REACTIONS OF HYDROGEN ATOMSWITH BENZENE AND TOLUENE
3971
The Reactions of Hydrogen Atoms with Benzene and Toluene Studied by Pulsed Radiolysis: Reaction Rate Constants and Transient Spectra in the Gas Phase and Aqueous Solution’
by Myran C. Sauer, Jr., and Barry Ward2 Chemistry Division, Argonne National Laboratory, Argonne, Illinois
6’0439 (Received M a y 19, 1967)
The rate constants at 25” for the gas-phase reactions of hydrogen atoms with benzene and toluene have been determined by the technique of pulsed radiolysis and are, in units of M-’ sec-’, 0.37 X lo8 and 1.0 X lo8, respectively. The activation energy in the case of benzene is about 3 kcal/mole. I n aqueous solution, the corresponding rate constants have been determined to be 1.1 X lo9 and 2.6 X lo9, respectively. The limits of error on these rate constants are ca. *20%. The rate constants were measured by following, as a function of time, the formation of the optical absorption due to radicals produced when hydrogen atoms, produced by an electron pulse, react with the aromatic compound. Benzene and toluene yield the cyclohexadienyl radical and the methylcyclohexadienyl radical, respectively, with absorption maxima a t 302 and 307 mp, respectively, in the gas phase. The half-widths of the absorption peaks are about 30 mp. I n aqueous solution, the absorption maxima are 311, (e311 (5.4 f 0.5) X loa M-’ cm-I) and 315 mp, respectively, with half-widths similar to those in the gas phase. The combination of two cyclohexadienyl radicals proceeds at least as fast as once every ten collisions in the gas phase a t 25”, indicating a “loose” activated complex. I n the liquid phase, the combination is diffusion controlled.
Introduction The use of the pulsed-radiolysis technique as a means of producing oxygen atoms in the gas phase and studying their reaction with molecular oxygen has been describedaar4 When a system consisting of about 50 atm of argon and a few centimeters of oxygen was irradiated with a 1-psec pulse of 12-15-Nev electrons, a “pulse” of oxygen atoms was found to be produced essentially simultaneously. The formation of ozone due to the reaction of this pulse of oxygen atoms with molecular oxygen was observed and analyzed to obtain information on the rate constant of the reaction 0 O2 Ar + 0 3 Ar. The present work describes how the same technique has been used to produce a pulse of hydrogen atoms by pulsed radiolysis of a system containing, for example, 50 atm of argon and 5 atm of hydrogen. By introducing a few centimeters of benzene or toluene, the reac-
+
+
+
tions of hydrogen atoms with these materials were followed after the electron pulse by measuring the rates of formation of the optical absorptions of the resulting radicals. Because of the limited number of reactions for which rate constants have been determined in both the gaseous and condensed phasesj5we have used similar techniques to investigate the rates of reaction of hydrogen atoms with benzene and toluene in aqueous solution.
(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) Shell Research Ltd., Thornton Research Centre, Chester, England. (3) M. C. Sauer, Jr., and L. M. Dorfman, J. A m . Chem. SOC.,86, 4218 (1964). (4) M. C. Sauer, Jr., and L. M. Dorfman, ibid., 87, 3801 (1965). (5) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y.,1965, p 199.
