J . Phys. Chem. 1990, 94, 2447-2453
2447
Bromine Atom Complexes with Bromoalkanes. Their Formation in the Pulse Radiolysis of Di-, Tri-, and Tetrabromomethane and Thelr Reactivity with Organlc Reductants Lian C. T. Shoute and P. Neta* Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: July 5, 1989; In Final Form: October 16, 1989)
Bomine atoms were produced in the pulse radiolysis of neat dibromomethane (DBM) and bromoform and of cyclohexane solutions containing DBM, bromoform, carbon tetrabromide, or ethyl bromide. The Br atoms form complexes with dimethyl sulfoxide (& 425 nm), with aromatic compounds, and with oxygen-containing compounds. In the absence of other complexing agents, since Br atoms do not abstract H from the solvents rapidly, they form complexes with their parent bromo compounds. The absorption maxima of these complexes are at 365 nm for C2H5Br-Br,390 nm for CH2Br2.Br,425 nm for CHBr3.Br, and 480 nm for CBr4.Br. The stability of RBr-Br appears to increase with the number of Br atoms in the molecule. These complexes act as oxidants toward p-methoxyphenol, 1,3,5-trimethoxybenzene,triphenylamine, and N,N,N',N'-tetramethyl-p-phenylenediamine. The rate constants for the oxidations were about 1O'O M-' s-l with CH2Br2.Brbut only of the order of 108-109 M-I s-I with CBr,.Br. The initial products of the oxidation are the ion pairs between the radical cation of the organic substrate and the Br- ion. In the case of p-methoxyphenol the initial ion pair releases HBr under neutral or basic conditions to form the neutral p-methoxyphenoxyl radical. The Br atom complexes are also capable of abstracting H from weak C-H bonds. The benzylic and allylic hydrogens in hexamethylbenzene and cyclohexene are abstracted with rate constants near lo9 by CH2Br2.Br and near lo7 M-' s-l by CBr4.Br. The behavior of Br atoms is compared with that of CI and I atoms.
Introduction Haloalkanes are frequently used as solvents for the electrochemical and radiolytic oxidation of solutes. Although there are a larger number of studies on the species responsible for the oxidation of solutes in chl~roalkanes,~-~ little information is available on the radiolytic transient species in bromoalkanes. The present study was undertaken to examine the behavior of shortlived intermediates produced in the radiolysis of dibromomethane (DBM) and related systems. Recent studies on the radiolysis of dichloromethane indicated that both [CH2C12]*+and C1 atoms are formed in the primary processes'P2
--
CH2C12
CH2C12
[CH2C12]'+ 'CH2Cl
+ e-
(1)
+ C1
(2) In the subsequent steps the radical cation deprotonates to yield 'CHCI,, the e- reacts with the solvent to yield 'CH2C1 and C1-, and the C1 atoms also react with the solvent by H abstraction to give 'CHC12 and HC1. The 'CHC12 and 'CH2Cl radicals react with O2to produce the corresponding proxy1 radicals, which are relatively long lived and do not react with the solvent. When only low concentrations of solutes are present in dichloromethane, the solute may be oxidized solely by the peroxyl radicals. On the other hand, when solutes are present at high concentrations, they may intercept the short-lived [CH2C12]*+and C1 species and undergo additional reactions. The rate constants for these rapid reactions were difficult to determine directly by following the decay or buildup of absorbing species and were often measured by com(1) Alfassi, Z. B.; Mosseri, S.;Neta, P. J. Phys. Chem. 1989, 93, 1380. (2) Emmi, S. S.; Beggiato, G.; Casalbore-Miceli, G. Radiar. Phys. Chem. 1989,33, 29. Emmi, S. S.; Beggiato, G.; Casalbore, G.; Fuochi, P. G. Fifrh Tihany Symp. Radiaf. Chem., Proc. 1982, 611. (3) Bibler, N. E. J. Phys. Chem. 1971, 75, 24; 1973, 77, 167. Biihler, R. E.; Hurni, B.Helv. Chim. Acra 1978, 61,90. Mehnert, R.; Brede, 0.;BUS, J.; Naumann, W. