Environ. Sci. Techno/. 1995,29,2322-2332
Kinetic and Mechanistic Study of the Atmospheric Chemiw of B J O R N G. KLOTZ, ARWID BIERBACH, IAN BARNES,* AND KARL H. BECKER Physikalische ChemielFachbereich 9, Bergische Universitdt, Gesamthochschule Wuppertal, 0-42097 Wuppertal, Germany
2,4-Hexadienedial and its alkylated derivatives, often referred to as muconaldehydes, have been postulated as primary ring cleavage products in the oxidation of simple alkylated aromatics. Some aspects of the atmospheric chemistry of several muconaldehydes, photolysis and reactions with OH and 03,have been investigated in a 1080-L reaction chamber at 298 f 2 K using in situ FT-IR spectroscopy. Rate coefficients have been determined for the reaction of OH radicals with the following muconaldehydes using the relative kinetic method (in units of lo-’* cm3 molecules-’ s-’): €,€-2,4- hexadie nedia I, 88 f 3; €,Z-2,4- hexad ienedial, 109 k 11; €,€-2-methyl-2,4-hexadienedial, 118 f 2. The major products of both the OH radical and the 03 reactions are, in all cases, E-butenedial and glyoxal and in the case of €,€-2-methyl-2,4-hexadienedial also methylglyoxal. Muconaldehydes have been postulated as intermediates in the oxidation of alkylated benzenes; if formed in this oxidation, their major atmospheric sink will be reaction with OH radicals.
the reaction of OH radicals with toluene using a triplequadruple tandem-MS technique 2,4-hexadienedial has been tentatively identified along with 6-0~0-2,4-heptadienal and hydroxy-6-0~0-2,4-heptadienal(5,6‘). In the aqueous phase, E,E-2,4-hexadienedial has been identified as a product of the irradiation of oxygen-containing saturated aqueous solutions of benzene with X-rays as early as 1959 (7). Wei et al. (8) have identified the same compound as a product of the UVirradiation of oxygen-containingliquid benzene. However, in a recent pulse radiolysis study of the reaction of OH with benzene in oxygenated aqueous solution, no evidence could be found for the formation of muconaldehydes using HPLC and GClMS techniques (9). Another aspect of the significance of 2,4-hexadienedials is their occurrence in the metabolism of benzene in animals (10). 2,4-Hexadienedialsare highly toxic compounds, the LDso of E,E-2,4-hexadienedialis 6.7 mglkg for mice (11). These compounds have recently been given a great deal of attention in studies of the toxicity of benzene since they are thought to be responsible for the hematotoxicity and myelotoxicity and possibly the carcinogenicity attributed to this compound (12). In light of the high toxicity of muconaldehydes and their probable occurrence as intermediates in the oxidation of aromatics, the Z,Z-, E&’-, and E,E-isomers of 2,4-hexadienedial and the Z,Z- and E,E-isomers of 2-methyl-2,4hexadienedial have been synthesized, and some aspects of their atmospheric chemistry have been investigated. Results are presented on the U V and IR spectra of the compounds, their photochemistry,and kinetic and mechanistic data on their reactions with OH radicals. Preliminary results from studies of reactions of the compounds with O3 are also presented. To our knowledge, this study represents the first report of the gas-phase atmospheric chemistry of muconaldehydes.
