K. Sehested, H. Corfitzen, H. C. Christensen,and E. J. Hart
31 0
Rates of Reaction of 0-, OH, and H with Methylated Benzenes in Aqueous Solution. Optical Spectra of Radicals K. Sehested,' H. Corfltzen, Danish Atomic Energy Commission,Research EstablishmentRisd, DK-4000 Roskilde, Denmark
H. C. Christensen, AB Atomenergi, Studsvik, Fack, S-611 ol Nyk6ping, Sweden
and E. J. Hart Argonne National Laboratory, Argonne, Illinois 60439 (Received April 29, 1974; Revised Manuscript Received October 8, 1974) Publication costs assisted by the Danish Atomic Energy Commission
Benzyl type radicals are formed in aqueous alkaline solutions by reaction of 0- with methylated benzenes uia abstraction of an H atom from a methyl group. The same radicals are formed in acid solutions but in an indirect way. Water elimination from the OH addition complex involving a proton takes place. Besides adding to the benzene ring the OH radicals also react by direct abstraction of an H atom from a methyl group. The amount of abstraction is directly proportional to the number of methyl groups in the hydrocarbon. H atoms add to methylated benzenes to form cyclohexadienyl radicals. The rate constants for 0-, OH, and H reacting with methylbenzenes are of the order of 2 X lo9, 7 X lo9, and 3 X lo9 M - l sec-l, respectively. The rate for the water elimination reaction varies for the different compounds and the values are listed. Recent experiments indicate that this reaction has an intermediate step with a species absorbing above 400 nm. This species is tentatively assigned as a cation radical.
Introduction Recently we reported1 that the benzyl radical is formed by pulse radiolysis of aqueous toluene solutions. The formation is a direct abstraction of a methyl H atom by 0- in strongly alkaline solutipn or an indirect reaction by water elimination from the primarily formed OH addition product in acid solutions. The objective of the present study is to show that similar reactions take place in aqueous solutions of methylated benzenes and to explore the mechanism of the benzyl formation in acid solutions in more detail. In aqueous toluene solutions a few per cent of the OH radicals abstract an H atom from the methyl group forming the benzyl radical directly. The ratio of this reaction to the addition reaction was examined as a function of the number of substituted methyl groups. Some work has been published on the photolysis and radiolysis in pure methylbenzenes and with the benzenes dissolved in organic s01vents,~-~ but very little is published about these compounds in aqueous solutions, probably because of their poor solubility in water.
Experimental Section Materials. The methylated benzenes were obtained from Merck, BDH, KochLeight, Fluka, K & K, and were of purest available quality and used without further purification. The perchloric acid was reagent grade from G. F. Smith and sodium hydroxide from Merck. The water was triply distilled and all glassware was preheated to 450'. Irradiation. The pulse radiolysis system on the Ris$ Linac was described previously.1+6The essential features are the Linac delivering 0.2-4-psec single pulses of 11-MeV electrons with a peak current of 0.25 A as a maximum, an The Journal of Physical Chemistry, Vol. 79, No. 4, 1975
Osram XBO 450-W xenon lamp used in pulsed operation, a Zeiss MM 12 double quartz prism monochromator, an EM1 95586 photomultiplier tube, and a Tektronix 555 doublebeam oscilloscope. In some experiments the set-up on the 2.3-MeV Febetron was used. The pulse is about 20-nsec duration and the detection system is 5-10 times faster than that on the lineac. Dosimetry. The current induced in a coil surrounding the electron beam was used for relative dosimetry. The absolute dose was measured with the hexacyanoferrate(I1) dosimeter' using g (eaq- OH) = 5.25 and ~ ( 4 2 0nm) 1000 M - l cm-1. Preparation of Solutions. Solutions of liquid methylated benzenes were deaerated and saturated with a gas by bubbling the gas through the solution with excess benzene in a 100-ml syringe. Solutions of solid compounds were prepared in a 1-1. flask by repeated evacuation and saturation with the appropriate gas. The pH of the solutions was adjusted with perchloric acid or sodium hydroxide and was measured on a Radiometer digital pH meter, PHM52. Solubility. Determidation of the solubilities of the methylated benzenes was carried out by gas chromatography on a Perkin-Elmer, Model F 11, equipped with a Carbowax 1500 column for the lower methylated compounds and a SE 30 column for the higher ones. The samples were taken from the saturated solution, which also contained M tert- butyl alcohol as a reference, and injected into the gas chromatograph. The calibration was done with the pure compounds. The solubilities in neutral and alkaline solution are shown in Table I. Corrections. The absorption spectra were corrected for depletion of solute (S) in the wavelength range where the solute absorbs light (X 5270 nm). G (-S) was assumed to be 5.9 [= g(eaq-) g(H) g(OH)] in NzO-saturated unbuffered solutions. In 0.5 M NaOH a G (-S) of 6.5 was used.8