Volume 7 1 , Number 18 November 1967
3972
Experimental Section General. Detailed descriptions of the techniques of high-pressure pulse radiolysis and the methods of spectrophotometric observation have been given earlier.4~6 However, the pressure gauge and the freeze-out arm were rern0vt.d from the high-pressure irradiation cells used in this investigation since it was suspected that they might cause errors in the results due to incomplete mixing in the “dead-ends.” Useful results mere also obtained from a 17 cm long quartz irradiation vessel containing a total pressure of only 1 atm. Jlaterials. The argon (Linde) and hydrogen @‘ationa! Cylinder Gas) were used without purification directly from the cylinder, the purity of the argon having been checked in an earlier in~estigation.~As for the hydrogen, the stated purity was 99.97%, with less than 10 ppni O2 and 5 ppm hydrocarbons. The benzene. and toluene (Phillips research grade) and chlorobenzene (La Pine and Co.) were degassed by several trap-to-trap distillations on a conventional vacuum line before vaporizing iiito the irradiation cell. The deuterated benzene and toluene (llerck Sharp and Dohme) were handled in a similar manner. Aqueous Solutions. The aqueous solutions of the aromatic compounds were prepared in nominal concentrations by first dissolving the hydrocarbon in methanol (Baker) and then adding an aliquot of this solution to triply distilled water. Degassing was effected in 100-cc syringes by adding argon to the solution and shaking vigorously for about 1 min before expelling the gases. After five or six such cycles, the oxygen concentration is about M . The technique of filling and emptying the 4-cm quartz irradiation cell using a differential gas pressure has been described p r ~ i o u s l y . ~The experimental arrangement for monitoriitg the optical transmission of the solutions was almost identical with that used in the gas-phase experiments except that the path length (within the irradiation vessel) of the light beam was only 8 cm as coinpared to 26 cm. The determination of the hydrocarbon concentrations was not sinilde since the high volatility of the solutes caused immediate loss from a solution on exposing it to a liquid-gas interface. Eventually, the analytical method adopted was gas chromatography on a silicon gun1 rubber column at 50”. Standard solutions were prepared in 100-cc syringes, containing glass mixing disks but no gas space, by injection of an appropriate amount of hj.drocarbon from a calibrated microsyringe into a known volume of methanol. This operation was followed by z u n identical one in which a portion of the hydrocarbon--methanol mixture was injected into water (to give a solution of required concentration). Signal The Journal of Physical Chemistry
MYRANC. SAUER, JR.,
ANI)
BARRY WARD
strengths from the degassed experimental solutions, prepared by normal volumetric techniques, were then compared with those from the standard solutions. Using this method, we estimate the uncertainty in the hydrocarbon concentrations to be not more flo%, including a correction for the loss of solute which occurs when the solution is forced into an empty cell. The solutions were adjusted to a pH of about 3 with sulfuric or perchloric acids in order to convert the solvated electrons produced by the pulse into hydrogen atoms and prevent possible radical formation by reaction of the aromatics with the electrons followed by protonation of the resulting anion. Xethanol was added to the system, not only to facilitate solution of the aromatics, but also to scavenge hydroxyl radicals* which add to the benzene rings ( k = 4.3 X lo9 kf-l sec-l) to form the hydroxycyclohexadienyl radical, absorbingg in the same region as the product of the hydrogen atom reaction. Unfortunately, methanol also reacts with hydrogen atoms :it a non-negligible ratel0 ( k = 1.G X lo6 111-l sec-l) such that under optimum conditions (benzene X, methanol -2.5 x lo-? 111) only about 70% of the hydrogen atoms formed add to the benzene. In a case such hs this, the experimentally determined pseudo-firqt-order rate constant is given by k1 [benzene] I:, [niethanol] where kl and k2 are the second-order rate constants for the reactions of hydrogen atoms with benzene and methanol. Since IC? and the alcohol Concentration are known, it is a simple matter to apply a correction to the kinetic measurements. In order to minimize this correction, experiments were repeated using completely deuterated methanol (lIerck Sharp and Dohme) as an OH scavenger. Deuteration has the effect that while the rate constant for OH reaction is little changed (from 4.P to 2.511 X 108 M-1 sec-I), that for the hydrogen atom reaction decreases from 1.6 x lo6 lo t o 8 X lo4 J1-’ sec-l.ll Here we have assumed that the reactivity of CD,OD is the same as that of CD30H; we feel this is a reasonable assumption since radical attack on alcohols occurs mainly a t a C-H b0nd.l’ Because of this decrease in
+
(6) M. C. Sauer, Jr., S. Arai, and L. 42, 708 (1965).
M. Dorfmnn, J . Chem. Phys.,
(7) E. J. Hart and J. W. Boag, J . Am. Chem, Soc., 84, 4090 (1962). (8) G. E. Adams, J. W. Boag, J. Currant, and B. D. Micliael, “Pulse Radiolysis,” M. Ebert, et al., Ed., Acsdemic Press, NeF Tork, N. T., and London, 1965, p 131. (9) I.. M. Dorfman, I. A. Taub, and R. E. Buhler, J . Chem. Phys., 36, 3051 (1962). (10) J. P. Sweet and J. K. Thomas, J . Phys. Chem., 68, 1363 (1964). (11) OH CD8OD: M. Anbar, D. Meyerstein, and P. Neta, J . Chem. SOC.,[B],742 (1966); H CDzOD: M.Anbnr and D . Meyerstein, J . Phys. Chem., 68, 3184 (1904).
