processes, Le., formation of a solute radical cation and a neutral radical, pulse radiolysis studies were carried out in a NZsaturated condition. The initial faster decay was not observed in the N2 saturated solution. Therefore, this must be due to reaction of 0 2 with neutral radical. Under these conditions, the absorption at 325 nm consists of H and OH adduct. The solute radical cation is not stable. The initial faster decay of the transient band observed on pulse radiolysis of an aqueous solution of bromobenzene (2 x mol dm-3, HC104 = 3 mol dmP3) was found to depend on the oxygen concentration. The bimolecular rate constant for reaction of 0 2 with this radical was determined to be 1.5 x lo9 dm3 mol-' s-l. When [HC104] was more than 3 mol dm-3, another transient absorption band was observed with ,Ima= 550 nm. Figure 2b shows the variation in the absorbance at 550 nm as a function of [HClOd]. The absorbance at 550 nm was observed to increase, and the plateau value could not be observed even at 9.8 mol dm-3 HC104. This may not represent the true variation of absorbance with H+ concentration as the 'OH radical yield would not remain constant in this high concentration of HC104. In acidic conditions, a part of radiation energy would also be absorbed by HC104, and the yield of 'OH radicals would decrease with increasing concentration of HC104. Based on electron density distribution, 78% of radiation energy would be absorbed by water in 3 mol dm-3 aqueous solution of HC104. Therefore, @(OH) would be equal to 2.3. This value of G('0H) would further decrease at higher [HC104]. Figure 4a shows the transient optical absorption spectrum obtained on pulse radiolysis of an 0 2 saturated aqueous solution
J. Phys. Chem., Vol. 99, No. 17, 1995 6521
Figure 4. Transient optical absorption spectrum obtained on pulse radiolysis of an 0 2 saturated aqueous solution of bromobenzene ( 2 x mol d&) in (a) 8 and (b) 3 mol dm-3 HC104 and (c) 8 mol dm-3 HzS04. 0.032
1
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!
t
.a
0
20 Time
(JJS)
Figure 5. Absorption time signal obtained on pulse radiolysis of an 0 2 saturated solution of bromobenzene (1.9 x mol dm-7 in 8 mol dm-3 HC104 at (a) 550 nm, (b) 290 nm, and (c) 290 nm in 3 mol dm-3 HClO4.
of bromobenzene (2 x mol dm-3) in 8 mol dmP3HC104. The transient band with Ama = 550 nm was observed to decay by first-order kinetics with a rate constant of 1.4 x lo4 s-' (Figure 5a). The intensity of this band remained independent of solute concentration in the range of 5 x 10-4-8 x mol dmP3. The rate constant for the reaction of *OH radicals with bromobenzene in 8 mol dm-3 HC104 was determined by formation kinetic studies at 550 nm. The pseudo-first-order rate constant (kObs) was found to increase linearly with solute concentration (1-5) x mol dm-3. The bimolecular rate constant determined from the slope of a linear plot of kobs versus solute concentration was 8.9 x lo9 dm3 mol-' s-l. The transient optical absorption spectrum obtained on pulse radiolysis of an 0 2 saturated aqueous solution of bromobenzene (Figure 4a) also showed another band with A, = 320 nm. The decay of this band was not from a single component and was different from that of the 550 nm band. Therefore, absorption in this region would not be due to only the contribution of transient species absorbing at 550 nm. Figure 4b represents the transient optical absorption spectrum on pulsing an 0 2 saturated aqueous solution of bromobenzene (2 x mol
Mohan and Mittal
6522 J. Phys. Chem., Vol. 99, No. 17,1995 dm-3) in 3 mol dm-3 HC104. The absorption band with ,A = 550 nm was not observed, and the nature of the transient absorption spectrum in the 270-320 nm region was different from that obtained in 8 mol dm-3 HC104 (Figure 4a). The absorption time signal at 290 nm in 8 mol dm-3 HC104 was also different (Figure 5b) from that observed in presence of 3 mol dmV3€IC104 (Figure 5c). The decay of the transient band at 550 nm remained unchanged in NZsaturated solution. As mentioned previously, H+-catalyzed dehydration of the OH adduct would yield a solute radical cation. The 550 nm band is therefore assigned to the solute radical cation C6H5Bf+ (Scheme 1). The decay of absorption at 290 nm was of a different nature in NZ and 0 2 saturated conditions. Since the decay was not of a single component, it must consist of solute radical cation and OH adduct. Since the absorbance at 550 nm has not reached saturation value, only a fraction of the OH adduct is converted to the radical cation. Although the absorbance at 550 nm increased with [HC104] (Figure 2b), it remained nearly constant at 325 nm (Figure 2a). It may be due to the fact that, as more of OH adduct is converted to solute radical cation, the decrease in absorbance due to OH adduct is compensated by absorbance of the solute radical cation in this region of wavelength.
