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A new transitory product in the ozonolysis of trans-2-butene at atmospheric pressure
Volume 156, number
I
CHEMICAL
PHYSICSLETTERS
24 March I989
A NEW TBANSITORY PRODUCT IN THE OZONOLYSIS OF ‘I’RANS-2-RUTENE AT ATMOSPHERIC PRESSURE 0. HORIE and G.K. MOORTGAT Max-Planck-lnstitutfijr Chemie, Division ofAtmospheric Chemistry, Postfach 3060, D-6500 Maim, Federal Republrc of Germany Received 21 October
1988; in final form 6 January
1989
A previously unidentified transitory species, tentatively assigned as hydroxyethyl formate, CH3CH (OH)-0-CHO, was formed as a major product in the ozonolysis of trans-2-butene at atmospheric pressure. A continuous stirred-tank reactor was used to analyze reaction products via molecular-beam sampling and matrix isolation FTIR spectroscopy. CH,CHO, HCHO, CO2, CO, CH30H, CH, and HZ0 were the main, HCOOH and CH*CO the minor, products. CH$OOH and propene ozonide were detected as trace components.
1t Introduction
The mechanism of ethene ozonolysis can be described by the formation of primary ozonide and its subsequent decomposition into the vibrationally excited Criegee intermediate H&Of (the asterisk indicates vibrational excitation) and a stable product HCHO. The unimolecular decomposition/isomerization and the collisional stabilization of this excited Criegee intermediate H2COt will lead to the formation of welldefined reaction products [ 1]. (In this paper, the term “Criegee intermediate” represented by the generic structure R,R,C02 should be interpreted as refering to either of the three isomeric species: dioxymethylenes R,R&O-O’, dioxiranes R, R&-O-O , I 1 or methylenebis (oxy )s
have shown recently that the reaction pathways of the excited Criegee intermediate CHsHCOI formed in the ozonolysis of trans-Zbutene should include c&her (less important) channels besides the usual ester channel. They proposed an O-atom channel and the hydroperoxide channel which are represented by the unique indicator products diacetyl and glyoxal, respectively. The subsequent reaction pathways of the collisionally stabilized Criegee intermediate CHjHCOZ, though not discussed by Martinez and Herron [ 61, also present interpretative difficulties. Kiihne et al. [ 71 observed the formation of butene ozonide in their matrix isolation study of the cis-2-butene ozonolysis at atmospheric pressure: “3C CH3HE-0-i) + CH3CHO+
R, R2C-O’ ,
o-o \I
A R
A
(butene
where R, =R2 =H in the ethene ozonolysis, and R, =CHs and R2=H in the 2-butene ozonolysis [ 2,3 1. Appropriate structures will be explicitly specified when necessary to describe mechanistic details. ) This simple picture becomes less applicable for the interpretation of the ozonolysis of propene, butenes, and higher alkenes [ 4,5]. Martinez and Herron [ 61 0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
H
\I 0
(1) CH3
ozonide)
Using a photo-ionization mass spectroscopy, Martinez et al. [8,9] observed that the concentration of butene ozonide (a product assigned to the m/e= 104 peak) increased with the concentration of CH$HO added to the OS + trans-2-butene system at low pressure (4 Torr). Niki et al. [lo] on the other hand did not mention the formation of butene ozonB.V.
39
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CHEMICAL PHYSICS LETTERS
ide in the O,+cis-2-butene reaction at atmospheric pressure, although they observed the formation of propene ozonide when HCHO was added to the reaction system: o-o
Hlc \I
cn,wd-o-6 + HCHO
---j
H \/
A0A
H
(2) H
In the ethene ozonolysis, hydroxymethyl formate (HMF), CH,(OH)-0-CHO, is known as a major transitory product, formed by the reaction of stabilized methylenebis(oxy)s H>C-0’
,
d* and HCHO \
2. Experimental
J f
HCHO
+
CHZ(OH)-O-CR0
A
H
tion of a similar transitory reaction product has not been reported. If this transitory species is formed from the Criegee intermediate CHJHC02 and the main product CH,CHO, an association product such as hydroxyethyl acetate (HEA), CH,CH (OH )-OCOCHj, may be a favourable structure by analogy to HMF. The purpose of this study was to carry out the ozonolysis of trans-Zbutene at atmospheric pressure and to attempt a complete analysis of the reaction products. The effect of the addition of SOZ, HCHO and CHJCHO was also examined. A continuous stirred-tank reactor was constructed for this purpose, which enabled us to analyze transitory species as well as stable products via molecular-beam sampling and matrix isolation IR spectroscopy [ 141.
[2,1 l-13]:
0’
H
0. In the butene
(3)
(HMF)
ozonolysis,
however, the observa-
The ozonolysis of trans-2-butene was carried out in a continuous stirred-tank reactor (CSTR) at atmospheric pressure and room temperature. A schematic diagram of the apparatus is shown in fig. 1.
Fig. I. A schematic diagram of the apparatus.