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 992; 1980, 84, 63; Radiochem. Radioanal. Lerr. 1982, 51, 45. Gremlich, H.-U.; Ha, T. K.; Zumofen, G.; Biihler, R. E. J . Phys. Chem. 1981,85, 1336. Van den Ende, C. A. M.; Luthjens, L. H.; Warman, J. M.; Hummel, A. Radiar. Phys. Chem. 1982, 19,455. Sumiyoshi, T.; Sawamura, S.; Koshikawa, Y.; Katayama, M. Bull. Chem. Soc. Jpn. 1982, 55, 2346. Biihler, R. E. Radiar. Phys. Chem. 1983, 21, 139. Washio, M.; Tagawa, S.; Tabata, Y . Radiar. Phys. Chem. 1983, 21, 239. (4) Burrows, H. D.; Greatorex, D.; Kemp, T. J. J. Phys. Chem. 1972, 76, 20. (5) Grodkowski, J.; Neta, P. J. Phys. Chem. 1984, 88, 1205.
petition with the solvent or with other solutes. Dimethyl sulfoxide was found1V6to be a useful competitor for this purpose because C1 atoms were found' to form a complex, DMSOC1, which exhibits intense absorption at 400 nm. Bromine atoms are expected to be formed in the radiolysis of bromoalkanes, but unlike the case of C1 atoms, H abstraction by Br atoms is thermodynamically unfavorable since the H-Br bond is weaker than most C-H bonds.* In the present study we find that Br atoms form complexes with DMSO as well as with bromoalkanes and that these complexes oxidize a variety of organic compounds with very high rate constants and can also abstract hydrogen from weak C-H bonds.
Experimental Sectiong Dibromomethane (DBM) was a Fluka puriss reagent and was used as received, bromoform was from Fisher and was purified by vacuum distillation and passing through activated alumina, carbon tetrabromide was from Eastman and was recrystallized from acetone/water, and ethyl bromide was from Fisher and was purified by passing on a column of activated alumina. Cyclohexane and dioxane were analytical grade from Mallinckrodt and were passed through activated alumina. Dimethyl sulfoxide (DMSO) was Aldrich Gold Label, and pyridine and benzene were analytical grade from Mallinckrodt. Triphenylamine (TPA) was obtained from Aldrich and was further purified by recrystallization from ethyl acetate/ethanol. N,N,N',N'-Tetramethyl-pphenylenediamine (TMPD) was also from Aldrich and was purified by sublimation. 1,3,5-Trimethoxybenzene (TMB) and cyclohexene were from Aldrich, hexamethylbenzene (HMB) was from Eastman, and phenol and p-methoxyphenol (PMP) were from Sigma. Fresh solutions were prepared before each experiment, and samples were transferred with a syringe to the irradiation cell. The pulse radiolysis apparatus was described before.1° It utilizes (6) Sumiyoshi, T.; Miura, K.; Hagiwara, H.; Katayama, M. Chem. Lerr. 1987, 1429. (7) Sumiyoshi, T.; Katayama, M. Chem. Lett. 1987, 1125.
(8) Bond dissociation energies are as follows: H-C1, 103.1; H-CHCI,, 99.0; H-Br, 87.6; H-CHBr,, 103.7; H-CBr,, 96.0; H-cyclohexyl, 95.5 kcal/mol (Handbook of Chemistry and Physics, 67th ed.;CRC Press: Boca Raton, FL, 1986). (9) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified are necessarily the best available for the purpose.
0022-3654/90/2094-2447$02.50/00 1990 American Chemical Society
2448
The Journal of Physical Chemistry, Vol. 94, No. 6 , I990 O 8 / I
"
"
"
"
'
I
'
"
500
400
"
'
Shoute and Neta
1
600
X,nm Figure 1. Optical absorption spectrum of the DMSO-Br complex formed in the pulse radiolysis of dimethyl sulfoxide (0.5 M) in dibromomethane solution.
