Environ. Sci. Technol. 2000, 34, 2875
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Comment on “Nighttime Tropospheric Chemistry: Kinetics and Product Studies in the Reaction of 4-Alkyl- and 4-Alkoxytoluenes with NO3 in Gas Phase” SIR: The nitrate radical (NO3) plays an important role in atmospheric chemistry. Kinetic and mechanistic data for reactions of NO3 with aromatic compounds are required to model smog formation. Bolzacchini et al. (1) have reported product yields for reactions of NO3 radicals with a series of 4-alkyl- and 4-alkoxytoluenes. In product studies, the observed yields of the primary products must be corrected for loss via reaction with NO3 radicals. No such corrections were made by Bolzacchini et al. (1). After being corrected for secondary chemistry, the product yields reported by Bolzacchini et al. (1) are not physically reasonable; we believe this deserves comment:
NO3 + aromatic f primary products
(1)
NO3 + primary products f secondary products (2) The appropriate rate equations can be solved analytically (2), and a correction factor to account for loss via reaction 2 can be computed. The expression is
[(
correction factor )
1
)
k2 1k1
x
((1 - x)[(1 - x){k2/k1 - 1} - 1])
]
where x is the fractional consumption of the aromatic compound, and k1 and k2 are the bimolecular rate constants of reactions 1 and 2, respectively. Bolzacchini et al. (1) identified several benzaldehyde derivatives as products; these compounds are very reactive toward NO3 radicals. Reaction of NO3 radicals with 4-ethyltoluene was reported to give the following products (and molar yields): 4-ethylbenzaldehyde (0.16 ( 0.02), 4-methylbenzaldehyde (0.13 ( 0.02), 4-methylstyrene (0.04 ( 0.01), 4-methyl acetophenone (0.17 ( 0.02), alcohols (0.05 ( 0.02),
10.1021/es000865c CCC: $19.00 Published on Web 05/27/2000
2000 American Chemical Society
and nitro- and nitrate derivatives (0.14 ( 0.03). Product yields were not corrected for secondary loss via reaction with NO3 radicals. Reaction of NO3 radicals with benzaldehyde proceeds via abstraction of the aldehydic H-atom (3); methyl- and ethylbenzaldehyde will be as reactive as benzaldehyde toward NO3 radicals. Reaction of NO3 radicals with styrene proceeds via electrophilic addition to the >CdCH2 double bond. Methylstyrene will be at least as reactive as styrene. Using k(NO3 + 4-ethyltoluene) ) 7.7 × 10-16 1 and k(NO3 + benzaldehyde) ) 2.6 × 10-15 cm3 molecule-1 s-1 3 [based upon k(NO3 + propene) ) 9.49 × 10-15 4], we calculate that, under the conditions employed by Bolzacchini et al. (1) with 67% conversion of 4-ethyltoluene, the yields of 4-methyland 4-ethylbenzaldehyde need to be revised upward by factors of 5.2. This correction leads to a carbon balance of 191%, which is not reasonable. NO3 radicals are expected to react at least 2000 (1.5 × 10-12 4,5/7.7 × 10-16 1) times more rapidly with 4-methylstyrene than with 4-ethyltoluene. The observation of 4 ( 1% yield of 4-methylstyrene by Bolzacchini et al. (1) is not reasonable. Similarly, the reported 4-tertbutylbenzaldehyde yield of 32 ( 1% from the NO3 reaction with 4-tert-butyltoluene needs correction upward by a factor of 4.2, which together with the other reported products gives a carbon balance of 192%, which is not reasonable.
Literature Cited (1) Bolzacchini, E.; Meinardi, S.; Orlandi, M.; Rindone, B.; Hjorth, J.; Restelli, G. Environ. Sci. Technol. 1999, 33, 461. (2) Meagher, R. J.; McIntosh, M. E.; Hurley, M. D.; Wallington, T. J. Int. J. Chem. Kinet. 1997, 29, 619. (3) Atkinson, R.; Carter, W. P. L.; Plum, C. N.; Winer, A. M.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1984, 16, 887. (4) Atkinson, R. J. Phys. Chem. Ref. Data 1997, 26, 215. (5) Atkinson, R.; Aschmann, S. M. Int. J. Chem. Kinet. 1988, 20, 513.
T. J. Wallington* Ford Motor Company SRL-E3083 Dearborn, Michigan 48121-2053
L. Frøsig and O. J. Nielsen Department of Chemistry University of Copenhagen, KL5 Universitetsparken 5 2100 København Ø, Denmark ES000865C
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