+
+
REACTIONS OF HYDROGEN ATOMSWITH BENZENE AND TOLUENE
the hydrogen atom reaction rate, the correction in the experiments with CDaOD was less than 3%, and for this reason the results from this system are more reliable. While the contribution of the hydroxycyclohexadienyl radical to the absorption could approach 5% of the maximum in these runs, its effect on the formation curves will be negligible since, to a good approximation, this absorption will be a constant cancelling out in the expression used in the first-order plot. Treatment of Data. The reactions after the pulse were monitored on an oscilloscope by displaying the intensity of transmitted light as a function of time. Photographs of the oscillographic traces were analyzed by a completely automatic, computerized technique, details of which may be found in ref 12, 13, and 14. Preparation of Gas-Phase Samples. The samples were prepared by first letting into the irradiation vessel the required pressure of the aromatic compound, followed by the desired pressures of hydrogen and/or argon. The samples were allowed to stand overnight to ensure complete mixing. A few samples were run in which excess liquid benzene or toluene was present. It was found that the rate of transient formation was larger than expected, based on the vapor pressure of benzene a t room temperature and 1 atm. This anomaly is accounted for by the elevation of saturated vapor pressures caused by high gas pre~sure.'~ Product Analysis. Gas chromatographic analysis of the products formed during the pulsed radiolysis of gasphase samples was carried out by trapping the nonvolatile contents of the irradiation vessel in a stainless steel U-shaped trap containing some glass wool, at liquid nitrogen temperature. This was done on a vacuum line so that the argon and hydrogen were pumped away. After 2 hr or more of distillation from the vessel to the trap, the trap was placed directly before the column on a flame detector apparatus. A 6-ft silicon-gum rubber column, temperature programmed from 25 to 210°, was used for the dimer analysis. An 8-ft P,P'-oxydipropionitrile column a t room temperature was used for the cyclohexadiene analysis. An analysis of products from the aqueous solutions was not attempted.
3973
I
I
I
I
I
I
I
I
I
I 300
I
I
I
I
I
I
I .o
W
u 2 a m a
5: m a
W
2
0.5
2 J
W
a
2 70
330
WAVELENGTH ( m p )
Figure 1. Gas-phase spectra of cyclohexadienyl (A) and methyl cyclohexadienyl (B) radicals. (Both spectra are arbitrarily normalized to 1.0 at maximum absorption.)
Results Transient Spectra. Figure 1 shows the spectra ob-
tion a t lower wavelengths due to another species, possibly a permanent product. The assignment of these spectra to the cyclohexadienyl radical, CsH,. ,,A( 302 mp), and methylcyclohexadienyl radical, C7H9. (A, 307 mp), will be discussed. Although the wavelength of maximum absorption did not vary, for a given aromatic compound, from one sample to another, or with changes in pressure, the precise shape and half-width of the absorption did. No general trends could be established with changes in pressure or composition. (Most of the results gave half-width of about 30 mp, but a few variations of about *20% occurred.) The spectra shown in Figure 1 are intended to indicate only the approximate shapes of the absorptions. Since a band-pass of 3.2 mp was used in the spectrophotometric determination of these spectra, some structural features may be lost. The use of completely deuterated benzene or toluene also had no effect on the wavelength of maximum absorption,
served when samples of argon and hydrogen (ratio about lO:l), or just hydrogen, containing much smaller amounts (ca. 7 cm) of benzene or toluene were pulse irradiated. The spectra were the same over the entire time range of the transient formation and decay, except that the decay a t 270-280 mp was slightly slower than a t wavelengths longer than 280 mp, indicating absorp-
(12) S. Arai and M. C. Sauer, Jr., J. Chem. Phys., 44, 2297 (1966). (13) M.C. Sauer, Jr., Argonne National Laboratory Report, ANL7113,Oct 1965. (14)M. C . Sauer, Jr., Argonne National Laboratory Report, ANL7146,Jan 1966. (15) S. Robin and B. Vodar, Discussions Faraday SOC., 15, 233 (1953).
