Colussi, Singleton, Irwin, and Cvetanovic
1900
Absolute Rates of O ~ y g e n ( ~ P Atom ) Reactions with Benzene and Toluenela A. J. Col~ssi,'~ D. L. Singieton,lc R. S. Irwin, and R. J. CvetanoviC' Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K 1A OR9
(Received February 25, 1975)
Publication costs assisted by the Netlonal Research Council of Canada
The absolute values of the rate constants, k2, of the bimolecular reactions of ground state oxygen atoms, O(3P), with benzene and toluene have been determined by a phase-shift chemiluminescence technique using modulated mercury photosensitized decomposition of nitrous oxide to generate O(3P).The rate constants determined over the temperature interval 298-462OK are given by the Arrhenius expressions ka(benzene) = (1.09 f 0.64) X 1O'O exp(-4.20 f 0.43 kcal mol-'/RT) and k2(toluene) = (2.30 f 0.11) X 1 O l o exp(-3.86 f 0.04 kcal mol-llRT) in units of M - l sec-l. The uncertainties in A and E correspond to 95% confidence limits.
Introduction While extensive information on the mechanism and the rates of the reactions of the ground state oxygen atoms, O(3P), with olefins is now available, the information on the corresponding reactions with aromatic compounds is still very limited. The mechanisms of the reactions with toluene and benzene have been studied previously in this laboratory but only relative reaction rates and activation energies, determined in competitive experiments with cyclopentene, could be obtained a t that time.2 Few absolute rate measurements have been reported for benzenes-5 or for substituted benzene^.^ The present work deals with the determination of the absolute values of the rate constants for benzene and toluene employing a modulation technique recently developed and used in this laboratory in similar studies with olefins.6--9 While this work was in progress, an article by Atkinson and Pitts'O reported the rate constants for the O(3P) reactions with benzene and toluene a t room temperature, also obtained by the modulation technique. Experimental Section The experimental arrangement and the general procedure have been described in previous publications,6-8 and only the more important points will be mentioned here. The O(") atoms were generated by the mercury photosensitized decomposition of N20 using a sinusoidally modulated mercury lamp. The concentration of O(3P)was monitored photometrically by measuring the NO2 chemiluminescence from their reaction with the NO added to the reactant gas mixture, The phase shift between the incident 254-nm radiation and the NO2 chemiluminescence was measured with photomultipliers and a lock-in amplifier. The phase shift, 4, is related to the chemical rate processes byfi tan 4 = 2~~(k2[aromatic] + kn[NO][M])-' where u is the modulation frequency and k2 and k 3 are the rate constants for reactions 2 and 3 in the sequence
Hg("P1) + N 2 0
-+
Hg('So)
o(V)+ aromatic O(:'P) + NO
+M
-
-+
+ Nz + O("P)
(1)
products
(2)
+M
(3)
NOz*
The Journal of Physical Chemistry, Wol. 79, No. 18, 1975
NOz*
-+
NO2 + hu