40
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CHEMICAL PHYSICS LETTERS
The reactor consists of a 2 QPyrex bulb equipped with a Teflon stirrer, two oppositely placed quartz windows, inlet ports for the reactants and the carrier gas, and a sampling and pumping port. The reaction mixture was withdrawn through a molecular-beam sampling unit designed according to the principle developed by Campargue [ 15 1, and deposited on a cold finger maintained at 7 _+1 K by a liquid-helium cryostat. Ozone was prepared in an electric discharge of pure oxygen (Linde, stated purity 99.999%). The ozone concentration was determined by the absorption at 254 nm in an optical cell placed upstream of the reactor. The optical absorption through the reactor was used for monitoring the effect of mixing and the approach to steady-state conditions. Trans-Zbutene (Messer-Griesheim, 99%) was metered through a needle valve and its concentration measured by a pressure-drop method. SO, (4.8% in synthetic air), CH&HO, and HCHO, which was generated by pyrolyzing trioxane at 450°C [ 161, were diluted in Ar (Linde 99.999%). The mixture of Ar and O2 was used as the carrier gas in which O2 was maintained at (20 f 1 )W during a run. The reaction products isolated in the matrix were analyzed using a Fourier-transform infrared spectrometer (Bomem DAO3.0 1). 128 scans were made with the spectral resolution of 0.5 cm-‘. Additionally, a gas-chromatographic analysis was performed for hydrocarbon fractions. Quantitative analysis of the products and the reactants was made by calibrating the matrix isolation FTIR spectra using standard techniques. For a gaseous species, the integrated intensity of a typical absorption band of the matrix isolation spectra was correlated with the concentration of the species in the bulb determined from the data of the pressure-drop measurements and the total flow rate of the carrier gas at a given pressure. For a liquid compound, a dilute gas mixture consisting of the compound at a known vapour pressure and Ar was prepared, and the same procedure used for the gaseous compound was followed for the calibration. Pyrolysis of 3-buten- l-01 at 560 ’ C [ 16 ] was used to generate an equimolar mixture of propene and HCHO for the calibration of the latter. Since the ratio of the band intensities of the two compounds is constant, the calibration of propene enabled the same for HCHO. Similarly, pyrolysis of acetic an-
24 March I989
hydride at 450’ C [ 161 was used to generate an equimolar mixture of ketene and acetic acid. Under the experimental conditions, the mole ratios of the matrix to reactants and products were typically in the range 5 x 10’ to 1 x 104. Therefore, the complex formation among them was considered negligible. However, the possibilities of complex formation between alkene and water [ 17 1, CO* and water [ 18 1, and water molecules [ 191 were carefully examined. Also, the formation of gaseous dimers of HCOOH and of CHJOOH [20] was examined and compared with their monomer spectra in matrix isolation [ 2 1,221, The technique of matrix isolation FTIR spectroscopy has been developed in this laboratory and applied to the trace gas analysis [ 23,241 as well as to kinetic studies [ 25 1. Actual sampling of the reaction mixture was performed by activating a shutter located in front of the cryostat chamber for a certain duration, typically 280 s, after the system reached a steady state. The range of the experimental conditions was: T= 294-30 1 K, p= 745-768 Torr, t (average residence time) = 40 s, [03]0= (2.4-10.5) x 10” cm-3, [transd-C,H,], =(1.94-18.9)X10i5 cme3, [SOZ]o=7.18x10L5 cm-3, [HCHOlo=5x 10” cm-‘, and [CH&HO], =16x lOI cm-3.
3. Results and discussion A steady-state condition was reached about 15 min after turning on the flow of gas through the reactor. At this stage, no rapid or long-term fluctuations in the O3 absorption were observable. After about 30 min of steady-state operation, the reactor wall was visibly covered with thin opaque film. This was also detected by the gradual increase in the observed optical absorption. This phenomenon seemed to be enhanced in the presence of the added HCHO. The thin film was removed by 1 h of pumping after the measurement. The experimental apparatus used in this work was first tested with respect to the ethene ozonolysis at atmospheric pressure. Details of these measurements will be published separately [ 26 ] _ The reaction system gave results which were consistent with the published data on the absolute product concentrations and on the fraction of the stabilization ofthe 41
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156, number 1
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PHYSICS
initially formed excited Criegee intermediate H&O; [ 12,13,27]. This ratio, r, taken to be equal to r= [ HMF] / ( [ HCHO] + [ HMF] ), was 0.38 and 0.44 at [03],,= 1.0~ lOI and 2.1 X 10” cm-3, respectively, with [C,H4],,=2.0X lOI cm-’ [26]. In the literature, we find r-0.37 [ 121, r=0.35+0.05 [ 13 1, and 0.39 + 0.05 [ 271. The use of this experimental method for the ozonolysis of trans-2-butene was justified on the basis of these results. Fig. 2a shows a portion of a typical matrix isolation FTIR spectrum of the 03+trans-Zbutene reaction system. CH,CHO, HCHO, C02, CO, CH,OH, CH4, and HZ0 were identified as the major stable products. HCOOH and CHzCO (the intense C-0 absorption at 214 1.7 cm- ’ near the CO band is not shown in fig, 2a) were minor products, and CH,COOH and propene ozonide were detected as trace components. There are several unidentified absorption bands in the spectra (fig. 2a). However, acetoin ( 3-hydroxy-2-butanone), CH,C( =O)CH(OH)CHJ, and diacetyl (2,3_butanedione), CH,C(=O)-C(=O)CH,, were not detected in contrast to the low-pressure ozonolysis of trans-2-butene [ 6 ]_ The formation of glyoxal (ethanedial), CHOCHO, was not conclusively excluded from the re-
LETTERS
24
action products for want of the reference spectra. Glycolaldehyde, CHI (OH)CHO, which was observed by Niki (cited in ref. [ 6 ] as a private communication) could not be excluded for the same reason. However, the contribution of the unidentified absorption bands to the total product concentration was estimated to be less than about 5% based on the assumption that the band strength was equal for all species. The quantitative data of the observed products are summarized in table 1, along with those of other series of experiments. The most interesting finding was the formation of a new product (compound Y) whose main absorption lines and their probable assignment are listed in table 2. Three sets of experiments were performed in order to characterize this product. Because of a striking similarity of its IR absorption bands to those of HMF [ 13,26 1, which is known to disappear with the added SOZ [ I 11, SOZ was added to the reaction system. The resultant matrix isolation FIJR spectra are shown in fig. lb. The addition of SO* eliminated compound Y completely. The results suggested that the new species should be an association product between the stabilized Criegee intermediate, possibly methyl methylenebis(oxy), and CH,CHO, analogous to the case of HMF in ethene ozonolysis [ 2,1 l131: H3C
0' \ i + CH3CHO +
CH,CIi(OH)-0-COCH3
,O\ H
_.. 800
1300
1550
1800
wovenumber. cm-’
Fig. 2. Matrix isolation FTlR spectra of the O3+trans-2-butene system. (a) [0,],=9.2~10~~cm-~, [C4Hslo=6.9x 10’5cm-3, f=40s.
(b)
[S02],,=7.2x
[0,]0=8.3~10’scm-‘,
[C,Hs],=6.6x
10” CII-~, reaction time t=40
10’5cm-‘,
s. The identified
bands are marked with circles. Closed circle, HEF (see text); I, 0,; 2, C,H,; 3, CH,CHO; 4, HCHO; 5, CH,OH; 6, CH,; 7, H,O; 8, CH,COOH; 9, HCOOH; 10,02; 11, SO*. 4L
March 1989
0.
(4)
(HEA)
The addition of CH&HO should therefore increase the formation of compound Y. It was found, however, that the addition of CH&HO did not affect the yield of this compound, as seen in table 1. Thirdly, since Niki et al. [lo] observed the formation of propene ozonide when HCHO was added to the 03+cis-Zbutene system, HCHO was added to see the effect on compound Y as well as on propene ozonide. The added HCHO increased the formation of compound Y markedly. In contrast, the increase in propene ozonide was insignificant. It must be pointed out that the matrix isolation FTIR spectra of propene ozonide formed in this system and those observed in the O,+ C2H1+CHsCH0 system [ 261 agreewell with the gas-phase spectra reported by Niki et al. [ lo]. The tenfold increase in [HCOOH] can
Volume 156, number I
CHEMICAL PHYSICS LETTERS
24 March 1989
Table 1 The product distribution in the ozonolysis oftrans-2-C,Hs. T=295-299 K, p= 745-765 Torr, 1~40 s Experiment [O,l,“’
[CJ&I, =’ [added] al 0,
con”.