50-ns pulses of 2-MeV electrons from a Febetron 705 accelerator. The dose per pulse was between 5 and 15 Gy in most experiments, as determined by dosimetry with N,O-saturated aqueous KSCN solutions." The kinetic spectrophotometric detection system consisted of a Varian 300-W xenon lamp, a 2-cm optical path length irradiation cell, a Kratos high-intensity monochromator, an RCA 4840 photomultiplier, and the appropriate shutters, lenses, and optical filters. The signals were digitized with a Tektronix 7612 transient recorder and analyzed by a PDP 11/34 minicomputer. All experiments were carried out at room temperature, 22 f 2 O C .
Results and Discussion In this study we investigated the formation and reactions of Br atoms in the radiolysis of bromoalkane solutions. Radiolysis of bromoalkanes, by similarity with the results on CH2C12,CHC13, and CCI4, is expected to lead to ionization and C-Br bond scission:
-
CH2Br2
[CH2Br2]'+ + e-
-
CH2Br2
'CH2Br
- 0
(3)
+ Br
(4) Since the C-Br bond dissociation energy is considerably lower than that of the C-CI bond, radiolytic C-Br dissociation may be more pronounced than that of C-Cl in analogous solvents. In the secondary steps, CI atoms react rapidly with the solvent by H abstraction whereas Br atoms are less likely to do so since such a process will be endothermic for most solvents. Dimethyl Sulfoxide. The CI atoms produced in CH2CI2, CHC13, and CCl., were found to react with DMSO, with k5 = 7 X lo9 M-' s-l, to yield a transient adduct absorbing at 400 nm:6 CI (CH3)2SO (CH,),S(CI)O (5)
O'
L i
I
O'
400
500
600
700
h,nm Figure 2. Optical absorption spectra of Br atom complexes with bromoalkanes: (a) neat CH,Br2, (b) CH2Br2(0.29 M) in cyclohexane, (c) CH3CH2Br(0.54 M) in cyclohexane, (d) CHBr3 (0.23 M) in cyclohexane, (e) CBr4 (0.05 M) in cyclohexane. All solutions under air, dose per pulse 15 Gy, absorbance monitored within 1 ps after the pulse.
-
reaction) and that the decrease in yield may be due to incomplete complexation of Br with DMSO or to additional processes (see below). This reaction served as reference for measuring the rate constants The DMSOSBr complex was also observed in the pulse radiolysis for reactions of C1 atoms with CH2C12and with various ~ o l u t e s . ' ~ ~ of bromoform and carbon tetrabromide. The former was used These observations suggest that a similar complex of Br atoms as the solvent whereas in the latter case, since CBr4 is a solid, the with DMSO could be formed in the radiolysis of DBM/DMSO studies were carried out with cyclohexane solutions. Under these solutions. conditions, the most likely route for production of Br atoms is via Pulse radiolysis of DMSO in DBM showed formation of a excitation of CBr, by energy transfer from excited cyclohexane: transient species with an optical absorption centered at 425 nm (Figure 1). This spectrum, which is slightly red-shifted from that c-Hx c-Hx* + c-Hx'' + e(7) Br + (CH3)2S0 (CH,),S(Br)O (6) c-Hx* + CBr, c-Hx CBr4* (8) of DMSOCI, is ascribed to DMSO-Br. The rate of formation CBr4* 'CBr3 + Br' (9) of this complex is very high, near 1Olo M-' s-I, and could not be determined in our system. The absorbance at 425 nm decreased When DMSO was present in the above solutions, an absorption by a factor of 1.7 as the concentration of DMSO was changed with a maximum at 425 nm was observed, indicating the formation of DMSO-Br complex. In both cases, however, the tail of the UV from 0.5 M to 0.5 mM, decreasing more sharply at lower absorption was shifted to slightly higher wavelengths as compared [DMSO]. An attempt to treat the decrease in yield as a comto the spectrum observed in DBM. The absorption of DMSO-Br petition between DMSO and DBM for the Br atoms (as was done decayed over ca. 100 ps in a