Volume 71. Number 18 November 1967
MYRANC. SAUER,JR.,AND BARRYWARD
3974
and the half-width did not change more than would be expected from the variations noted above for the nondeuterated compounds. When samples of argon plus benzene or toluene without hydrogen gas were irradiated, an absorption about 30-50y0 as strong was observed in the same spectral region, with the same wavelength of maximum absorption. The spectra were not appreciably different in shape except for a tendency toward stronger absorption a t wavelengths below the maxima. Ilowever, there was an initial absorption which disappeared within about 5 psec after the pulse and which had a maximum absorption around 330-340 mp in the case of benzene. -4. similar, but weaker, absorption was also present, in the case of toluene. The identity of the species responsible for these rapid decays is unknown. The decay of the main absorption was similar to that observed in the experiments where hydrogen was present (see later section on transient decay). For the sake of completeness, it should be mentioned that L'blank" experiments on argon or hydrogen alone gave no absorbing transients. These facts will be discussed later. The absorption spectra obtained from the reaction of
I .o
1
-
1
I
I
I
I
I
I
w
hydrogen atoms with benzene (A, 311 mp) and toluene (Am,, 315 mp) in aqueous solution are shown in Figure 2. These spectra were taken a t about 20 psec after the pulse; the decay was considerably slower a t lower wavelengths than at the wavelength of maximum absorption and higher. This indicates either an absorption due to a product or the presence of another transient which absorbs a t lower wavelengths but decays slowly. The half-widths are somewhat uncertain because of the increase in absorption at low wavelengths, but are not appreciably different from the gas-phase results. We estimate that a maximum of 5% of the absorption is due to CsHsOH and CHzOH formed in reactions of OH and H with the aromatic solute and methanol. The spectrum of the methanol radical .CH20H, in an aqueous solution of 2.5 X lod2 M CHBOH is included in Figure 2. The absorption of CHzOH is considerably weaker than that of the cyclohexadienyl radical (see later section on absorption coefficients), but it has been plotted on a comparable scale for convenience. Rate Constantsfor Hydrogen Atom Reactions. Figures 3 and 4 show oscilloscope traces representing the concentration of transient as a function of time when a system containing 53 atm of argon, 7 atm of hydrogen, and 0.1 atm of benzene is pulse irradiated. The initial relatively sharply curved part represents the transient formation, and the subsequent, more slowly changing part of the trace represents the decay of the transient. As will be shown, the formation of the transient is due to the reaction of hydrogen atoms with benzene and the decay is due to the dimerization and disproportionation
V
z Q
m E
0
cn
0.5 w
> -
2 J w
[r
Figure 3. Formation of cyclohexadienyl (gas phase), higher intensity pulse; X = 302 mp.
I
2 3
I
I
I
I
I
I
I
270 280 290 300 310 320 330 340 WAVELENGTH ( m p )
Figure 2. Spectra in aqueous solution: A, cyclohexadienyl radical; B, methylcyclohexadienyl radical; C, .CHlOH radical. A and B are both arbitrarily normalized to 1.0 at maximum absorption, and C is arbitrarily normalized to 0.5 a t 270 mp. The actual absorption coefficient of C is much smaller than shown, relative to A and B: €Aa1' 5.4 x 103 M-' cm-1; d l l 25 M-1 cm-1.
The Journal of Physical Chemistry
Figure 4. Formation of cyclohexadienyl (gas phase), lower intensity pulse; X = 302 mp.
REACTIONS OF HYDROGEN ATOMSWITH BENZENE AND TOLUENE
of the resulting cyclohexadienyl radical. Similar curves are obtained in the case of toluene. The analysis of such curves for the rate constants of the reactions of hydrogen atoms with the aromatic compounds must take into account the fact that the transient undergoes decay by radical-radical reaction even during the time period of its formation. Experimentally, one can minimize the importance of this fact by decreasing the electron pulse intensity and increasing the amplification of the signal. That is, the rate of the pseudo-first-order reaction of hydrogen atoms with the aromatic compound is proportional to the initial hydrogen atom concentration, but the rate of the secondorder radical-radical reaction will depend on [radica1l2. The result of this effect can be seen by comparing Figures 3 and 4. An iterative method for computer analysis of such curves (where decay is not negligible) has been described14and was used to determine rate constants from curves such as Figure 3.16-1* Figure 5 shows the optical density (circles) as determined from the trace shown in Figure 4, and the corrected optical density (triangles), as a function of time. The corrected optical density is the actual optical density corrected to the hypothetical situation in which no decay of the cyclohexadienyl radical occurs. Figure 6 shows the first-order plot for the approach to the plateau of the corrected optical density curve. The slope of this plot is equal to the
