J. Phys. Chem. 1995,99, 8669-8672
8669
CH3CO Reactions with Cl2 and 0 2 : More Evidence for HCl Elimination from the CH3CHClO Radical E. W. Kaiser" and T. J. Wallington Ford Motor Company, Research Laboratory, Mail Drop 3083/SRL, Dearbom, Michigan 48121 -2053 Received: January 6,1995; In Final Form: March 16, 1995@
The rate constant for the reaction of CH3CO with Cl2 (kl) has been measured relative to that with 0 2 (k2) using UV irradiation of mixtures of CH3CH0, C12, 0 2 , and N2 to generate C1 atoms, which react with CH3CHO to form CH3CO. Consumption of CH3CHO and formation of CH3COC1 were monitored by FTIR spectroscopy and used to determine kJk2 = 7.9 f 0.5 at 700 Torr and 296 K. The C1-initiated oxidation of mixtures of C2H5C1, C12, 0 2 , and N2 results in a monotonic increase in CH3COC1 yield as the [C12]/[02] ratio increases consistent with the presence of the CH3CO radical. A lower limit to kJk2 deduced from these experiments is 40% smaller than that measured above. These results support CH3CO as an intermediate in the oxidation process formed by molecular elimination of HC1 from the CH3CHC10 radical (CH3CHC10 C H F O HC1). During these experiments the rate constant for reaction of CH3CHC1 with Cl2 (k3) relative to that with 0 2 (k4) was also measured (k3/k4 = 0.42 f 0.02).
-
+
Introduction In two recent papers, we have presented evidence obtained both by FTIR spectroscopy' and by time-resolved, diode-laser IR and UV spectroscopy2 that the chlorine-atom-initiated oxidation of C2H5Cl unexpectedly proceeds via molecular elimination of HCl from the alkoxy radical CH3CHC10 (e.g., CH3CHC10 CH3CO HCl). Such a mechanism requires that the CH3CO radical be formed. While several pieces of evidence support this fact (e.g., the presence of low yields of CH3C(O)OOH in the FTIR experiments and CH3C(O)OO in the UV measurements), the acetyl radical is of such importance to the proposed mechanism that its role will be examined further in this paper. The C1-initiated oxidation of CH3Cl proceeds by a similar molecular elimination (CH2C10 HCO HC1) at low 0 2 d e n ~ i t y . ~For this latter reaction, the presence of the HCO radical was confirmed by observing that the dependence of the HCOCl yield (formed from HCO Cl2) on the [C12]/[02] ratio was identical to that predicted from literature values of the rate constants for the reactions HCO Cl2 and HCO 0 2 . In the previous FTIR experiments on C2H5C1 oxidation, the 0 2 density was too large to permit detection of CH$2OC1 formation from reaction 1
-
+
-
+
+ +
CH3C0
+
+ C12- CH3COC1+ C1
(1)
because reaction 2 was the dominant sink for CH3CO radicals. CH3C0
+ 0, - CH3C(0)O0
(2)
In the current experiments, the variation of the CHFOC1 yield is determined as a function of the [C12]/[02] ratio at a sufficiently low 0 2 density that reactions 1 and 2 have comparable rates. The rate constant ratio, k1lk2,deduced from these experiments, is then compared to that obtained from a similar study of the competitive chlorination of CH3CO in which CH3CO is formed by the well established reaction of C1 with CH3CHO. These CH3CHO experiments provide the first measurement of the ratio @
The ratio k3/k4 was also measured as a part of these experiments to provide necessary corrections to the experimental data.
kllk2.
Abstract published in Advance ACS Abstracts, May 1, 1995.
0022-365419512099-8669$09.00/0
+ C12- CH3CHC12+ C1 CH3CHC1+ 0, - CH3CHC10,
CH3CHC1
(3) (4)
Experimental Section The experimental apparatus has been described in detail previously.' It consists of an evacuable 140 L Pyrex chamber interfaced to an FTIR spectrometer with a total path length of 27 m. The reactor is surrounded by 16 fluorescent UV (BLB) bulbs which photolyze molecular chlorine to begin the reaction. In these experiments, the reactor is evacuated and then filled with the desired partial pressures of Cl2 (Matheson, 99.99%), 0 2 (Scott, UHP), CH3CH2C1 (Matheson, 99.7%, degassed) (or CHFHO), and diluted to 700 Torr with N2 at ambient temperature (296 K). A spectrum is taken prior to irradiation, and the mixture is then irradiated in two or three stages to produce the desired final consumption of the organic reactant. At the end of each irradiation period, another spectrum is taken to assess the reactant loss and the amount of CH3COC1 and CH3CHCl2 formed. Calibration of CH3COCl (Aldrich, 99%) and CH3CHC12 (Eastern Chemical) absorbances is obtained by expanding a known partial pressure of the pure compound into the reactor and diluting it to 700 Torr with air. Unless stated otherwise, error limits represent 2a statistical uncertainty only.