(% )
C4Hs conv. (96 )
8.32 6.58 7.18
8.41 6.62 0
96.5 85.6
96.3 85.7
Effect of CH,CHO
Effect of HCHO
Effect of SO2
9.18 6.93 0 97.0 85.6
8.56 6.79 0
9.20 6.76 5 96.4 85.2
8.57 6.65 16
97.0 83.8
97.9 81.4
96.7 40.3 23.0 14.7 11.8 12.0 1.8 0.4 2.5 63.0
394 b’ 52.3 22.0 20.9 16.0 12.0 2.0 1.4 4.1 59.6
Product yield relative to the converted C.,H, (mol 94) CH,CHO Cc& co HCHO CH,OH CH, CH,CO CHTCOOH HCOOH HEF ”
80.8
80.8 35.9 21.4 Il.4
35.8 19.5 7.5 5.5 12.1 0 4.5 5.6 0
IO.0 IO.6 1.7 0.5 2.7 62.9
91.1 38.3 21.9 14.1 10.2 11.5 1.7 0.4 2.4 59.2
99.4 39.0 32.0 109 b1 13.4 13.1 1.7 0.8 25.8 102.0
bJ The total concentration ( = added + formed).
a) Theconcentrations are in units of IOtJ cm-‘. ‘) Qualitative estimates only (see text).
Table 2 The main IR absorption bands assigned to hydroxyethyl formate, CH3CH(OH)-0-CHO,
in Ar/02=80/20
matrix
Wave number (cm- ’ )
Intensity
Probable assignment
1745 1336, 1332 (doublet) 1193 1160 1116 931 854,850 (doublet)
S
C-O stretch of formate C-O-H deformation of primary/secondary alcohol C-O-C asymmetric stretch of ester (formate) C-O-C asymmetric stretch ofester (formate) C-OH stretch of secondary alcohol
w m s S
w W
be attributed to the secondary reactions of HOz and HCHO in the presence of O2 [ 26 1. The main source of HCHO is considered to be subsequent oxidations of CH, and CH,O radicals which are formed in the unimolecular decomposition of the excited CH,HCO; intermediate [ 11. These results indicate that compound Y is not a product between CH&HO and CH3HC02 intermediate as expected, but is formed between HCHO and CH3HC02 intermediate, and has a structure similar to that of HMF. Most of the absorption lines listed in table 2 are consistent with the suggested
hydroxyethyl structure, formate (HEF): CH$ZH( OH)-0-CHO. The low-frequency vibrations may correspond to the ring puckering of the isomeric form of HEF similar to the cis form of HMF [ 131. There is an alternative structure for compound Y, hydroxymethyl acetate (HMA), CHI( OH)-0-C(=O)CH, which could be derived via the decomposition and subsequent isomerization of propene ozonide [ 111. However, since the increase in propene ozonide formation with the addition of HCHO was insignificant while that of compound Y was remarkable, this pathway was con43
Volume 156, number
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PHYSICS LETTERS
sidered unlikely. Rather, a direct addition of the collisionally stabilized methylenebis (oxy ) radical to HCHO yielding HEF may be the more favourable pathway by analogy to the formation of HMF in the ethene ozonolysis [ 21: o*
HJC \ I C
/ \ H
HJC
\r
O...H
I
c--o. .c=o
+ HCHO +
I
I 0’
9
n
H
H3C-CH(OW-0-CHO (HEF)
(5)
The frequencies listed in table 2 are distinctly different from those of propene ozonide, which seems to be formed exclusively by the reaction of dioxymethylene radical with CH&HO [ 26 1: 0-G
H
\ /
c.
I H
H2C
t
-\/ CHjCHO
-72
o-o
If
\I
A 0 AE H
(6)
Above all, propene ozonide possesses no GO group ( 1745 cm-‘). Also the characteristic frequencies of alcohol around 1330 cm-’ do not belong to propene ozonide. It should be mentioned here that the formation of HEF was also observed in the ozonolysis of cis-2-butene and propene [26], The formation of this transitory product in the ozonolysis of 2-butenes and propene has not been reported in previous studies [6,7,9,10,28]. In a static experiment with the 03+cis-2-C4Hs+HCH0 system, Niki et al. [lo] mentioned that several weak bands, notably one near 1200 cm-‘, remained unassigned. A careful look at the residual spectrum of the reaction mixture reveals that some of the unassigned bands, roughly at 1336, 1323, 1194 and 1166 cm-’ (read from fig. 1 of ref. [ 10 ] ) coincide with the HEF frequencies of table 2. They might belong to HEF, although the intensity is too small to be certain. The approximate concentration of HEF listed in table 1 was estimated on the basis of a 1OOWcarbon balance in the absence of the added compounds. It should be noted that the increase in [ HEF] with the addition of HCHO implies that the 100% carbon 44
24 March 1989
balance is not applicable for an accurate estimation of the concentration of HEF. In the absence of the additional HCHO, a large fraction of the stabilized Criegee intermediate CH3HC0z would still be present in the reaction system or heterogeneously lost to the reactor wall. The coverage of the reactor wall with the thin film observed may be due to the formation of some products with low vapour pressures on the reactor wall. What fraction of the stabilized intermediate reacts with HCHO is difficult to assess. Therefore, the HEF concentration in table 1 should be considered to be only qualitative. When the stable products are considered, the results of this study shown in table 1 give the first quantitative analysis of the majority of the reaction products in the ozonolysis of trans-2-butene at atmospheric pressure. The ozonolysis of cis-2-butene carried out under similar conditions used in this study gave essentially the same product distribution as that obtained for trans-2-butene ozonolysis [ 261. This is because the decomposition of the primary ozonide gives the same initial products CH,HCO; and CH,CHO for both isomers. The single, partially quantitative analysis of the main products at atmospheric pressure is attributed to Niki et al. [ lo]. In the presence of 10 ppm HCHO, the initial reaction mixture consisting of 10 ppm cis-Zbutene and 5 ppm O3 gave, at 50% conversion of C4Hs, about 13% CH4, 5% HCOOH, 8% CHXOH, and 37% CO, relative to the reacted C4Hs. Except for HCOOH whose formation in the presence of HCHO could be affected by the secondary reactions involving HOZ radicals (see above), the yield of these products is in good agreement with ours. However, our results differ from those of Niki et al. [lo] in several aspects. They stated that CH&OOH was never observed (presumably also in the presence of the added SO,). Our results with added SO2 show a remarkable increase in [ CH,COOH 1. They also estimated an 18% yield of propene ozonide relative to the reacted butene after the addition of HCHO. Our results show that the yield of propene ozonide was negligible, while that of HEF was large. One important difference was that the initial concentrations of the reactants in our system were almost two orders of magnitude larger than those used by Niki et al. Also, in our system the reaction was kept mixed mechanically whiIe in their