Assignment of Transient Species The primary reactive species produced on radiolysis of an aerated acidic aqueous solution are H02, 'OH, and C104 radicals. HOz radicals could not be the source of reaction with bromobenzene as (1) the transient band was also observed in N2 saturated solutions where HO2 radicals would not be produced, and (2) the transient band was not observed in the presence of tert-butyl alcohol, an efficient *OHand weak H atom scavenger. Moreover, the HO2 radical is a mild oxidizing agent with a redox potential value of 1.0 V,32 and even a stronger one-electron oxidant such as Br2.- (see text) failed to produce a similar transient band. The C104 radical also could not be responsible for this band as it absorbs at Am= = 335 nm.33 Independent pulse radiolysis studies of 0 2 saturated aqueous solutions of HC104, in the absence of bromobenzene, did not show similar absorption bands. The reaction of C104 radicals with bromobenzene was indirectly studied on pulse radiolysis of an 0 2 saturated neutral aqueous solution of NaC104 (6.5 mol dm-3) containing bromobenzene (2.0 x mol dm-3) which showed the absence of a transient band at 550 nm. Therefore, the 550 nm band observed in the presence of HClO4 could not be due to reaction of c104 radicals with bromobenzene. If this band is due to reaction of 'OH radicals (in presence of high concentration of H+) with bromobenzene, then it should also be produced in the presence of another acid. The transient optical absorption spectrum obtained on pulse radiolysis of an 0 2 saturated aqueous solution of bromobenzene (2 x mol dm-3) in 8 mol dm-3 H2S04 (Figure 4c) was similar to that observed in 8 mol dm-3 HC104. The intensity of the 550 nm band was slightly lower than that produced in 8 mol dm-3 HC104, which should be due to a lower Hammett acidity function (Ho) of H z S O ~ If. ~Br-/ ~ Br2 were formed on thermaVphotochemical reaction of acidic aqueous solution, then the transient band on pulse radiolysis should have been observed at 360 nm and not at 550 nm. Even its intensity would have increased with solute concentration. Therefore, within the experimental time, bromobenzene is stable and does not decompose in the presence of a high concentration of HC104.
0 300
350
L50 WAVELENGTH
550 (nm)
650
700
Figure 6. Transient optical absorption spectrum obtained on pulse mol radiolysis of a Nzsaturated solution of bromobenzene (4.5 x dm-3) in DCE. Inset shows absorption time signal at 570 nm.
Effect of Substituent on Formation of Radical Cation H+-catalyzed dehydration of OH adducts of substituted benzene is found to depend on the nature of the substituents. In the case of benzene, even 10 mol dm-3 HzS04 was not able to form solute radical cations.21 Solute radical cations of methoxy substituted benzenes are observed at pH = 4.22,23The *OHradical induced reactions with phenyl substituted carboxylic acid have also shown the formation of solute radical cations.35 Because of the higher electron affinity of bromine as compared to hydrogen, removal of OH- by H+ would be possible at higher concentrations of H+ in the case of bromobenzene. At low H+ concentration, solute radical cations would immediately decay by deprotonation. Their lifetime could be increased at high H+ concentration. Alkyl bromides are also observed to undergo H+-catalyzed dehydration at a higher acid concentration as compared to alkyl iodides.17 H+-catalyzed dehydration of OH adducts of fluoroiodo and chloroiodo alkanes have also been observed at much higher acid concentrationas compared to iodo alkanes, and this is explained by the presence of fluorine and chlorine, high electron affinity groups.36 The formation of the solute radical cation of iodobenzene was observed at pH = 2.36E Therefore, it may be concluded that the higher concentration of acid required for acid-catalyzed dehydration of the OH adduct of bromobenzene would be due to the high electron affinity of bromine. y-Radiolysis of bromobenzene in a glassy matrix of 3-methylpentane at 77 K has shown the formation of optical absorption bands with A,, = 280 and 550 nm and are assigned to C6H5Br'+.5 1,ZDichloroethane (DCE) has been frequently employed as a solvent for the study of solute radical cations of organic compounds due to the high ionization potential of DCE ( 11.12 eV).37 The yield of solvent cation is increased because the electrons produced by ionizing radiations are scavenged by the parent-molecule and undergo dissociative electron capture reaction. The ionization potential of bromobenzene (8.98 eV) is lower than that of DCE. Therefore, it should be possible for solvent cations to transfer their charge to solute molecules. Pulse radiolysis of a Nz saturated solution of bromobenzene in DCE showed a transient band with A, = 570 nm and increasing absorption below 300 nm (Figure 6). This band was observed to decay by first-order kinetics with t112 = 10 ps. The intensity of this band remained independent of solute concentration in the region of 2 x -1x mol dmV3. The band was not observed on pulse radiolysis of a N2 saturated solution of benzene in DCE, suggesting that the band observed with bromobenzene at 570 nm is not due to any transient species generated on addition of CH2CHzCl radicals (formed on dissociative electron capture by solvent) to the n system. Such radical species are expected to absorb at