second-order process with 2k/t = for the case of Cl atoms in CH2CI2,using eq 10 in ref 1) did not 2 X lo6 cm s-I. The products of this decay process have not been give a linear relationship, indicating that Br atoms do not react examined, but a second-order reaction is likely to result in the with DBM by H abstraction (due to the endothermicity of this formation of Br,. This further indicates that the complexed Br atoms do not react with the solvents by H abstraction. (IO) Neta, P.; Huie, R. E. J . Phys. Chem. 1985, 89, 1783. Complexes of Br A t o m with RBr. Pulse radiolysis of the above ( I 1 ) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem. 1980,84, 2088. solutions in the absence of DMSO resulted in the formation of
+
-
--
-
-
+
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2449
Bromine Atom Complexes with Bromoalkanes
radiolysis of benzene gives a high yield of triplet states, which transfer energy to the bromo compounds to form the Br atoms. The transient spectra monitored in benzene solutions were found to be identical for CBr4, CHBr3, and CH2Br2,with a peak at 530 nm (Figure 3). This corresponds to the known complex of Br atoms with b e n ~ e n e , ~which ~ * ' ~appears to be formed more favorably under these conditions than the RBrSBr complexes (eq 11). Upon diluting the benzene with cyclohexane, keeping [CBr,]
.06 a, 0
c (3 .04 I l L
2O .02 a
Br' 01'
'
'
400
'
'
500
'
'
600
'
'
700
'
I
X,nm Figure 3. Optical absorption spectrum of the Br atom complex with benzene. Monitored within 1 ks after the pulse radiolysis of benzene containing 0.05 M CBr4.
intense transient absorptions which were different for the different bromo compounds and thus cannot be assigned to free Br atoms. The absorption maxima were at 365 nm for C2H5Br,390 nm for CH2Br2,425 nm for CHBr,, and 480 nm for CBr, (Figure 2). The first and last compounds were examined in cyclohexane while the two liquid bromomethanes were examined both in cyclohexane and as neat solvents. The following additional results lead to the conclusion that these absorptions can be ascribed to the complexes of Br atoms with the parent bromo compounds: Br'
+ RBr
-
RBr-Br
(10)
(a) The transient species are produced in neat CHzBr2and in cyclohexane solutions of all four bromo compounds but not in solutions of CBr4 in CC14 or in CH2C12. This indicates that the precursor of the species observed is the Br atoms and not the radical cations formed in halogenated hydrocarbons. Furthermore, radical cations of the analogous chloromethanes have much shorter lifetimes (in the nanosecond range) than the species observed here (in the microsecond range). (b) The transient species are produced in cyclohexane by energy transfer from the excited solvent molecule to the bromo compound which results in the formation of Br atoms. The radical cation of cyclohexane cannot transfer its charge to the bromo compounds since the ionization potential of cyclohexane is 9.9 eV while those of the bromo compounds are between 10.3 and 10.5 eV.I2 The yield of the CBr4.Br complex was found to increase when cyclohexane was replaced with Decalin and bicyclohexyl, in accord with the higher radiolytic yield of the excited states in these s01vents.l~ The spectrum recorded in the pulse radiolysis of CBr, in dioxane was red-shifted by 20-30 nm, and the absorbance at the peak was about half that observed in cyclohexane. The total yield of excited states in dioxane is similar to that in cy~lohexane'~ so that Br atoms are expected to be produced. The shift in spectrum, however, suggests that in this case Br forms a complex with dioxane rather than with CBr, since the former is in large excess. Similarly, the transient absorption recorded with CBr4 in acetone had peaks at 400 and
