The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 1095
Communications to the Editor
(6) R. N. Jones, W. F. Forbes, and W. A. Mueller, Can. J , Chem., 35, 504 (1957). (7) P. F. Mountain and S.Walker, Adv. Mol. Relax. Processes, 7, 105 (1975). (8) S. P. Tay and S . Walker, J . Chem. Phys., 64, 1634 (1975). (9) C. K. McLellan and S. Walker, Can. J. Chem., 55, 583 (1977). (10) M. Davies and J. Swain, Trans. Faraday SOC.,67, 1637 (1971). (1 1) M. Davies and A. Edwards, Trans. Faraday Soc., 63, 2163 (1967). (12) B. Ostie, "Statistics in Research", 2nd ed,Iowa State University Press, Ames, Iowa, 1963.
(13) P. F. Mountain and S. Walker, Can. J . Chem., 52, 3229 (1974). (14) A. L. McClelian, "Tables of Experimental Dipole Moments", W. H. Freeman, San Franclsco, Calif., 1963. (15) L. E. Sutton, Chem. Soc., Spec. Pub/., No. 11 (1965); No. 18 (1958). (16) A. Lakshmi, S.Walker, N. A. Weir, J. H. Caiderwood, J. Chem. SOC., Faraday Trans. 2 , 74, 727 (1978). (17) C. K. McLelian, Ph.D. Thesis, University of Man., Canada, 1977. (18) A. Kwaja, private communication. (19) M. A. Mazid, private communication.
COMMUNICATIONS TO THE EDITOR Hydroxyl Radical Reactions in the Gas Phase. Products and Pathways for the Reaction of OH with Toluene Publication costs assisted by the Environmental Protection Agency
Sir: Aromatic hydrocarbons are important constituents of polluted urban atmospheres1 and the extent to which they contribute to the formation of photochemical smog is of considerable concern.26 Of particular interest are the reactions of hydroxyl radical with aromatics, since they are the major route for involvement of these hydrocarbons in the chemistry of the troposphere. The kinetics of gasphase OH-aromatic reactions have recently been determineda6-11The products and precise mechanisms of the reactions remain undetermined; however, to identify them is essential for determining the fate of aromatics in the environment and developing accurate chemical models of urban airsheds. For alkylbenzenes, we suggest two major reaction pathways. Shown for the case of toluene, these are benzylic hydrogen atom abstraction, reaction 1,and radical addition O H t C,H5CH3
-
H20 t C G H ~ C H ~
C,H,CH,*
I
to the aromatic ring, reaction 2. Davis et aL8 investigated the pressure dependence of the OH-toluene reaction at 298 K and concluded that k l / ( k l + k,) is less than 0.5. On the basis of the temperature dependence of the same reaction, Perry et al.ll deduced that at ambient temperature this ratio was 0.14 f 0.06. In neither case were the actual reaction products identified. Hydroxyl radicals were generated from hydrogen atoms formed in an argon discharge flow system by reaction 3. H
+ NO, -+OH t NO
and secondary reactions negligible. Products were collected by condensation in a cold trap or by adsorption onto a Chromosorb G-packed column. Gas chromatographymass spectroscopy and flame ionization gas chromatography were used to identify and quantitatively analyze the products of the reaction. The OH-toluene reaction was run 20 times under conditions of varying initial NOz, Oz, and total pressure. In all cases the only major gas phase products were benzaldehyde, benzyl alcohol, m-nitrotoluene, and 0-,m-, and p-cresol. Nitrotoluene and total cresols formed in rougly equal amounts, and predominated over benzaldehyde and benzyl alcohol. o-Cresol predominated over the para and meta isomers. Although the relative amounts of the individual products depended on the reaction conditions, the ratio (C6H5CH0+ C6HSCHzOH)/(total products) was invariant at 0.149 f 0.019. The total amount of products was approximately 5 X lo-'% of the toluene remaining, consistent with the low conversion expected for the system. Two minor products were p-methylbenzoquinone and an unidentified compound less volatile than toluene. Each of these amounted to approximately 0.75% of the total products. No phenol was observed (less than 0.5% of products); thus, displacement of the CH, group by OH is not an important reaction. To account for these observations, we propose reactions 1 , 2 , and 4-10. Reactions 4-8 are analogous to the known
(3)
Argon (5 Torr) was used as the carrier gas. The linear flow velocity was 1 X lo3 cm s-l. The method used is described in detail e1swhere.l2-lEApproximately 0.1 Torr of toluene and 1-10 Torr of Op, to trap the radicals formed in reactions 1and 2, were introduced 10 cm downstream from the point of addition of NO2, thus ensuring that reaction 3 would be complete. Since the amount of toluene initially added is large compared with the amount of OH available (1 X Torr),lgconversion of toluene to products is low 0022-3654/78/2082-1095$01.00/0
+ 0,
-+
C,H,CH,O;
C,H,CH,O,* t NO C,H,CH,O* t NO, C,H,CH,O* + 0, C,H,CHO + HO; C,H,CH,O. + NO C,H,CHO t HNO +
-
-+
C,H,CH,O. t NO, C,H,CHO I + 0, HOC,H,CH, t HO,. -+
+ HNO,
+
I
+ NO,+m-NO,C,H,CH,t
H,O
(4) (5 1
(6) (7 1 (8) (9)
(10)
reactions of methyl,20methylperoxy,21~22 and methoxyZ3J4 radicals. Reaction 9 closely resembles the reaction of cyclohexadienyl radicals with 02.25 Reaction lop6as well as similar addition-elimination reaction^^^^^^ have been observed in solution, although rate constants have not been measured. Given the above mechanism, the distribution of the individual products should depend on the reaction conditions (i.e,, the relative amounts of NOz and 0,) but the amount of abstraction relative to addition product should not." Thus, our ratio (C6H6CH04- C6H5CHzOH)/(total products) is a valid measure of the amount of reaction 1 relative to total reaction, i.e., kl/(kl + k z ) . 0 1978 American Chemical Society
1096
The Journal of Pbysical Chemistry,
Vo/.82, No. 9, 1978
From the dependence of the yields of cresol from reaction 9 and nitrotoluene from reaction 10 on the ratio of N02/O2, we have estimated k I o / k 9to be (4.4 f 0.5) X lo3. Thus, in the atmosphere, reaction 2 will lead essentially only to cresols because the ambient ratio of [O,]/[NO,] is usually well above lo5. Reaction 1 would be followed exclusively by reaction 4 and by reactions 5 and 6 producing benzaldehyde, NO2, and HOz.. Neither benzaldehyde nor cresol has been observed to accumulate in the troposphere, suggesting that both of these initial products are subject to further reaction.
