J . Phys. Chem. 1988, 92. 4008-4008
4008
Comment on “Flash Photolysis Resonance Fluorescence Investigation of the Gas-Phase Reactions of OH Radicals with a Series of Aliphatic Ketones over the Temperature Range 240-440 K”’ Sir: The recent flash photolysis-resonance florescence study of Wallington and Kurylo’ dealing with the kinetics of the gas-phase reactions of the OH radical with ketones has greatly expanded the literature data base concerning this class of organic compounds. For 2- and 3-pentanone and 2-hexanone the absolute room temperature rate constants determined by Wallington and Kurylo’ can be compared to rate constants obtained by using a relative rate t e c h n i q ~ e , ~and , ~ significant discrepancies, outside of the combined stated experimental error limits, are evident.’ The relative rate constant measurements2 were carried out in a 60-L Teflon chamber, and problems ascribed to first-order wall losses of the ketones during the irradiations were observed.’ To attempt to resolve these discrepancies between these relative rate and absolute rate constant measurements, we have redetermined the rate constants for the reactions of the OH radical with 2- and 3-pentanone and 2-hexanone using a relative rate method. To avoid wall effects, experiments were carried out in a 6400-L all-Teflon chamber, equipped with black light irradiation. The experimental technique was otherwise as described previously,’ and cyclohexane was again used as the reference organic. The ketones and cyclohexane were monitored by gas chromatography (GC), and all experiments were carried out at 296 f 2 K and 740-Torr total pressure of dry pure air. The experimental data obtained from CH30NO-NO-air irradiations of 2-pentanone + 2-hexanone + cyclohexane (two irradiations), 3-pentanone cyclohexane (two irradiations), and 3-pentanone 2-hexanone + cyclohexane (one irradiation) [these mixtures being chosen to avoid interferences on the G C column used] are plotted in Figure 1 in accordance with the equation
+
+
[ketone],, [ketone], where [ketone],, and [cyclohexane],, are the concentrations of ketone and cyclohexane, respectively, at time to, [ketone], and [cyclohexane], are the corresponding concentrations at time t , and k , and kz are the rate constants for the reactions of the OH radical with the ketone and cyclohexane, respectively. No indications of wall effect problems were evident, and least-squares analysis of the data yields the rate constant ratios k , / k z = 0.681 f 0.034, 0.281 f 0.019, and 1.22 f 0.06 for 2-pentanone, 3-pentanone, and 2-hexanone, respectively, at 296 f 2 K (where the error limits are two least-squares standard deviations). These rate constant ratios are in reasonable agreement with those previously determined at 299 f 2 K of 0.626 f 0.018,0.245 & 0.044, and 1.21 f 0.08, respectively,2 showing that the wall effects previously observed in the smaller chamber used’ were reasonably well taken into account. Using a rate constant for the reaction of the OH radical with cyclohexane of kz = 7.3 1 X lo-’’ (1) Wallington, T. J.; Kurylo, M. J. J . Phys. Chem. 1987, 91, 5050. (2) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Pitts, J. N., Jr. Int. J . Chem. Kinet. 1982, 14, 839. (3) Atkinson, R. Chem. Rev. 1986, 86, 69.
0022-3654/88/2092-4008$01.50/0
-
-
-z
06
2 - HEXANONE
r/
-
+
W
0
+ W
I
I
02
0 in([
I 04
I 06
1
CYCLOHEXANE],^/ [CYCLOHEXANE],)
Figure 1. Plots of eq I for the reactions of the OH radical with 2- and 3-pentanone and 2-hexanone, with cyclohexane as the reference organic.
cm3 molecule-’ s-I at 296 K3 leads to rate constants kl of (in units of cm3 molecule-’ s-I) 4.98 f 0.25, 2.05 f 0.14, and 8.92 f 0.44 for 2-pentanone, 3-pentanone, and 2-hexanone, respectively. These rate constants can be compared to those determined by Wallington and Kurylo,’ also at 296 K, of (in units of lo-’’ cm3 molecule-’ s-I) 4.00 f 0.29, 2.74 f 0.13, and 6.64 f 0.56, respectively, showing discrepancies of 20-35% which are not systematically high or low. Furthermore, rate constant ratios of k(2-hexanone)/k(2-pentanone) and k(2-hexanone)/k(3-pentanone) can be obtained directly from the CH30NO-NO-cyclohexane-air irradiations involving 2-hexanone + 2-pentanone and 2-hexanone + 3-pentanone, without the intermediacy of cyclohexane as the reference organic. Thus, while the rate constant ratio of k(2-hexanone)l k(2-pentanone) = 1.80 f 0.04 obtained from least-squares analysis of the present 2-pentanone and 2-hexanone data from CH30N0-N0-2-pentanone-2-hexanone-cyc10hexane-air irradiations is in good agreement with that of 1.66 f 0.19 from the absolute rate constant data of Wallington and Kurylo,‘ the rate constant ratio of k(2-hexanone)/k(3-pentanone) = 4.22 f 0.18 obtained from the CH30NO-NO-3-pentanone-2-hexanonecyclohexane-air irradiations is significantly higher than that of 2.42 f 0.24 derived from the absolute rate data.’ Our directly determined rate constant ratios of k(2-hexanone)/k(2-pentanone) = 1.80 f 0.04 and k(2-hexanone)/k(3pentanone) = 4.22 f 0.18 are in excellent agreement with the ratios of 1.79 f 0.13 and 4.34 f 0.37, respectively, derived from the individual rate constant ratios k,/k2, showing that our entire data set is self-consistent. Clearly, there are significant discrepancies between the absolute’ and relative rate constants for these three ketones which are not dependent on the choice of the rate constant for the reference organic used in the relative rate measurements, and these must be ascribed to systematic errors in at least one of these kinetic studies. Statewide Air Pollution Research Center Roger Atkinson* Sara M. Aschmann University of California Riverside, California 92521
0 1988 American Chemical Society