Volume 156,number 1
CHEMICALPHYSICSLETTERS
system the reactants were introduced into a cylindrical cell and there was no forced mixing. However, addition of HCHO should affect both systems in the same way. No explanation for the difference in the results can be given at present. Kllhne et al. [ 7 ] observed the formation of butene ozonide in their matrix isolation study of cis-Zbutene ozonolysis with the initial reactant concentrations of about 1 x lOI* crne3, about 100 times larger concentrations than used in this work. We found no absorption lines at the wave numbers given by Ktihne et al. as belonging to the butene ozonide. Niki et al. [lo] did not mention the observation of butene ozonide in their FTIR spectra. It seems that CH,HCO, intermediate reacts much more slowly with CH&HO than with HCHO. Our results indicate that the reaction of the stabilized Criegee intermediate CH,HCOz with HCHO gives HEF preferentially and not propene ozonide. The subsequent reactions of the stabilized Criegee intermediate may vary for different ranges of the initial concentration of the reactants. This possibility should be investigated in detail. The low-pressure study of Atkinson et al. [ 28 ] using photoionization mass spectroscopy listed HCHO, CHICHO, CH,CO, glyoxal, HCOOH, CH30H and H202 as observed products. CO and CO1 were not detectable by the method used. In a recent low-pressure (4 Torr total pressure of 02) study with photoionization mass spectroscopy, Martinez and Herron [ 61 gave two sets of quantitative data on the main reaction products of the trans-2-butene ozonolysis. With [O~]0=3.5~10’5 cm-’ and [C4Hs]o= 3.4x 1Ol4 cmp3, the product yield in percent of the reacted butene at t= 3 s (butene conversion= 86%) was: CH3CH0, 74; HCHO, 36; HzO, 3 1; COZ, 25; acetoin, 16; CH4, 6, and CHICO, 2. The other experiment with [0,10=4.7x lOI cmp3 and [C,Hslo=2.5x 1Or5 cmp3 gave the yield at t=3 s (0, conversion 87%) in the same order as above: 72, 39, 5, 15, 8, 8, and 3. They gave the upper limit of the CO formation as < 0.2 mol per mol of butene consumed. They also observed the formation of diacetyl and glyoxal (the product assigned to the m/e 58 peak). Methanol was not listed as a product, although its formation was mentioned in their earlier study [9].
24 March 1989
On the basis of the yield of CH4 (6-ll%), Martinez and Hen-on [ 61 concluded that the ester channel contributed at least 101 of the CH,HCOt decomposition. Similarly, based on the yield of glyoxal and diacetyl, the hydroperoxide channel and the O-atom channel were estimated to contribute > 10% and < 5% of the total CH3HCOZ decomposition, respectively. At atmospheric pressure, the O-atom channel seems inoperative since diacetyl was not formed. Similarly, from consideration of the formation of glyoxal (see above), the hydroperoxide channel could be at most a few percent of the total CH3HCOT decomposition. On the other hand, CO?, CO and CH30H as well as CH, are the products of CHjHCO: decomposition and contribute to the ester channel. Our results shown in table 1 indicate that at atmospheric pressure, the major fraction of the total CH,HCOf decomposes through the ester channel. A greater fraction of CHpHCOt is collisionally stabilized at atmospheric pressure than at lower pressure (e.g. 4 Torr), resulting in a very small branching ratio for the O-atom formation. A detailed discussion of all participating reactions will be given elsewhere [ 261. Finally, the addition of SO1 to the 03/alkene systems has been known to produce aerosol whose main component is H2S04 [ 27,29-311. However, little attention has been paid to the effect of the SO2 addition on the organic reaction products, in particular organic acids [27,29]. The increase in [HCOOH] and [CH,COOH] in this reaction system had not been reported before. This seems the first case of the ozonolysis of higher alkenes in which the increase in the organic acids was observed with the added SO,. The increase in [CH,CHO] and [ HCHO] in the presence of SO, was not found (table 1 ), in contrast to the earlier observations for higher alkenes including trans-2-butene by Cox and Penkett [ 291. Whether these results suggest that SO2 reacts with HEF as well as with CH3HC02 under the present conditions is not clear. To assess the direct effect of SO2 on the increase in the acid concentrations, the effect of the addition of both water and SO2 on the ratio of the organic acid to aldehydes should be examined [ 321. Further studies are underway.
45
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CHEMICAL
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft by its Sonderforschungsbereich 233, “Dynamik und Chemie der Hydrometeore”. We thank Dr. Steve Wilson for numerous discussions.
PHYSICS LETTERS
24 March 1989
[ 131 H. Niki, P.D. Maker, CM. Savage and L.P. Breitenbach, Phys. Chem. 85 ( 198 1) 1024.
[ 1410.
Horie and G.K. Moortgat, Quadrennial Symposium, August 4-l 3,1988, Gottttingen, FRG.
unimolecular
reactions,
NSRDS-NBS
2 1 (1977)
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p. 3 14.
[ 17] A. Engdahl and B. Nelander, J. Phys. Chem. 90 ( 1986) 4982. [ 181 L. Fredin, B. Nelander and G. Ribbegard, Chemica Scripta 7 (1975)
[ 1 ] R. Atkinson and A. Lloyd, J. Phys. Chem. Ref. Data 13 (1984) 315. [ 21 J.T. Herron, R.I. Martinez and R.E. Huie, Intern. J. Chem. Kinetics 14 (1982) 201. [ 31 L.B. Harding and W.A. Goddard III, J. Am. Chem. Sot. 100 (1978) 7180. [4] RI. Martinez and J.T. Herron, J. Phys. Chem. 91 (1987) 946. [ 51 H. Niki, P.D. Maker, CM. Savage, L.P. Breitenbach and M.D. Hurley, J. Phys. Chem. 91 (1987) 941. [6] RI. Martinez and J.T. Herron, J. Phys. Chem. 92 (1988) 4644. [ 7) H. Kilhne, M. Forster, J. Hulliger, H. Ruprecht, A. Bauder and H.-H. Giinthard, Helv. Chim. Acta 63 ( 1980) 197 1. [8] RI. Martinez, R.E. Huie and J.T. Herron, Chem. Phys. Letters 72 (1980) 443. [9] RI. Martinez, J.T. Herron and R.E. Huie, J. Am. Chem. Sot. 103 (1981) 3807. [ IO] H. Niki, P.D. Maker, CM. Savage and L.P. Breitenbach, Chem. Phys. Letters 46 ( 1977) 327. [ 111 Fu Su, J.G. Calvert and J.H. Shaw, J. Phys. Chem. 84 ( 1980) 239. [ 121 C.S. Kan, Fu Su, J.G. Calven and J.H. Shaw, J. Phys. Chem. 85 (1981) 2359.
