HE
J O U R N A L O F
PHYSICAL CHEMISTRY Registered in US.Patent Office
0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 26
DECEMBER 28, 1978
Rate Constants for the Reactions of OH Radicals and Ozone with Cresols at 300 f 1 K Roger Atkinson," Karen R. Darnall, and James N. Pitts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1 (Received June 13, 1978) Publication costs assisted by the University of California, Riverside
Rate constants for the reaction of OH radicals with 0 - , m- and p-cresol, which are products of the NO, photooxidation of toluene, have been obtained from their rates of disappearance, relative to those for n-butane and neopentane, in irradiated NO,-organic-air mixtures at atmospheric pressure and 300 f 1 K. The rate constants for m- and p-cresol, relative to that for o-cresol, are as follows: m-cresol, 1.42 f 0.08; p-cresol, 1.10 f 0.05. The rate constant determined for o-cresol is h3(o-cresol)= (4.7 f 0.4) x lo-'' cm3 molecule-'^-^, leading cm3 molecule-' cm3 molecule-l s-l and h3(p-cresol)= (5.2 f 0.5) X to h3(m-cresol) = (6.7 f 0.7) X SKI. Reaction of these cresols with O3 was observed to be slow, with rate constants for all three isomeric cresols cm3 molecule-I s-l. of -6 X
Introduction The gas phase reactions of the hydroxyl radical with aromatic compounds are of fundamental interest as well as being of importance in chemical kinetic modeling of photochemical air po1lution.l Cresols are formed from the reaction of OH radicals with toluene under simulated atmospheric c~nditions,l-~ and react further to yield such products as the hydroxynitrot~luenes.~ Since under atmospheric conditions the predominant reaction of the aromatics is with the OH radical,6 a knowledge of the OH radical rate constants for the isomeric cresols is necessary. T o date the only literature data available for the cresols are rate constants for the reaction of OH radicals with o-cresol over the temperature range 299-423 K, obtained using a flash photolysis-resonance fluorescence technique.' Comparison of the room temperature rate constant for the reaction of OH radicals with o-cresol ( h = 3.41 f 0.68) x cm3 molecule-1 s-ll7 with that for toluene ( h N 6.1 X lo-'' cm3 molecule-' s - ' ) ' , ~ ~ 'shows that o-cresol is much more reactive than toluene toward OH radicals and hence secondary reactions of the cresols will be important in the NO, photooxidation of toluene. In this work rate constants for the reaction of OH radicals with 0 - , m-, and p-cresols have been determined a t 300 f 1 K from the rates of disappearance of the cresols in irradiated NO,-organic-air mixtures. 0022-3654/78/2082-2759$0 1.OO/O
Experimental Section The experimental techniques used to determine OH radical rate constants have been described previously."J2 Irradiations were carried out in a -5500-L Teflon bag placed inside a 6400-L all-glass environmental chamber, which was maintained at 300 f 1 K during the irradiations. The cresols and neopentane and n-butane were monitored as a function of irradiation time by gas chromatography (FID) using a 6 f t X 1/4 in. 0.1% SPlOOO/Carbopack C column, temperature programmed from 363 to 473 K a t 20 K per min for the cresols, and a 36 ft X l / * in. stainless steel column of 10% 2,4-dimethylsulfolane on C-22 Firebrick (60-80 mesh) followed by a 18 in. X 1/8 in. stainless steel column of 10% Carbowax 600 on C-22 Firebrick (60-80 mesh) operated a t 273 K for the alkanes. The initial reactant concentrations were neopentane and nbutane, -0.045 ppm; o-cresol, -0.25 ppm; rn-cresol or p-cresol, -0.25 ppm; NO, -0.35 ppm; and NO2, -0.10 ppm. Four 5-h irradiations were carried out, with analyses of the cresols every hour and of the two alkanes every 0.5 h. Because of gas chromatographic interferences between m-cresol and p-cresol, two irradiations were carried out with 0 - and p-cresol, and two with 0- and m-cresol. In nonirradiated experiments no losses of the cresols were observed within the analytical errors of *5% over a time period of 18 h. In addition two nonirradiated experi-
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1978 American Chemical Society
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The Journal of Physical Chemistry, Vol. 82, No. 26,
ments (initial concentrations of the cresols being o-cresol, -0.25 ppm; m- or p-cresol, -0.25 ppm) were carried out in the presence of 0.5-0.8 ppm of O3 to determine the magnitude of the rate constants for the reactions of the cresols with 03.
Results and Discussion The concentrations of 0- and p-cresol, or o- and m-cresol, and of neopentane and n-butane were monitored in irradiated NO,-cresol-alkane-air mixtures at atmospheric pressure and 300 f 1 K. With the present experimental technique dilution due to sampling has been shown to be negligible," and the observed disappearance of the alkanes during the irradiations is solely due to reaction with the OH radical" -d In [alkane]/dt = k[OHl (1)
Darnall, and
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,
(hz - k , ) ~ o t [ O Hdt l (111)
and In ([o-cresol],,/[o-cresol],) = h30Jt[OH] dt (IV) t0
J. N. Pitts
r
004
008
006
[n-BUTANElIo[NEOPENTANE],
[ n - B U T A N E I 1 [NEOPENTANE] 10
where h is the rate constant for the reaction HzO t products OH + alkane In the present irradiation it was observed Chat plots of In [alkane] against time (or of In ([n-butane]/[neopentane]) against time, which eliminates errors due to small differences in sampling volume) were curved, with the OH radical concentrations increasing monotonically during the irradiations. The OH radical concentrations obtained from -d In ([n-butanel/[neopentane])/dt = (ha- k,)[OH] (11) where hl and hz are the rate constants for the reaction of the OH radical with neopentane and n-butane, respectively, during the irradiations were in the range (1-4) X lo6 molecule ~ m - ~ , From the observed disappearance rates (-20% loss over 5 h) of the cresols in the presence of 0.5-0.8 ppm of 03, observed rate constants for the reaction of O3 with all three isomeric cresols of heff 6 X cm3 molecule-l s-' (to within approximately a factor of 2 ) were obtained. The values of kegare of course not necessarily the rate constants for the initial reaction of O3 with the cresols, because of the probable occurrence of reactions of species formed in the initial reaction with the cresols. However, these values are of the same order of magnitude as h 2 X cm3 molecule-l s-' obtained from O3 decays in the presence of excess m-cres01.l~ At the O3 (50.12 ppm) and the OH radical (- (1-4) X l o 6 molecule ~ m - concentrations ~) observed in the irradiated NO,-cresol-alkane-air mixtures, the reaction of O3 with the cresols can be calculated to contribute