NO** + M NO2 + M Flow rates of N20 and NO were determined by calibrated flow meters. The aromatics were introduced into the flow system by bubbling various fractions of the N2O flow through the aromatic at about 19O. The flow rates of the aromatics were determined in an initial series of experiments (series A) by condensing the flowing gas mixture for a known interval and measuring the aromatic manometrically after distilling off the more volatile N2O and NO. In a second series of experiments (series B), carried out after a number of further improvements in the phase shift apparatu^,^ the concentrations of the aromatics were determined by observing their uv absorption in a 10.0-cm cell upstream from the reaction cell with a Beckman DU-2 spectrophotometer. Benzene was monitored at 247.1 nm, and toluene at 260.3 nm. The absorbance calibrations were done by expanding mixtures, accurately made with gas burets, of the aromatic and NzO into the absorption cell. Since the apparatus used to prepare and transfer the calibration mixture was not completely grease-free, some of the aromatic could have been lost before reaching the absorption cell. To check this possibility, a grease-free gas handling system was incorporated around the absorption cell. Mixtures were made again with a gas buret. The two procedures gave the same calibration curve. The absorbance of both aromatics was independent of the N20 partial pressures used in the kinetic experiments. Flow rates of the aromatics were varied by a t least a factor of 4 (except for toluene a t 298OK where it was varied by a factor of 3). The flow rate of NO was varied by about a factor of 2, and the total pressure was varied between 40 and 70-80 Torr. In series A experiments, the rate of the 254-nm light absorption in the reaction cell was about 2 X 1014 quanta cm-3 sec-1, as determined by the amount of nitrogen formed during the mercury sensitized decomposition of N20. In series B experiments, the rate was about 5 x 10l2 quanta cm-3 sec-1, as determined by gas chromatographic analysis of the reaction products of O(3P) with butene-1. In series B, less than 0.01% of the aromatics was consumed by reaction with O(:'P). The temperature of the reaction cell or gas mixture was measured with thermocouples. A t 470°K the difference in the two temperatures was less than 4'. Gas chromatographic analysis showed less than 0.01% +
1901
Rates of O(3P) Atom Reactions with Benzene and Toluene 2
4
8
6
IO
12
16
14
18x10-3
TABLE I: Rate Constants, kz,of the Reactions of Ground State Oxygen Atoms with Benzene and Toluene
1
Temp, Aromatic ~~~~
Benzene Toluene
OK
10-'k 2, SeC-lU
AT1
10-''k3, M - * sec-"
~
449 473 498 373 423 473
Series A 1.29i0.05 1.83i0.14 2.23 10.08 1.54i0.08 2.68i0.11 4.73 i0.17
2.30f 0.16 2.87i0.38 2.66 *0.25 3.23 k0.21 3.43 *0.22 2.08i0.17
Series B 298 0.0930i0.0045 3.96+0.21 359 0.300*0.005 2.97k0.10 462 1.13 i0.03 2.56 i 0.06 Toluene 298 0.342i0.015 4.60*0.09 363 1.10i0.04 3.40*0,12 462 3.45k0.12 2.37 i 0.10 a The indicated uncertainties are the least-squares estimates of the standard deviations. Benzene
Flgure 1. Plots of 2au/[benzene] tan 4 vs. [NO][M]/[benzene] at three temperatures. (At 298'K 0.072 < [benzene] < 0.40, 0.013 < [NO] < 0.033, 1.5 < [N20] < 3.2, 700 < v < 1250 Hz; at 359'K 0.040 < [benzene] < 0.34, 0.020 < [NO] < 0.031, 1.8 < [NzO] < 3.1. 800 < u < 1300 Hz; at 462'K 0.041 < [benzene] < 0.191, 0.10 < [NO] < 0.25, 1.3 < [NzO] < 2.0, 2000 < u < 5000 Hz. Units of concentration are mM.)
L
I
I
0
IO
0.5
Figure 2. Plots of 2~u/[toluene] tan 4 vs. [NO][M]/[toluene] at three temperatures. (At 298'K 0.024 [toluene] 0.11, 0.013 [NO] 0.088, 2.0 [NzO] < 4.2, 800 < u 2000 Hz; at 363'K 0.013 < [toluene] < 0.089, 0.035 < [NO] 0.17, 1.7 < [N20] 3.5, 1250 < u < 1400 Hz; at 462'K 0.012 [toluene] < 0.071, 0.096 < [NO] 0.28, 1.3 [N20] < 2.5, 2000 < u 4000 Hz. Units of concentration are mM.)