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[ 151 R. Campargue, J. Phys. Chem. 88 (1984) 4466. [ 161SW. Benson and H.E. O’Neal, Kinetic data on gas phase
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1713.
1221 R.L. Redington, J. Mol. Spectry. 65 (1977) 171. (231 J.P. Burrows, D.W.T. Griflith, G.K. Moortgat and G.S. Tyndall, J. Phys. Chem. 89 ( 1985) 266. [24] D.W.T. Griffith andG. 59.
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to be published. and H. Akimoto,
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[ 281 R. Atkinson, B.J. Finlayson and J.N. Pitts Jr., J. Am. Cham. Sec. 95 (1973) 7592. [29] R.A. Cox and S.A. Penkett, J, Chem. Sot. Faraday Trans. I 68 (1972) 1735.
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CHEMICAL
PHYSICSLETTERS
24 March I989
A NEW TBANSITORY PRODUCT IN THE OZONOLYSIS OF ‘I’RANS-2-RUTENE AT ATMOSPHERIC PRESSURE 0. HORIE and G.K. MOORTGAT Max-Planck-lnstitutfijr Chemie, Division ofAtmospheric Chemistry, Postfach 3060, D-6500 Maim, Federal Republrc of Germany Received 21 October
1988; in final form 6 January
1989
A previously unidentified transitory species, tentatively assigned as hydroxyethyl formate, CH3CH (OH)-0-CHO, was formed as a major product in the ozonolysis of trans-2-butene at atmospheric pressure. A continuous stirred-tank reactor was used to analyze reaction products via molecular-beam sampling and matrix isolation FTIR spectroscopy. CH,CHO, HCHO, CO2, CO, CH30H, CH, and HZ0 were the main, HCOOH and CH*CO the minor, products. CH$OOH and propene ozonide were detected as trace components.
1t Introduction
The mechanism of ethene ozonolysis can be described by the formation of primary ozonide and its subsequent decomposition into the vibrationally excited Criegee intermediate H&Of (the asterisk indicates vibrational excitation) and a stable product HCHO. The unimolecular decomposition/isomerization and the collisional stabilization of this excited Criegee intermediate H2COt will lead to the formation of welldefined reaction products [ 1]. (In this paper, the term “Criegee intermediate” represented by the generic structure R,R,C02 should be interpreted as refering to either of the three isomeric species: dioxymethylenes R,R&O-O’, dioxiranes R, R&-O-O , I 1 or methylenebis (oxy )s
have shown recently that the reaction pathways of the excited Criegee intermediate CHsHCOI formed in the ozonolysis of trans-Zbutene should include c&her (less important) channels besides the usual ester channel. They proposed an O-atom channel and the hydroperoxide channel which are represented by the unique indicator products diacetyl and glyoxal, respectively. The subsequent reaction pathways of the collisionally stabilized Criegee intermediate CHjHCOZ, though not discussed by Martinez and Herron [ 61, also present interpretative difficulties. Kiihne et al. [ 71 observed the formation of butene ozonide in their matrix isolation study of the cis-2-butene ozonolysis at atmospheric pressure: “3C CH3HE-0-i) + CH3CHO+
R, R2C-O’ ,
o-o \I
A R
A
(butene
where R, =R2 =H in the ethene ozonolysis, and R, =CHs and R2=H in the 2-butene ozonolysis [ 2,3 1. Appropriate structures will be explicitly specified when necessary to describe mechanistic details. ) This simple picture becomes less applicable for the interpretation of the ozonolysis of propene, butenes, and higher alkenes [ 4,5]. Martinez and Herron [ 61 0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
H
\I 0
(1) CH3
ozonide)
Using a photo-ionization mass spectroscopy, Martinez et al. [8,9] observed that the concentration of butene ozonide (a product assigned to the m/e= 104 peak) increased with the concentration of CH$HO added to the OS + trans-2-butene system at low pressure (4 Torr). Niki et al. [lo] on the other hand did not mention the formation of butene ozonB.V.
39
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CHEMICAL PHYSICS LETTERS
ide in the O,+cis-2-butene reaction at atmospheric pressure, although they observed the formation of propene ozonide when HCHO was added to the reaction system: o-o
Hlc \I
cn,wd-o-6 + HCHO
---j
H \/
A0A
H
(2) H
In the ethene ozonolysis, hydroxymethyl formate (HMF), CH,(OH)-0-CHO, is known as a major transitory product, formed by the reaction of stabilized methylenebis(oxy)s H>C-0’
,
d* and HCHO \
2. Experimental
J f
HCHO
+
CHZ(OH)-O-CR0
A
H
tion of a similar transitory reaction product has not been reported. If this transitory species is formed from the Criegee intermediate CHJHC02 and the main product CH,CHO, an association product such as hydroxyethyl acetate (HEA), CH,CH (OH )-OCOCHj, may be a favourable structure by analogy to HMF. The purpose of this study was to carry out the ozonolysis of trans-Zbutene at atmospheric pressure and to attempt a complete analysis of the reaction products. The effect of the addition of SOZ, HCHO and CHJCHO was also examined. A continuous stirred-tank reactor was constructed for this purpose, which enabled us to analyze transitory species as well as stable products via molecular-beam sampling and matrix isolation IR spectroscopy [ 141.
[2,1 l-13]:
0’
H
0. In the butene
(3)
(HMF)
ozonolysis,
however, the observa-
The ozonolysis of trans-2-butene was carried out in a continuous stirred-tank reactor (CSTR) at atmospheric pressure and room temperature. A schematic diagram of the apparatus is shown in fig. 1.
Fig. I. A schematic diagram of the apparatus.