Results and Discussion
CHJCHOExperiments. Irradiation of a Cl2,02, CH3CH0, N2 mixture results in the formation of C1 atoms. The C1 atom
abstracts the aldehydic hydrogen in greater than 99% yield: providing a ckan source of CH3CO radicals. The rate constant ratio kllk2 can be determined from the loss of CH3CHO (A[CH3CHO]) and the amount of CH3COCl formed using equation A. Experiments were performed in which the 0 2 partial [CH3COC1] k, [C121 k2 [O,] A[CH,CHO] - [CH,COCl] 0 1995 American Chemical Society
(A)
Kaiser and Wallington
8670 J. Phys. Chem., Vol. 99, No. 21, 1995 TABLE 1: Selected Data for C1-InitiatedAcetaldehyde Oxidation at 700 Torr of N2 [CH$HO]o (mTorr)
[02]0 (Torr)
[Clzlo (mTorr)
A[CH3CHO] (mTorr)
[CH3COCl] (mTorr)
75.8
3.62
445
11.4 22.0
5.33 10.8
71.4
10.6
156
6.2 14.1
0.62 1.5
Lad
(s)a 10 20 30 60
Nominal UV irradiation time. The UV lamps were not equipped with preignition coils and the exact irradiation times may vary.
0
Y
I
0
I
0.6
Om
I 0
U
a
0.4
u
pressure was kept constant at 4 Torr, and the Cl2 was varied from 117 to 980 mTorr. A second series of experiments was performed with constant Ch (170 mTorr) and variable 0 2 (1.6510.6 Torr). The percentage consumption of CH3CHO was varied from 7% to 50% during the course of the seven experiments. For each experiment, two data points were taken with the percentage consumption differing by approximately a factor of 2-3. The values of kllk2 calculated using eq A were independent of mixture composition and percentage of CH3CHO consumption. The two data points from each of two representative runs are presented in Table 1. These data were obtained at extremes of the compositions tested. In Figure 1, the right side of eq A is plotted vs [C12]/[02] for the seven experiments. Each data point represents the average of the two irradiation times included for each mixture. The slope of the plot is equal to k,/k2 (=7.9 & 0.5). The quoted error is 2 standard deviations from the mean of the fit to the data. Note that the values of [C12]/[02] in Figure 1 have been corrected where necessary for consumption of Cl2 during the reaction. This correction was always less than 5%. CH&!H&!l Experiments. The C1-initiated oxidation of C2H5C1 as it pertains to these experiments involves reactions 1-4 plus the following:'
-
+ HCl C1 + CH3CH2Cl CH,CH2C1 + HC1 CH,CHC10, + RO, CH3CHC10 + RO + 0, CH3COCl + ROH + 0, C1+ CH3CH2Cl
-
CH,CHClO CH,C(O)OO CH,C02
CH,CHClO,R
+ 0,
CH3CHC10H iR-,O
-
+ RO,
-
CH3CHC1
CO,
CH3C0 i- HCl CH3C02
+ RO + O2
+ methyl products
(5)
(6) (7a) (7b) (7c) (74
(8) (9) (10)
RO2 refers to all possible peroxy radicals (e.g., CH3CHC102, CH2ClCH202, CH3O2, HOz, CH3C(O)OO). On the basis of data and citations in ref 1, reaction 5 accounts for 82% of the C&C1 consumption and reaction 6 18%. The CHzCH2Cl radical does not fonii CH3COCl' and will not be discussed further. Reaction 7a is the predominant path for the reaction of CH3CHClO, with peroxy radicals, as deduced from the FTIR product data.] However, the yield of CH3COC1 for small [C121/[0~1 ratios was constant at %16% in our previous experiments.' This formation of CH3COCl is independent of the [Cl2]/ [Oz] ratio and, therefore, cannot involve reaction 1. Remeasurement at a [Cl,]/[O,] ratio of 0.006 during the current
\
'2 0 0
0.2
Om
I 0
U
0.0 0.00
0.05
0.10
0.15
[C~,l/[O,I Figure 1. Plot of [CH3COCl]/{A[CH3CHO] - [CH3COCl]> versus [C12]/[02]observed following UV irradiation of mixtures of CH3CH0, Clz, 02,and N2 at 700 Torr.
experiments gave a CH3COCl yield of (17 rt l)%; based on the value of kl/k2 measured above, %3%of this 17% yield results from CH3CO reaction with Cl2 under these conditions. We believe that the source of the remaining %14% CH3COC1 yield is the sum of small contributions from the reactions of several R02 radicals with CH3CHC102 (7b). Support for this suggestion is provided by C1-initiated cooxidations of CH3CH2Cl with a sufficient excess of either C& (150: 1) or HZ(2100:1) to ensure that '50% of the CH3CHC102 radicals react with CH302 or HO2 (based on computer modeling of the kinetics using rate constants for analogous reactions). In the presence of C h , the CH3COC1 yield was 22 f 1% of the total C2H5C1 consumed, while in the presence of H2, the yield was 19 rt 2%. These results confirm that reactions of CH3CHC102 with CH3O2 and with HO2 do produce modest yields of CH3COCl. HO2 and CH3O2 are formed from the methyl radicals generated in reaction 10 and HO2 is also produced during the generation of chloroacetaldehyde following reaction 6 (see more complete mechanism in Table 1 of ref 2). As additional confirmation that this CH3COC1 background results from RO2 reactions, addition of NO (which removes R02) in the previous FTIR experiments at high 0 2 partial pressure decreased the CH3COCl yield to below the detectability limit (
