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Response to 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 correction of observed yields of some products of the reaction of the NO3 radical with 4-alkyl or 4-alkoxytoluenes performed by Wallington et al. (1) is based on the assumption that these compounds are formed as primary products. Quantitative Analysis of Starting Materials and Reaction Products. Calibration of the analytical system for starting materials and carbonyl compounds (trapping, elution, evaporation, gaschromatographic separation) was necessary because the recovery from the trap was measured in a previous paper (2) to be 9-21% from charcoal traps and 78-94% from XAD-2 (polymer styrene-divinylbenzene) traps, depending from the nature of the compound to be trapped. The calibration gave the following linear plots:
y ) 1.561x + 0.383 y ) 0.893x - 2.568
for 4-isopropyltoluene 1
for 4-isopropylbenzaldehyde 13
The results shown in Table 2 of our paper result from this procedure. Are Artifacts Formed during Sampling, Elution, and Gaschromatographic-Mass Spectrometric Analysis? Scheme 1 shows the reactivity of 4-ethyltoluene 1 with NO3. The secondary benzyl radical 2 will form 4-methylacetophenone 4 and 4-ethylbenzyl alcohol 5 by reaction with dioxygen or 1-(4-methylphenyl)-ethan-l-nitrate 6 by reaction with NO2 after the formation of a benzyloxy radical intermediate. The primary benzyl radical 3 will form 4-ethylbenzaldehyde 7 and 4-ethylbenzyl alcohol 8. It was not possible to locate 4-ethylbenzyl nitrate 11 in the complex reaction mixture. It is well-known that secondary and tertiary benzylic nitrates are thermally unstable (3), so it is likely that 4-methylstyrene 9 will be formed by elimination of nitric acid from the nitrate 6 (as mentioned in our paper) during the analytical procedure,
SCHEME 2
where the compounds are exposed to high temperatures. It cannot be excluded that 9 is formed in the gas phase as well, where it may be the precursor of 4-methylbenzaldehyde 10, but 10 may also be formed by elimination of CH3 from a secondary benzyloxy radical. An analogous situation occurs with 4-isopropyltoluene 12 (Scheme 2). Alcohols and carbonyls formed in this reaction derive from the intermediate benzyl radical 14 formed in all the mechanistic alternatives (Scheme 3). This will be transformed into a peroxy radical 15, which will disproportionate to carbonyls 16 and alcohols 17. Alternatively, peroxy radicals 15 may react with NO3 to form alkoxy radicals 18 and then carbonyl compounds 16. Peroxy radicals 15 do also react with NO2 to form peroxynitrates 19. These may be an additional source of carbonyls by loss of nitric acid. In the present study, the NO3 radical was produced by the thermal dissociation of N2O5 under conditions where the initial concentration of NO2 was a few ppmV. At the end of the reaction, the concentration of NO2 was 20 ppmV or more. Under these conditions, the reaction between peroxy radicals 15 and NO2 apparently becomes a main reaction of peroxy radicals 15, and the equilibrium between peroxy radicals 15 and peroxynitrates 19 appears to be shifted so much versus the peroxynitrate that it must be considered as being a stable intermediate. Taking as an example the simple case of the ethylperoxy radical and the corresponding peroxynitrate at 5 ppmV of NO2 and at 298 K, the ratio [peroxynitrate]/ [peroxyradical] would be 5 × 10-3 (4). The rate constant of the reaction C2H5OO + NO3 is 2.50 × 10-12 cm3 molecule-1 s-1 (5). Hence, for a NO3 concentration of 1 ppbV (which appears to be relevant to these experiments), the liftetime of the peroxynitrate should be ∼54 min, assuming that other reactions of C2H5OO can be neglected. Thus, peroxynitrates
SCHEME 1
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 13, 2000
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2000 American Chemical Society Published on Web 05/27/2000
SCHEME 3
of the experiment, and a simple correction for their reaction with NO3 radicals is not possible. Furthermore, as mentioned in our paper, there are good reasons to assume that the styrenes are in part artifacts deriving from benzyl nitrates during sampling and analysis.
Literature Cited (1) Wallington, T. J.; Frøsig, L.; Nielsen, O. J. Environ. Sci. Technol. 2000, 34, 2875-2875. (2) Chiodini, G.; Rindone, B.; Polesello, S.; Cariati, F.; Hjorth, J.; Restelli, G. Environ. Sci. Technol. 1993, 27, 1659-1664. (3) Ferris, A.; McLean, K. W.; Marks, I. G.; Emmons, W. D. J. Am. Chem. Soc. 1953, 75, 4078-4088. (4) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref Data 1997, 26, 521-1011. (5) Biggs, P.; Canosa-Mas, C. E.; Fracheboud, J.-M.; Shallcross, D. E.; Wayne, R. P. J. Chem. Soc. Faraday Trans. 1995, 91, 817825. (6) Roberts, J. M. Atmos. Environm. 1990, 24A, 243-287.
Ezio Bolzacchini, Simone Meinardi, Orlandi, and Bruno Rindone*
Marco
Department of Environmental Sciences University of Milano Via Emanueli 15 I-20126 Milano, Italy have to be considered in the kinetic analysis. In fact, the characteristic absorption bands of ROONO2 (6) are observed in the IR spectra at ∼790 cm-1 (O-N stretch) and ∼1300 cm-1 (-NO2 stretch, symmetric). Another band at 1720 cm-1 (-NO2 stretch, asymmetric) is sometimes observed. Hence, neither the aldehydes nor the styrenes can be considered as being primary products in a kinetic analysis
Jens Hjorth and Giambattista Restelli Commission of the European Communities Joint Research Center Environment Institute 1-21020 Ispra (VA), Italy ES002002P
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