40
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The reactor consists of a 2 QPyrex bulb equipped with a Teflon stirrer, two oppositely placed quartz windows, inlet ports for the reactants and the carrier gas, and a sampling and pumping port. The reaction mixture was withdrawn through a molecular-beam sampling unit designed according to the principle developed by Campargue [ 15 1, and deposited on a cold finger maintained at 7 _+1 K by a liquid-helium cryostat. Ozone was prepared in an electric discharge of pure oxygen (Linde, stated purity 99.999%). The ozone concentration was determined by the absorption at 254 nm in an optical cell placed upstream of the reactor. The optical absorption through the reactor was used for monitoring the effect of mixing and the approach to steady-state conditions. Trans-Zbutene (Messer-Griesheim, 99%) was metered through a needle valve and its concentration measured by a pressure-drop method. SO, (4.8% in synthetic air), CH&HO, and HCHO, which was generated by pyrolyzing trioxane at 450°C [ 161, were diluted in Ar (Linde 99.999%). The mixture of Ar and O2 was used as the carrier gas in which O2 was maintained at (20 f 1 )W during a run. The reaction products isolated in the matrix were analyzed using a Fourier-transform infrared spectrometer (Bomem DAO3.0 1). 128 scans were made with the spectral resolution of 0.5 cm-‘. Additionally, a gas-chromatographic analysis was performed for hydrocarbon fractions. Quantitative analysis of the products and the reactants was made by calibrating the matrix isolation FTIR spectra using standard techniques. For a gaseous species, the integrated intensity of a typical absorption band of the matrix isolation spectra was correlated with the concentration of the species in the bulb determined from the data of the pressure-drop measurements and the total flow rate of the carrier gas at a given pressure. For a liquid compound, a dilute gas mixture consisting of the compound at a known vapour pressure and Ar was prepared, and the same procedure used for the gaseous compound was followed for the calibration. Pyrolysis of 3-buten- l-01 at 560 ’ C [ 16 ] was used to generate an equimolar mixture of propene and HCHO for the calibration of the latter. Since the ratio of the band intensities of the two compounds is constant, the calibration of propene enabled the same for HCHO. Similarly, pyrolysis of acetic an-
24 March I989
hydride at 450’ C [ 161 was used to generate an equimolar mixture of ketene and acetic acid. Under the experimental conditions, the mole ratios of the matrix to reactants and products were typically in the range 5 x 10’ to 1 x 104. Therefore, the complex formation among them was considered negligible. However, the possibilities of complex formation between alkene and water [ 17 1, CO* and water [ 18 1, and water molecules [ 191 were carefully examined. Also, the formation of gaseous dimers of HCOOH and of CHJOOH [20] was examined and compared with their monomer spectra in matrix isolation [ 2 1,221, The technique of matrix isolation FTIR spectroscopy has been developed in this laboratory and applied to the trace gas analysis [ 23,241 as well as to kinetic studies [ 25 1. Actual sampling of the reaction mixture was performed by activating a shutter located in front of the cryostat chamber for a certain duration, typically 280 s, after the system reached a steady state. The range of the experimental conditions was: T= 294-30 1 K, p= 745-768 Torr, t (average residence time) = 40 s, [03]0= (2.4-10.5) x 10” cm-3, [transd-C,H,], =(1.94-18.9)X10i5 cme3, [SOZ]o=7.18x10L5 cm-3, [HCHOlo=5x 10” cm-‘, and [CH&HO], =16x lOI cm-3.
3. Results and discussion A steady-state condition was reached about 15 min after turning on the flow of gas through the reactor. At this stage, no rapid or long-term fluctuations in the O3 absorption were observable. After about 30 min of steady-state operation, the reactor wall was visibly covered with thin opaque film. This was also detected by the gradual increase in the observed optical absorption. This phenomenon seemed to be enhanced in the presence of the added HCHO. The thin film was removed by 1 h of pumping after the measurement. The experimental apparatus used in this work was first tested with respect to the ethene ozonolysis at atmospheric pressure. Details of these measurements will be published separately [ 26 ] _ The reaction system gave results which were consistent with the published data on the absolute product concentrations and on the fraction of the stabilization ofthe 41
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156, number 1
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PHYSICS
initially formed excited Criegee intermediate H&O; [ 12,13,27]. This ratio, r, taken to be equal to r= [ HMF] / ( [ HCHO] + [ HMF] ), was 0.38 and 0.44 at [03],,= 1.0~ lOI and 2.1 X 10” cm-3, respectively, with [C,H4],,=2.0X lOI cm-’ [26]. In the literature, we find r-0.37 [ 121, r=0.35+0.05 [ 13 1, and 0.39 + 0.05 [ 271. The use of this experimental method for the ozonolysis of trans-2-butene was justified on the basis of these results. Fig. 2a shows a portion of a typical matrix isolation FTIR spectrum of the 03+trans-Zbutene reaction system. CH,CHO, HCHO, C02, CO, CH,OH, CH4, and HZ0 were identified as the major stable products. HCOOH and CHzCO (the intense C-0 absorption at 214 1.7 cm- ’ near the CO band is not shown in fig, 2a) were minor products, and CH,COOH and propene ozonide were detected as trace components. There are several unidentified absorption bands in the spectra (fig. 2a). However, acetoin ( 3-hydroxy-2-butanone), CH,C( =O)CH(OH)CHJ, and diacetyl (2,3_butanedione), CH,C(=O)-C(=O)CH,, were not detected in contrast to the low-pressure ozonolysis of trans-2-butene [ 6 ]_ The formation of glyoxal (ethanedial), CHOCHO, was not conclusively excluded from the re-
LETTERS
24
action products for want of the reference spectra. Glycolaldehyde, CHI (OH)CHO, which was observed by Niki (cited in ref. [ 6 ] as a private communication) could not be excluded for the same reason. However, the contribution of the unidentified absorption bands to the total product concentration was estimated to be less than about 5% based on the assumption that the band strength was equal for all species. The quantitative data of the observed products are summarized in table 1, along with those of other series of experiments. The most interesting finding was the formation of a new product (compound Y) whose main absorption lines and their probable assignment are listed in table 2. Three sets of experiments were performed in order to characterize this product. Because of a striking similarity of its IR absorption bands to those of HMF [ 13,26 1, which is known to disappear with the added SOZ [ I 11, SOZ was added to the reaction system. The resultant matrix isolation FIJR spectra are shown in fig. lb. The addition of SO* eliminated compound Y completely. The results suggested that the new species should be an association product between the stabilized Criegee intermediate, possibly methyl methylenebis(oxy), and CH,CHO, analogous to the case of HMF in ethene ozonolysis [ 2,1 l131: H3C
0' \ i + CH3CHO +
CH,CIi(OH)-0-COCH3
,O\ H
_.. 800
1300
1550
1800
wovenumber. cm-’
Fig. 2. Matrix isolation FTlR spectra of the O3+trans-2-butene system. (a) [0,],=9.2~10~~cm-~, [C4Hslo=6.9x 10’5cm-3, f=40s.