> [o(3P)] compared to the conditions when [o(3P)] >> [c,&].Both of these observations were interpreted in terms of a slow initial attack of O(3P) on benzene followed by rapid subsequent reactions of O(3P) with free radicals formed in the first step. Such an explanation may account for the differences between the present results and those of Atkinson and Pitts. As mentioned briefly before, i t could also account for at least a part of the larger values obtained for kz for both benzene and toluene in the initial set of experiments (series A), carried out in this laboratory. The rate of the 254-nm light absorption in our series A experiments was about the same as in Atkinson and Pitts' experiments. The potential importance of a consecutive attack by oxygen atoms on the primary reaction products can be estimateds by assuming that two free radicals are formed in the primary reaction and that two free radicals are also formed in the reaction of oxygen atoms with any free radical. The only net loss of free radicals is assumed to be by
recombination. If the secondary oxygen atom reactions and the radical recombination reactions occur a t nearly every collision, then it can be calculateds that between 0.5 and 3% of the oxygen atoms may have been consumed in secondary reactions in the present study, and 3-9% in Atkinson and Pitts' study. Thus the effect of secondary reactions may not have been entirely insignificant, although even this reaction mechanism, which probably exaggerates the loss of oxygen atoms, is not sufficient to reconcile completely the difference between the two sets of results. In this analysis, it was assumed that the rate of 254-nm light absorption was uniform throughout the illuminated volume of the reaction cell. However, the rate of light absorption, and hence the oxygen atom production rate, falls off exponentially along the length of the cell. The actinometry, of course, gives only the average rate of light absorption. In the volume near the front window of the cell, therefore, the estimated effect of secondary reactions would be larger. For the two reactions studied, addition of the O(3P) atoms to the aromatic ring and an independent hydrogen abstraction have to be considered as the potential reaction channels. The rate constants measured in the present work are for the overall reaction, i.e. they include not only the addition of O(3P) atoms but also H atom abstraction if it occurs. However, the high value of the C-H bond dissociation energy in benzene would make direct H abstraction unlikely, in agreement with the apparent constancy of the activation energy over a wide temperature interval (255498'K). The benzilic hydrogens in toluene, on the other hand, are much weaker and a contribution of some abstraction to the total rate cannot be ruled out. In both cases, however, the expected "abstraction" products (dibenzyl and diphenyl) were not detected a t room temperature and an attack of the electrophilic O(3P) atoms on the aromatic ring appeared to be the main reaction channel.2
Acknowledgments. The authors are grateful to Dr. S. Furuyama for helpful discussions and to Drs. R. Atkinson and J. N. Pitts for a copy of their manuscript (ref 15) before publication. References a n d Notes (a) Issued as NRCC No. 14836. (b) National Research Council of Canada visiting research officer, sponsored by Consejo Nacional de Investigaciones Cientiflcas y Tbcnicas of Argentina. (c) National Research Council of Canada Postdoctorate Fellow (1972-1974). (a) G. Boocock and R. J. Cvetanovic, Can. J. Chem., 39, 2436 (1961); (b) G. R. H. Jones and R. J. Cvetanovic, Can. J. Chem., 39, 2444 (1961). R. A. Bonanno, P. Kim, J. H. Lee, and R. B. Timmons, J. Chem. Phys., 57, 1377 (1972). I. Mani and M. C. Sauer, Jr., Adv. Chem. Ser.. No. 82, 142 (1968). L. I. Avramenko, R. V. Koiesnikova, and G. I. Savinova, lzv. Akad. Nauk. SSSR,Ser. Khim., 28 (1965). R. Atkinson and R. J. Cvetanovic. J. Chem. Phys.. 55, 659 (1971). R. Atkinson and R. J. Cvetanovic, J. Chem. Phys., 58, 432 (1972). S. Furuyama, R. Atkinson, A. J. Colussi, and R . J. Cvetanovic, ht. J. Chem. Kinet., 8, 741 (1974). D. L. Singleton, S.Furuyama, R. J. Cvetanovic, and R . S.Irwin, J. Chem. Phys., In press. R. Atkinson and J. N. Pitts, Jr., J. Phvs. Chem.. 78. 1780 (1974). R. J. Cvetanovic, R. P. Overend, a i d G. Paraskevopoulos, lnt.'J. Chem Kinet., Supplement (Proceedings of the Symposium on Chemical Klnetlcs Data for the Lower and Upper Atmosphere). 1975. R. J. Cvetanovic, Prog. React. Kinet., 2, 39 (1964). E. J. Y. Scott and E. W. R. Steacie, Can. J. Chem., 29, 233 (1951). (14) R. J. Cvetanovic, Adv Photochern., 1, 115 (1963). (15) R. Atkinson and J. N. Pitts, Jr., J. Phys. Chem., 79, 295 (1975).
The Journal of Physical Chemistry, Vol. 79,NO. 18, 1975