0.025
0.023
@
0.02 I
> 0.01 9 0)
Z
0.0
a
0
0.0 0
0.0
0.01 I
0.00s
+
+
(16) In order to make a complete correction t o a curve such as Figure 3, the values of the rate constants for the reactions between two hydrogen atoms and between hydrogen atoms and the aromatic radical must be known relative to the rate constant for the reaction between two aromatic radicals. The latter reaction is responsible for the decay in Figure 3. As will be shown, the gas-phase rate constant for this reaction is between 1Olo and 1011 M-1 sec-', with the lower value being more probable, and the liquid phase value is about 1.8 X lo0 M-1 sec-1. I n the gas phase, the reaction between two hydrogen atoms is dependent upon the third body, and for argon, the rate constant17 is 2.3 X l o Q M - 2 sec-1. Therefore, at the highest argon concentrations used in this work, the rate constant times argon concentration is about 10'0 M - 1 sec-1, which is about the same as the rate constant for reaction between two cyclohexadienyl radicals. In the analysis of experimental curves, it was found that when the H H reaction was given a relative value of from 0 to 1 times the cyclohexadienyl dimerization rate constant, there was little change in the result for the rate constant derived for the addition reaction. Data on the reaction rate constant of hydrogen atoms with the cyclohexadienyl radical are not available, so the reaction was assumed to proceed with the same rate constant as H H. Again, the exact value had little effect on the result obtained. In aqueous solution, the rate constant for reaction between two hydrogen atoms is known to be'* about 1Olo A.1-1 sec-1, which is higher than the value obtained for reaction of two cyclohexadienyl radicals of 1.8 X lo0 M-1 set-1 (see Discussion section). The value for kH+n. was assigned values intermediate to those for ~ H + H and ~ R + R ,but nearer to that for ~ H + H . When relative values (to ~ R + R ) of 10.0 for kH+H and 5.0 for kH+n were used, the first-order formation plots were no longer linear, and all indications are that relative of 5.0 for ~ H + Hand 2.5 for ~ H + Rare reasonable. The value obtained for the rate constants for the reaction of hydrogen atoms with benzene and toluene did not change by more than 15 to 20% when the relative values of ~ H + H and kH+n were changed from 1.0 and 1.0 to 10.0 and 5.0, so the effect of these values on the result is small. Also, the results from experiments which were done with the smallest pulses, where the corrections for H H and H R would be at a minimum, are in agreement with values which were obtained from the higher intensity pulses. (17) B. A. Thrush, Progr. Reaction Kinetics, 3, G4 (1965). (18) M.Anbar and P. Neta, Intern. J. A p p l . Radiation Isotopes, 16, 227 (1965).
+
t 0 i
rate constant for reaction of hydrogen atoms with benzene multiplied by the benzene concentration. In the case of curves such as Figure 4,where the decay is nearly negligible, the analysis was often carried out by the usual method4 of making a first-order plot of the approach to the uncorrected plateau. The rate constants obtained from the analysis of curves such as those shown in Figures 3 and 4 were in agreement. Table I summarizes the rate constants obtained for the reactions of hydrogen atoms with benzene and toluene. Although the rate constant for benzene seems to increase slightly with total pressure, the effect is almost within the experimental scatter, and apparently there is no marked effect of pressure on the rate constants. It is worthwhile to note that removal of hydrogen atoms by diffusion to the walls is ruled out by the data, since this would cause an increase in the rate constant at lower pressure. The average values of the rate constants and the experimental uncertainties may be stated as k(H CBHB)= (0.37 f 0.06) X 108, and k(H C6HsCH3)= (1.0 f 0.1) X 108, in units of J1-l sec-I.
+
P
W
3975
I 10
I
I
I
20
30
40
I 50
I 60
+
70
p sec
Figure 5. ODtical densities us. time, calculated from Figure 3: A, actual optha1 density; 0,corrected optical density.
+
V o l u m e 71. Yurnber 12
S o w n b e r 1967
MYRANC. SAUER, JR., AND BARRY WARD
3976
Table I : Summary of Rate Constants Obtained for Reactions of Hydrogen Atoms Reaction of H with
Conditions
Benzene Benzene Benzene Toluene Toluene Toluene-methyl-&
Gas Phase 0.9 atm of Ar, 0.12 atm of Hz, 0.13 atm,of benzene 6 atm of Ar, 0.8 atm of Hz, 0.09 atm of benzene 54 atm of Ar, 7 atm of HP, 0.09 atm of benzene 11 atm of Ar, 0.8 atm of HI, 0.03 atm of toluene 54 atm of Ar, 7 atm of H2, 0.03 atm of toluene 11 atm of Ar, 0.8 atm of H2,0.03 atm of toluene
k X 10-8, M-1
sec-1
0.31 f 0 . 0 5 0.36 f 0.05 0.43 f 0.05 0.94 f O . l 1.1 f O . l 0.94 & 0.2 Av values over a number of runs
Aqueous Solution: pH 3 2.5 x 10-2 M CHaOH, 7.9 X 10-6 M benzene 2 X lo-* M CD,OD, 6.1 X M benzene 2.5 x lo-* M CH30H, 2.9 X M toluene 2 x 10-2 M CDaOD, 3.3 X l o b s M toluene
Benzene Benzene Toluene Toluene
13 i2" 10 1" 39 i 6" 18 =k 2"
*
Corrected for reaction of hydrogen atoms with methanol (see text).