Experimental Section Introduction
Kinetic, Photolysis, and Product Studies. The experiments
Despite intensive research during the last two decades, the primary ring-opening products in the OH-initiated atmospheric oxidation of aromatic hydrocarbons have still not been identified. Several mechanisms have been postulated they all involve an initial addition of OH radicals to the ring to form hydroxycyclohexadienyl radicals and a subsequent addition of O2to these radicals to form the corresponding peroxyradicals (1-3). The OH adducts have been detected spectroscopically (2, 31, and rate coefficients are now available for the reaction of many of these adducts with 0 2 (1, 4). The possibilities suggested for the further reactions of the hydroxycyclohexadienylperoxy radicals include (i) rearomatization with the formation of phenol-type compounds and HOz; (ii) formation of a hydroxycyclohexadienyloxyradicalfollowed by ring opening with the formation of 2,4-hexadienedials, which are also referred to in the literature as muconaldehydes; and (iii) cyclization of the hydroxycyclohexadienylperoxy radical with a subsequent addition of a second O2 and further reaction to products. Although 2,4-hexadienedialshave not yet been positively identified in the atmospheric oxidation of aromatics, there is evidence from some studies that they might be primary ring-opening products. In gas-phase product studies of
concerning the kinetic and product studies of the reactions of the compounds with OH radicals and O3 were carried out in a 1080-L quartz-glass chamber that is surrounded by 32 low-pressure mercury lamps (UV lamps, Amax = 254 nm) and 32 superactinic fluorescent lamps (VISlamps, 320 < 1 < 480 nm, Am== 360 nm). Details of the experimental setup can be found elsewhere (13). The experiments on the photochemistry of the hexadienedialswere also carried out in the 1080-L reactor; however, the UV spectra of the compounds were measured in a 480-L Duran glass reactor in combination with a spectrometer (SPEX, focal length 2 cm) and a diode array detector (PAR 1412), details of which can be found elsewhere (14). The concentration-time profiles of reactants and products were monitored in the 1080- and 480-L reactors in situ by long-path FT-IR using optical path lengths of 492 and 51.6 m, respectively, and a spectral resolution of 1 cm-’. The hexadienedials were transferred to the reaction chambers by gently heating weighed quantities of the substance in a stream of dry nitrogen. All other substances were injected directly into the reactors using calibrated gas-tight syringes. The reactants and products were monitored at the following absorption wavenumbers (in cm-’): 2,Z-2,4-hexadienedial1,
2322 m ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 9, 1995
0013-936W95/0929-2322$09.00/0
0 1995 American Chemical Society
1702; E,Z-2,4-hexadienedia11, 1709; E,E-2,4-hexadienedia11, 1716;Z,Z-2-methyl-2,4-hexadienedial, 1701;E,E-2-methyl2,4-hexadienedial, 1712; E-2-butene, 2949, 963; 03,1050; E-butenedial, 1724; Z-butenedial, 1707; glyoxal, 2847; methylglyoxal, 2835; formaldehyde,2802; maleic anhydride, 1805; formic acid, 1105; CO, 2176. The experiments were performed at a total pressure of 1000 mbar of synthetic air and a temperature of 296 f 2 K using irradiation times between 5 and 150 min. The photolysis of methyl nitrite with the VIS lamps in the presence of NO and molecular oxygen was used to produce OH radicals (15):
+ hv - CH,CO + NO CH,O + 0, - CH,O + HO, HO, + NO - NO, + OH
CH30N0
In the kinetic studies as well as in the product studies, the initial concentration of CH30NO was typically at about 0.61.0 ppm (1 ppm = 2.46 x 1013molecules ~ m at- 298 ~ K). The concentrations of the carbonyls and the reference hydrocarbon E-2-butene were in the range 0.25-1 x 1014 molecules ~ m - Avalue ~. of k = 6.4 x lo-" molecules cm-, s-I (1) has been used as rate coefficient for the reaction of OH radicals with E-2-butene in the analysis of the kinetic data. The OH radicals generated in the photolysis systems will react with the carbonyls and also with the reference hydrocarbon:
+ carbonyl - products, k, OH + reference - products, k2 OH
Under the experimentalconditions, the loss ofthe carbonyls due to photodissociation and wall reactions was found to be negligible. The decay of the carbonyls and the reference hydrocarbon are therefore governed by the rate laws I and 11, respectively:
-
d [carbonyl] = k,[OH][carbonyl] dt
(1)
[carbonyl] '0)
[carbonyl]
=
2 (: In