(b)
[S02],,=7.2x
[0,]0=8.3~10’scm-‘,
[C,Hs],=6.6x
10” CII-~, reaction time t=40
10’5cm-‘,
s. The identified
bands are marked with circles. Closed circle, HEF (see text); I, 0,; 2, C,H,; 3, CH,CHO; 4, HCHO; 5, CH,OH; 6, CH,; 7, H,O; 8, CH,COOH; 9, HCOOH; 10,02; 11, SO*. 4L
March 1989
0.
(4)
(HEA)
The addition of CH&HO should therefore increase the formation of compound Y. It was found, however, that the addition of CH&HO did not affect the yield of this compound, as seen in table 1. Thirdly, since Niki et al. [lo] observed the formation of propene ozonide when HCHO was added to the 03+cis-Zbutene system, HCHO was added to see the effect on compound Y as well as on propene ozonide. The added HCHO increased the formation of compound Y markedly. In contrast, the increase in propene ozonide was insignificant. It must be pointed out that the matrix isolation FTIR spectra of propene ozonide formed in this system and those observed in the O,+ C2H1+CHsCH0 system [ 261 agreewell with the gas-phase spectra reported by Niki et al. [ lo]. The tenfold increase in [HCOOH] can
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CHEMICAL PHYSICS LETTERS
24 March 1989
Table 1 The product distribution in the ozonolysis oftrans-2-C,Hs. T=295-299 K, p= 745-765 Torr, 1~40 s Experiment [O,l,“’
[CJ&I, =’ [added] al 0,
con”.
(% )
C4Hs conv. (96 )
8.32 6.58 7.18
8.41 6.62 0
96.5 85.6
96.3 85.7
Effect of CH,CHO
Effect of HCHO
Effect of SO2
9.18 6.93 0 97.0 85.6
8.56 6.79 0
9.20 6.76 5 96.4 85.2
8.57 6.65 16
97.0 83.8
97.9 81.4
96.7 40.3 23.0 14.7 11.8 12.0 1.8 0.4 2.5 63.0
394 b’ 52.3 22.0 20.9 16.0 12.0 2.0 1.4 4.1 59.6
Product yield relative to the converted C.,H, (mol 94) CH,CHO Cc& co HCHO CH,OH CH, CH,CO CHTCOOH HCOOH HEF ”
80.8
80.8 35.9 21.4 Il.4
35.8 19.5 7.5 5.5 12.1 0 4.5 5.6 0
IO.0 IO.6 1.7 0.5 2.7 62.9
91.1 38.3 21.9 14.1 10.2 11.5 1.7 0.4 2.4 59.2
99.4 39.0 32.0 109 b1 13.4 13.1 1.7 0.8 25.8 102.0
bJ The total concentration ( = added + formed).
a) Theconcentrations are in units of IOtJ cm-‘. ‘) Qualitative estimates only (see text).
Table 2 The main IR absorption bands assigned to hydroxyethyl formate, CH3CH(OH)-0-CHO,
in Ar/02=80/20
matrix
Wave number (cm- ’ )
Intensity
Probable assignment
1745 1336, 1332 (doublet) 1193 1160 1116 931 854,850 (doublet)
S
C-O stretch of formate C-O-H deformation of primary/secondary alcohol C-O-C asymmetric stretch of ester (formate) C-O-C asymmetric stretch ofester (formate) C-OH stretch of secondary alcohol
w m s S
w W
be attributed to the secondary reactions of HOz and HCHO in the presence of O2 [ 26 1. The main source of HCHO is considered to be subsequent oxidations of CH, and CH,O radicals which are formed in the unimolecular decomposition of the excited CH,HCO; intermediate [ 11. These results indicate that compound Y is not a product between CH&HO and CH3HC02 intermediate as expected, but is formed between HCHO and CH3HC02 intermediate, and has a structure similar to that of HMF. Most of the absorption lines listed in table 2 are consistent with the suggested
hydroxyethyl structure, formate (HEF): CH$ZH( OH)-0-CHO. The low-frequency vibrations may correspond to the ring puckering of the isomeric form of HEF similar to the cis form of HMF [ 131. There is an alternative structure for compound Y, hydroxymethyl acetate (HMA), CHI( OH)-0-C(=O)CH, which could be derived via the decomposition and subsequent isomerization of propene ozonide [ 111. However, since the increase in propene ozonide formation with the addition of HCHO was insignificant while that of compound Y was remarkable, this pathway was con43
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PHYSICS LETTERS
sidered unlikely. Rather, a direct addition of the collisionally stabilized methylenebis (oxy ) radical to HCHO yielding HEF may be the more favourable pathway by analogy to the formation of HMF in the ethene ozonolysis [ 21: o*
HJC \ I C
/ \ H
HJC
\r
O...H
I
c--o. .c=o
+ HCHO +
I
I 0’
9
n
H
H3C-CH(OW-0-CHO (HEF)
(5)
The frequencies listed in table 2 are distinctly different from those of propene ozonide, which seems to be formed exclusively by the reaction of dioxymethylene radical with CH&HO [ 26 1: 0-G
H
\ /
c.