+
+
+
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Rates of Reaction of 0-, OH, and H with Methylated Benzenes TABLE I: Solubilities of Methylated Benzenes in Unbuffered and Alkaline Solutionsa Solubility neutral] alkaline, M x 103 Tolueneb (methylbenzene)
5.9/4.7
o-Xylene (1,2-dimethylbenzene)
2.1/1.4
m-Xylene (1,3-dimethyIbenzene)
1.7/0.95
p-Xylene (1,4-dimethylbenzene)
2.0/1.4
Mesitylene (lP3,5-trimethylbenzene) Hemimillitene (1,2,3-trimethylbenzene) Pseudocumene (192,4-trimethylbenzene) Durene (1,2,4,5tetramet hylbenzene)
0.53/0.37 0.75/0.62 0.70/0.67 0.04/0.03
Isodurene (1,2,3,5tetramethylbenzene)
0.40/0.16
Prehnitene (1,2,3,4tetramethylbenzene)
0.60/0.28
Pentamethylbenzene
0.05/0.02
Hexamethylbenzene
0.01
Methylbenzyl radical A,,
258 307 317.5 261 312 323 262 311 323 268 298 311 322 265 315 328 263 317 329 266 315 328 27 1 318 331 267 321 331 267 317 332 269 320 335 270 330
E,
OH adduct
M-1 cm-1
km,,
14,000 3)300 5,500 16,100 3,100 5,900 15,600 2,800 4,700 15,500 2,500 3,200 4,000 16,000 2,800 4,200 16,000 3,500 6,000 15,500 2,500 6, 000 12 000 2,700 3I 500 16)800 3,200 4 ,400 14,600 4,300 4,600 16,000 3,000 3,700 11,500 3,000
320
4,300
319
326
4,700
328
4,500
328
6,000
326
5,900
312
4,300
338
4,300
333
6,500
328
5,700
328
5,000
330
4,000
328
5,200
337
4,300
333
3,500
338
3,500
332
5,600
326
5,200
333
4,800
333
5,200
332
4,300
333
3,500
)
E,
M-l cm-1
H adduct k,,
E,
M-l cm-1 4,800
a Wavelengths of absorption maxima and corresponding extinction coefficients of substituted benzyl radicals, OH, and H adducts in aqueous solutions. Reference 1.
Results and Discussion Alkaline Solution. Methylbenzyl radicals were produced and studied in aqueous solutions by pulse radiolysis of alkaline NzO-saturated solutions (0.5 M NaOH). In this solution eaq-, H, and OH are converted into 0- within 0.1 ysec after the pulse and as shown p r e v i o u ~ l y 0~~~ reacts J~ preferentially with the side group by abstraction of an H atom. The methylbenzyl radicals are thus formed in a direct process, and the resulting spectra are similar and resemble the spectrum of the unsubstituted benzyl radica1.l As an example the spectrum of the 3,5-dimethylbenzyl radical is shown in Figure 1. The three main absorption peaks are shifted slightly (10-20 nm) to longer wavelength with increasing number of methyl groups (Table I). The extinction coefficients at the absorption maxima are calculated assuming G (0-) = 6.5,8 and the extinction of the peaks at 260-270 nm are very similar, about 15,000 M-l cm-l, for all compounds. As for the benzyl radical there is an absorption at longer wavelength (450-550 nm) from the methylbenzyl radicals with low extinction coefficients of 100-300 M-l cm-l. The rate constants for the reaction of 0- with methylbenzenes were determined by the increase at the absorption maxima with two-three different concentrations and a dose of about 200 rads. The rate constants for all compounds are of the order of 2 X lo9 M-l sec-l (Table
111) independent of the number of methyl groups, which indicate that the rate is diffusion controlled. The methylbenzyl radicals disappear in bimolecular reactions with 2k (1.5 f 0.5) X 109M-l sec-l (Table 111). Neutral Solution. In unbuffered NzO-saturated solutions the OH radical preferentially adds to the benzene ring forming a hydroxymethylcyclohexadienyl radical. A small part of the OH radicals react, however, with the methylated benzenes by abstraction of an H atom from a methyl group forming a methylbenzyl radical. The spectrum in neutral NzO-saturated solution is therefore composed of the spectrum of the OH adduct, the methylbenzyl radical, and a small part of the H adduct. The H and OH adducts have their maximum absorptions at 320-340 nm and a minimum at 260-270 nm where the methylbenzyl radicals have their main absorptions. Using the extinctions for these radicals found in alkaline solution and subtracting a certain percentage of this spectrum from that in neutral solution so that the resulting absorption curves become smooth around 260-270 nm, we compute the proportion of the abstraction reaction relative to the addition reaction. The amount of this abstraction reaction is directly proportional to the number of methyl groups. Figure 2 shows the uncorrected spectra obtained in neutral solutions for one, two, three, and four methyl groups substituted into the The Journal of Physical Chemistry, Vol. 79, No. 4, 1975
312
K. Sehested, H. Corfitzen, H. C . Christensen, and
E. J.
Hart
TABLE II: R a t e Constants for Benzyl Formation from OH Reacting with Methylated Benzenes by Direct Abstraction of a Methyl H Atom H abstrn % OH 6 .O
kabstrn,
Toluene" o-Xylene 12 .o rn-Xylene p - X ylene Mesitylene 18.5 Hemimillitene Pseudocumene 26 .O Durene Isodurene Prehnitene Pentamethylbenzene 34 . O Hexamethylbenzene