Gas-PhaseFT-IR Spectra of the Hexadienedial Derivatives. Figure 1, panels a-e, shows the IR spectra of the various hexadienedial derivatives in the range 2000-800 cm-I. The insets in the panels show the region from 3500 to 2600 cm-l. Also included in Figure 1 are the structural formulas of the compounds. It was not possible to transfer the hexadienedials quantitatively to the reaction chamber at room temperature. As a consequence, IR absorption coefficients for the compounds have been determined in a small heatable cell at temperatures between 355 and 385 K. The method and experimental setup used is described in detail in Mihalopoulos et al. (17). The following values have been determined for the IR absorption coefficients (base e): E,Z-2,4-hexadienedial (1709 cm-') = 59 x loT4ppm-' m-l (= 2.4 x lo-'' cm2 molecule-') E,E-2,4-hexadienedial (1716 cm-') = 59 x ppm-' m-' (=2.4 x cm2 molecule-')
'
reference] reference]t:)
Results and Discussion
E,E- 2 -methyl-2,4-hexadiene dial (17 12 cm- 1 = 41 x ppm-' m-' (=1.7 x lo-'' cm2 molecule-')
direference] = k,[OH] [reference] dt
After integration and combination of I and I1 eq I11 results:
In(
Methods of Preparation of the 2,4-Hexadienedial Derivatives. The Z,Z-isomers of each 2,4-hexadienedial and 2-methyl-2,4-hexadienedial were prepared using the procedure described by Golding et al. ( l a ,which involves the NaI04oxidation of the corresponding cyclohexadiene1,2-diols(Z-3,5-cyclohexadiene-1,2-diol and Z-3-methyl3,5-cyclohexadiene-l,2-diol, respectively), which were purchased from Fluka Chemicals. The resulting Z,Z-2,4hexadienedial was thermally isomerized into the E,Z-form and catalyticly into the E,E-form. Z,Z-2-Methyl-2,4-hexadienedial was thermally isomerized into the E,E-form. Purities of 99%, 98-99%, 97-98%, 99%, and 95% were determined for E,E-, E,Z-, and Z,Z-2,4-hexadienedialand E,E- and Z,Z-2-methyl-2,4-hexadienedial, respectively.These impurities were identified and quantified by 'H-NMR spectroscopy. In all cases, the impurities consisted of EIZisomers of the respective hexadienedials. Due to their low concentrations and the basically identical chemical behavior of the products and the impurities, no correction was made for them in the experiments.
(111)
where [carbonyl],,and [referenceltoare the concentrations of the reactant and reference organics, respectively, at time to; [carbonyl] and [reference] are the corresponding concentrations at time t; and kl and kZ are the rate coefficients for the reaction of OH with the carbonyl and reference compounds, respectively. Hence plots of In ([carbonyl]t o / [carbonyl]3 against In ([reference][,/ [reference][) should yield a straight line of slope k l / k2 with zero intercept. In the ozone experiments, as in the OH experiments, the initial concentrations of the hexadienedialswere in the range 0.25- 1 x l O I 4 molecules ~ m - Ozone ~ . concentrations were in the range of 15-18 ppm.
The calibrations were subject to a large degree of scatter due to the low volatility of the compounds, and the values should only be treated as approximations. The absorption coefficients are much higher than the values for the CO stretch of simple aldehydes, a-dicarbonyls, acrolein, and methacrolein, which are typically in the order of 15-23 x ppm-I m-l (18).Although the values are associated with a large degree of uncertainty, it is felt that the higher values are probably not unreasonable since as will be discussed below the values obtained for the gas-phase UV absorption cross-sections based on the IR absorption coefficients are in reasonable agreement with the more accurate liquid-phase UV absorption cross-sections. Furthermore, high values are also in accordance with the much higher measured UV photolysis frequencies for these compounds compared to those for simple aldehydes. The most prominent absorption in the IR spectra is the carbonyl band just above 1700 cm-I, which corresponds to an a,B-unsaturated aldehyde. In the region from 2700VOL. 29. NO. 9, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2323
I) Gas-phase UV-spectra of hexadienedials
a) Z,Z-2.4- hexadienedial
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c) E,E-2,4-hexadienedial
c) E,E-2-rnethyl-2,4-hexadienedial 200
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220
240
260
280
320
300
1
360
340
11) Liquid-phase UV-spectrum of E,E-2,4-hexadienediaI
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3400
3200
3000
2800
2600
1106 - 0
2000
1800
-
1600 1400 1200 Wavenumber (cm')
h
1000
f
FIGURE 1. Infrared spectra of hexadienedials in the range 2000-
800 cm-': (a) Z,Z-2,4-hexadienedial; (b) €,Z-2,4-hexadienedial; (c) €,€-2,4-haxadienediaI; (d) LZ-2-methyl-2.4-hexadienedial; (e) €,E2-methyl-2,4-hexadienedial.The insets in the panels show the region 3600-2600 cm-l. The structural formulas of the hexadienedials are also shown in the panels.