Acknowledgment. This research has been supported by Grant No. R-803846 of the US. Environmental Protection Agency .
References and Notes W. A. Lonneman, S. L. Kopczinski, P. E. Darlev, . and F. D. Sutterfield, Environ. Sci. Techno/., 8, 229 (1974). J. M. Heuss and W. A. Glasson, Environ. Sci. Tecbnol., 2, 1109 (1968). A. P. Altshuller, S. L. Kopczinski, D. Wilson, W. Lonneman, and F. D. Sutterfield, J . Air Po//ut. Control Assoc., 19, 291 (1969). A. P. Altschuller, S. L. Kopczlnski, W. A. Lonneman, F. D. Sutterfield, and D. L. Wilson, Environ. Sci. Techno/., 4 , 44 (1970). J. M. Heuss, G. J. Nebel, and B. A. D'Alleva, Envlron. Sci. Tecbnol., 8,641 (1974). F. D. Morris and H. Niki, J . Pbys. Cbem., 75,3641 (1971). D. A. Hansen, R. Atkinson, and J. N. Pitts, J . Pbys. Cbem., 79, 1763 (1975). D. D. Davis, W. Bollinger, and S. Fischer, J. Pbys. Cbem., 79, 293 (1975). G. J. Doyle, A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Environ. Sci. Techno/., 9, 237 (1975). A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, J. Pbys. Cbem., 80, 789 (1976).
Communications to the Editor (11) R. A. Perry, R. Atkinson, and J. N. Pitts, J. Pbys. Chem., 81, 296 (1977). (12) F. Kaufman and F. P. Del Greco, J. Cbem. Pbys., 35, 1895 (1961). (13) F. P. Del Greco and F. Kaufman, Discuss. Faraday Soc., 33, 128 (1962). (14) L. F. Phillips and H. I. Schiff, J. Cbem. Pbys., 37, 1233 (1962). (15) J. E. Breen and G. P. Glass, Int. J. Cbem. Kinet., 3, 145 (1970). (16)J. E. Breen and G. P. Glass, J. Cbem. Pbys., 52, 1082 (1972). (17) A. Pastrana and R. W. Carr, Int. J. Chem. Kinet., 6, 587 (1974). (18) W. E. Wilson, J. Phys. Chem. Ref. Data, 1, 535 (1972). (19) Estimate obtained from a detailed chemical model. (20) D. A. Parkes, Int. J . Cbem. Kinet., 9, 451 (1977). (21) R. Simonaitis and J. Heicklen, J. Pbys. Cbem., 78,2417 (1974). (22) C. T. Pate, B. J. Finlayson, and J. N. Pitts, J. Am. Cbem. Soc., 98, 6554 (1974). (23) J. R. Barker, S. W. Benson, and D. M. Golden, Int. J . Cbem. Kinet., 9, 31 (1977). (24) L. Batt, R. T. Milne, and R. D. McCulloch, Int. J. Cbem. Kinet., 9, 567 (1977). (25) D. G. Hendry and D. Schuetzle, J. Am. Cbem. Soc., 97, 7123 (1975). (26) E. Halfpenny and P. L. Robinson, J. Cbem. SOC., 939 (1952). (27) T. Suehiro, M. Hirai, and T. Kaneko, Bull. Cbem. SOC.Jpn., 44, 1407 (1971). (28) M. E. Kurz, R. L Fozdar, and S. S. Schukz, J. Org. Cbem., 39,3336 (1974). (29) Our experiments have been carried out over the pressure range 6-15 Torr Ar-0, and show no evidence of any pressure dependence. Davis et al. (ref 8) using He as the bath gas observed in this pressure range a bimolecular rate constant ( k , -t k2)equal to 0.8 of the high pressure value. Thus, the observed value of k l l ( k l k 2 ) would be 20% greater than the hlgh pressure limit value if He was used. Since our results were obtained in the presence of Ar-OP mixtures rather than He, k2 is expected to be very near the high pressure limit under the conditions of our expriments.
+
SRI International Menlo Park, California 94025 Received November 17, 1977
Richard A. Kenley John E. Davenport Dale G. Hendry'