I H
H2C
t
-\/ CHjCHO
-72
o-o
If
\I
A 0 AE H
(6)
Above all, propene ozonide possesses no GO group ( 1745 cm-‘). Also the characteristic frequencies of alcohol around 1330 cm-’ do not belong to propene ozonide. It should be mentioned here that the formation of HEF was also observed in the ozonolysis of cis-2-butene and propene [26], The formation of this transitory product in the ozonolysis of 2-butenes and propene has not been reported in previous studies [6,7,9,10,28]. In a static experiment with the 03+cis-2-C4Hs+HCH0 system, Niki et al. [lo] mentioned that several weak bands, notably one near 1200 cm-‘, remained unassigned. A careful look at the residual spectrum of the reaction mixture reveals that some of the unassigned bands, roughly at 1336, 1323, 1194 and 1166 cm-’ (read from fig. 1 of ref. [ 10 ] ) coincide with the HEF frequencies of table 2. They might belong to HEF, although the intensity is too small to be certain. The approximate concentration of HEF listed in table 1 was estimated on the basis of a 1OOWcarbon balance in the absence of the added compounds. It should be noted that the increase in [ HEF] with the addition of HCHO implies that the 100% carbon 44
24 March 1989
balance is not applicable for an accurate estimation of the concentration of HEF. In the absence of the additional HCHO, a large fraction of the stabilized Criegee intermediate CH3HC0z would still be present in the reaction system or heterogeneously lost to the reactor wall. The coverage of the reactor wall with the thin film observed may be due to the formation of some products with low vapour pressures on the reactor wall. What fraction of the stabilized intermediate reacts with HCHO is difficult to assess. Therefore, the HEF concentration in table 1 should be considered to be only qualitative. When the stable products are considered, the results of this study shown in table 1 give the first quantitative analysis of the majority of the reaction products in the ozonolysis of trans-2-butene at atmospheric pressure. The ozonolysis of cis-2-butene carried out under similar conditions used in this study gave essentially the same product distribution as that obtained for trans-2-butene ozonolysis [ 261. This is because the decomposition of the primary ozonide gives the same initial products CH,HCO; and CH,CHO for both isomers. The single, partially quantitative analysis of the main products at atmospheric pressure is attributed to Niki et al. [ lo]. In the presence of 10 ppm HCHO, the initial reaction mixture consisting of 10 ppm cis-Zbutene and 5 ppm O3 gave, at 50% conversion of C4Hs, about 13% CH4, 5% HCOOH, 8% CHXOH, and 37% CO, relative to the reacted C4Hs. Except for HCOOH whose formation in the presence of HCHO could be affected by the secondary reactions involving HOZ radicals (see above), the yield of these products is in good agreement with ours. However, our results differ from those of Niki et al. [lo] in several aspects. They stated that CH&OOH was never observed (presumably also in the presence of the added SO,). Our results with added SO2 show a remarkable increase in [ CH,COOH 1. They also estimated an 18% yield of propene ozonide relative to the reacted butene after the addition of HCHO. Our results show that the yield of propene ozonide was negligible, while that of HEF was large. One important difference was that the initial concentrations of the reactants in our system were almost two orders of magnitude larger than those used by Niki et al. Also, in our system the reaction was kept mixed mechanically whiIe in their
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CHEMICALPHYSICSLETTERS
system the reactants were introduced into a cylindrical cell and there was no forced mixing. However, addition of HCHO should affect both systems in the same way. No explanation for the difference in the results can be given at present. Kllhne et al. [ 7 ] observed the formation of butene ozonide in their matrix isolation study of cis-Zbutene ozonolysis with the initial reactant concentrations of about 1 x lOI* crne3, about 100 times larger concentrations than used in this work. We found no absorption lines at the wave numbers given by Ktihne et al. as belonging to the butene ozonide. Niki et al. [lo] did not mention the observation of butene ozonide in their FTIR spectra. It seems that CH,HCO, intermediate reacts much more slowly with CH&HO than with HCHO. Our results indicate that the reaction of the stabilized Criegee intermediate CH,HCOz with HCHO gives HEF preferentially and not propene ozonide. The subsequent reactions of the stabilized Criegee intermediate may vary for different ranges of the initial concentration of the reactants. This possibility should be investigated in detail. The low-pressure study of Atkinson et al. [ 28 ] using photoionization mass spectroscopy listed HCHO, CHICHO, CH,CO, glyoxal, HCOOH, CH30H and H202 as observed products. CO and CO1 were not detectable by the method used. In a recent low-pressure (4 Torr total pressure of 02) study with photoionization mass spectroscopy, Martinez and Herron [ 61 gave two sets of quantitative data on the main reaction products of the trans-2-butene ozonolysis. With [O~]0=3.5~10’5 cm-’ and [C4Hs]o= 3.4x 1Ol4 cmp3, the product yield in percent of the reacted butene at t= 3 s (butene conversion= 86%) was: CH3CH0, 74; HCHO, 36; HzO, 3 1; COZ, 25; acetoin, 16; CH4, 6, and CHICO, 2. The other experiment with [0,10=4.7x lOI cmp3 and [C,Hslo=2.5x 1Or5 cmp3 gave the yield at t=3 s (0, conversion 87%) in the same order as above: 72, 39, 5, 15, 8, 8, and 3. They gave the upper limit of the CO formation as < 0.2 mol per mol of butene consumed. They also observed the formation of diacetyl and glyoxal (the product assigned to the m/e 58 peak). Methanol was not listed as a product, although its formation was mentioned in their earlier study [9].
24 March 1989
On the basis of the yield of CH4 (6-ll%), Martinez and Hen-on [ 61 concluded that the ester channel contributed at least 101 of the CH,HCOt decomposition. Similarly, based on the yield of glyoxal and diacetyl, the hydroperoxide channel and the O-atom channel were estimated to contribute > 10% and < 5% of the total CH3HCOZ decomposition, respectively. At atmospheric pressure, the O-atom channel seems inoperative since diacetyl was not formed. Similarly, from consideration of the formation of glyoxal (see above), the hydroperoxide channel could be at most a few percent of the total CH3HCOT decomposition. On the other hand, CO?, CO and CH30H as well as CH, are the products of CHjHCO: decomposition and contribute to the ester channel. Our results shown in table 1 indicate that at atmospheric pressure, the major fraction of the total CH,HCOf decomposes through the ester channel. A greater fraction of CHpHCOt is collisionally stabilized at atmospheric pressure than at lower pressure (e.g. 4 Torr), resulting in a very small branching ratio for the O-atom formation. A detailed discussion of all participating reactions will be given elsewhere [ 261. Finally, the addition of SO1 to the 03/alkene systems has been known to produce aerosol whose main component is H2S04 [ 27,29-311. However, little attention has been paid to the effect of the SO2 addition on the organic reaction products, in particular organic acids [27,29]. The increase in [HCOOH] and [CH,COOH] in this reaction system had not been reported before. This seems the first case of the ozonolysis of higher alkenes in which the increase in the organic acids was observed with the added SO,. The increase in [CH,CHO] and [ HCHO] in the presence of SO, was not found (table 1 ), in contrast to the earlier observations for higher alkenes including trans-2-butene by Cox and Penkett [ 291. Whether these results suggest that SO2 reacts with HEF as well as with CH3HC02 under the present conditions is not clear. To assess the direct effect of SO2 on the increase in the acid concentrations, the effect of the addition of both water and SO2 on the ratio of the organic acid to aldehydes should be examined [ 321. Further studies are underway.
45
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CHEMICAL
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft by its Sonderforschungsbereich 233, “Dynamik und Chemie der Hydrometeore”. We thank Dr. Steve Wilson for numerous discussions.
PHYSICS LETTERS
24 March 1989
[ 131 H. Niki, P.D. Maker, CM. Savage and L.P. Breitenbach, Phys. Chem. 85 ( 198 1) 1024.
[ 1410.
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NSRDS-NBS
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