2900 cm-l, the aldehydic C-H stretching vibrations are visible, and in the region 3380-3400 cm-l the overtones of the CO stretch are visible. Stretching vibrations due to the C=C double bonds can be identified in the region from 1570-1600 cm-l and are somewhat lower than the values usually found for this vibration due to the a,B-unsaturated carbonyl character of the investigated compounds. For the E,E-compounds,the strong absorptions at around 1100 cm-1 can be attributed to C-H out-of-plane deformation vibrations, which are somewhat higher than those generally observed because of the conjugated diene-dialdehyde character of the 2,4-hexadienedial. W Absorption Spectra. The liquid-phase W absorption spectra of the isomers of 2,4-hexadienedial and 2-methyl-2,4-hexadienedial have been recorded in the range 200-400 nm in cyclohexane using a double-beam spectrometer (UV-21OAShimadzu). The recorded spectra and measured absorption coefficients are in good agreement with those already published in the literature (16). The 2324
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9.1995
n
4
1
l
d,,
0 200
, , , , , ,\, 260 280 300 Wavelength (nm)
,
220
240
,
,
320
~
340
360
FIGURE2. Liquid- and gas-phase UV spectra of the hexadienedials. The lower trace shows the liquid-phase spectrum of €,€-2,4hexedienedial recorded in cyclohexane. The upper trace shows the gas-phase spectra of (a) €,Z-2,4-hexadienediaI, (b) €,€-2,4-hexadienedial, and (c) €,H-rnethyl-2,4-hexadienedial. The absorption cross sections for the gas-phase spectra are only estimates; see text for details.
lower trace of Figure 2 shows, as an example, the liquidphase UV spectrum of E,E-2,4-hexadienedia11. In cyclohexane solution, the isomers of hexadienedial show a broad absorption between 240 and 300 nm with two maxima of approximately equal strength at 268 and 279 nm. The spectrum of 52-2-methyl-2,4-hexadienedial in cyclohexane shows a broad featureless absorption between 240 and 320 nm whereas that of the E,E-isomer shows two maxima at 280 and 292 nm. The values of the absorption cross-sections for the maxima of all the compounds are approximately equal with values of -6 x lo-" cm2molecule-'. The compounds all show a very weak and broad absorption between 300 and 450 nmwitha maximum at -360 nm. This band gives rise to the yellow color of the pure compounds and is a factor of -230 weaker than the absorptions in the 240-300-nm region. The upper trace of Figure 2 shows the gas-phase W absorption spectra of (a) E,2-2,4-hexadienedial1,(b) E,E-2,4-hexadienedia11, and (c)E,E-2-rnethyl-2,4-hexadienediala The gas-phase spectra show more structure compared to the liquid-phase spectra. As mentioned above, it has not been possible to transfer these compounds quantitativelyto the gas phase: the values of the absorption cross-sections given in the upper trace of Figure 2 are estimates based on a calculation of the concentration of the compound from the simultaneously
TABLE 1
UV Absorption Cross=Sectionsfor Muconaldehydes (in Units of cm2 molecule-’) for 290-355 nm Region and 5=nm Intervals wavelength interval (nm) 290-295 295-300 300-305 305-310 310-315 315-320 320-325 325-330 330-335 335-340 340-345 345-350 350-355
f,Z-hexadienedial 1.45 7.15 3.90 2.09 1.04 4.72 1.77 8.72 6.11 5.99 1.11 1.18 9.21
x lo-’* x x x x x x x x IO-” x IO-” x x x
E,€-hexadienedial 1.67 1.19 7.51 4.39 2.45 1.41 8.13 5.55 4.30 2.73 3.43 3.02 1.06
x x x x x x x x x x
lo-’’
x
x x
lo-”
€,€-2-methyl2.4-hexadienedial 1.74 8.54 5.57 4.29 2.99 1.88 1.12 6.70 4.25 2.66 1.83 1.11 4.12
x
x x x x x x x x lo-’’ x’IO-*O x x x IO-”
recorded IR spectra using values for the absorption coefficient of the carbonyl stretching frequency determined in the calibration experiments described above. The values should be treated with caution until more accurate calibrations can be performed. The good observed agreement between the cross-sections determined in the gas- and solvent-phases, however, suggests that the gas-phase absorption cross-sections are not unreasonable and are probably reliable to within a factor of 2. Table 1 lists the estimated average cross-sections for the hexadienedials for the region 290-355 nm (in units of cm2 molecule-’) for intervals of 5 nm. Assuming a photolysis quantum yield of unity, these cross-sections can be used to estimate photolysis frequencies Jg for the hexadienedials in the atmosphere. For noontime July 1 at a latitude of 40” N, the following photolysis frequencies have been calculated (19, 20):
suggests that the quantum yield for photodissociation will be very low for these molecules. This contrasts sharply with the photolysis behavior of unsaturated 1,4-dicarbonyls reported recently by Bierbach et al. (18)where photodissociation was a major pathway. It is expected that, for the hexadienedials, photolysis under atmospheric conditions will result primarily in photoisomerization. (B) PhotolysiswithWLamps. All of the hexadienedials were rapidly photolyzedwith the Wlamps. The photolysis frequencies were too fast to be measured accurately with all of the 32 W lamps switched on, and consequently measurements were made with only two lamps. The decay followed first-order kinetics and led to the following values of the photolysis frequencies in 1000 mbar of synthetic air: E,Z-2,4-hexadienedial k = (5.05 k 0.41) x
s-l
(t =
198 s)
E,E-2,4-hexadienedial k = (5.84 f 0.38) x
s-l
(t=
171 s)
E,E-2-methyl-2,4-hexadienedial k = (5.89 f 0.51) x s-l
(z = 170 s)
Replacing synthetic air with N2 as the bath gas had no significant effect on the measured photolysis frequencies, indicating that the decay of the hexadienedials is probably due mainly to photolysis and not to secondary reactions with radicals generated during the photolysis. The extremely short UVlifetimes of only about 3 min, even though only two of the 32 W lamps were in operation, can be explained by the structure of the 2,4-hexadienedials. These diene-dicarbonyl compounds have four conjugated doublebonds (two carbonyl and two C=C double bonds), which corresponds to a chromophore that strongly absorbs light in the ultraviolet region. This is confirmed by the solution and gas-phase W spectra shown in Figure 2, lower and upper traces, respectively. The products observed in the W photolysis of the E,Z-2,4-hexadienedial: Jg = 1.4 x s-l hexadienedials were also independent of whether the photolysis was performed in synthetic air or nitrogen as the bath gas. It should be noted that the system is never E,E-2,4-hexadienedial: Jg = 4.1 x s-l completely O2 free; even when using N2 as the bath gas, traces of O2 up to 100 ppm can be present in the reaction E,E-2-methyl-2,4-hexadiendial: lo= 4.2 x s-l system due to minor leaks. Of the products formed, only CO could be unambiguously identified. Figure 3, panels Photolysis of the Hexadienedials. (A) Photolysiswith a-c, shows the residual spectra for (a) E,Z-2,4-hexadieneVIS Lamps. The Z,Z-isomers of 2,4-hexadienedial and dial, (b) E,E-2,4-hexadienedia11,and (c) E,E-2-methyl-2,42-methyl-2,4-hexadienedial thermally isomerized to the E,Zhexadienedial. Among the most prominent features are isomers and had lifetimes (t= l/k, k is the measured firststrong ketene and carbonyl absorptions at approximately order decay coefficient) of 13 and 27 min, respectively. 2130 and 1760 cm-l, respectively. The ketene and carbonyl Because of the rapid thermal isomerization, photolysis absorptions disappear on continued irradiation, and a new experiments on the Z,Z-isomers were not attempted. carbonyl absorption at approximately 1730 cm-I appears. Irradiation of E,Z-2,4-hexadienedialand the E,Z-2-methylThe compound(s) giving rise to the various absorptions are 2,4-hexadienedial formed from Z,Z-2-methyl-2,4-hexadi- not presently known. Ketene-type compounds could be enedial in 1000 mbar of synthetic air with the VIS lamps formed by aphotolytically allowed 1,3-Hshift (suprafacial) resulted in the formation of the E,E-isomerswith first-order or 1,5-H shift (antarafacial): photoisomerization rate coefficients of (9.56 f0.25) x and (1.22 f 0.04) x s-l, respectively. No degradation of the two E,E-isomers,(E,E-2,4-hexadienedialand E,E-2methyl-2,4-hexadienedial) was observed on irradiation with all of the 32 superactinic lamps. The analysis of the W absorption spectra for the three compounds above predicts photolysis frequencies/,, of the order of s-l. Although these photolysis frequencies are fairly high, the lack of product formation in the VIS photolysis experiments VOL. 29, NO. 9, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY m 2325
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1800
1600
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FIGURE 3. Residual infrared spectra obtained for (e) €,Z-2,4hexadienedial, (b) €,€-2,4-hexadienediaI, and (c) €,&2-methyI-2,4hexadienedial after 5 min irradiation with two of the UV lamps and subtraction of absorptions due to the compounds themselves and
co.
Alternatively, photolytic loss of an aldehydic H-atom followed by reaction with 0 2 could result in ketene dicarbonyl compounds:
050
075
H
The compounds formed in the above reactions can account for the carbonyl and ketene absorptions; they do not account, however, for the CO formed in the system. Because of the rapid nature of the photolysis, it is difficult to decide from the present studies whether CO is a primary or secondary product. The lifetimes of the ketenes formed in the UV photolysis of the hexadienedials were of the order of 30-40 min, Le., an order of magnitude longer than the lifetimes of the hexadienedials. Experiments using the VIS photolysis of CH30NO as the OH source show that the rate coefficient for the reaction of the ketene with OH is of the 2326 * ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 9,
1995
,
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125
FIGURE 4. Plots of the OH kinetic data for (a) €,E-2,4-hexadienediaI, (c) F,Z-2,4-hexadienedial, and (c) €,€-2-methyl-2,4-hexadienedial plotted according to eq 111 as described in the text. TABLE 2
Rate Coefficient Ratios kllk2 and Rate Coefficients k1 for the Reaction of OH Radicals with Isomers of 2,&Hexadienedial and 2-Methyl-2,4-hexadienedial at 298 f 2 K in 1000 mbar of Synthetic Air k1lk2
€,Z-2,4-hexadienediaI 1.71 i 0.17 €,€-2,4-hexadienediaI 1.37 10.05 €,€-2-methyl1.84i0.03 2,4-hexadienedial
Y
100
In([E-2-butene],,/ [E-2-butene],)
compound
H
,d
E.E-2-methyl-2.4-hexadienedtal
25, 2.0:
2126
)
3
10" x
4 (298 K)
(cm3molecule-' 109 i 11 88 i 3 11812
s-l)
literature this work this work this work
same order of magnitude as those determined for the hexadienedials. OH Kinetic Studies. Figure 4, panels a-c, shows the kinetic data for E,E-2,4-hexadienedia11, E,Z-2,4-hexadienedial, and E,E-2-methyl-2,4-hexadienedial obtained relative to E-2-butene and plotted according to eq 111. Each plot represents a minimum of three experiments and shows reasonable linearity. The resulting rate coefficient ratios kllk2 and calculated rate coefficients kl are listed in Table 2. The value for E,Z-2,4-hexadienedial is only approximate due to the large contribution from the E,Z to E,E photoisomerization of this compound. The IR spectrum of E,E2,4-hexadienedialproduced by the photoisomerization of E,Z-2,4-hexadienedial is not very different from that of E,Z2,4-hexadienedial. This leads to large uncertainties in the computer-aided analysis of the IR spectral data and consequently to large uncertainties in the rate coefficient determination for the reaction of OH with E,Z-2,4-hexa-
dienedial. The first-order E,Z to E,E photoisomerization rate for 2,4-hexadienedial was measured in separate experiments and the value of
o,,ol
a) E,Z-2,4-hexadienediaI
- .,
~
"1
kh,(E,Z-2,4-hexadienedial) = (9.56f 0.25) x IO-'S-~ obtained was taken into account in the determination of the rate coefficient using the procedure described in Bierbach et al. (18). As can be seen from Table 2, the 2,4-hexadienedials undergo rapid reaction with OH radicals. The rate coefficient for the reaction of OH with E,E-2-methyl-2,4hexadienedialis -35% faster than that of the nonmethylated derivative E,E-2,4-hexadienedialSThis is reasonable since the positive inductive effect of the methyl group increases the electron density in the C-2/C-3 double bond, which promotes the OH addition to the double bond. Such an increase is also clearly evident between the rate coefficients for other dienes, e.g.,butadiene and 2-methyl-1,3-butadiene ( I ) . The rate coefficients for the hexadienedials are also of similar magnitude as those reported for a number of substituted penta- and hexadienes and also 2-methyl-1,3butadiene ( I ) . For the hexadienedials, the OH radical reaction can occur by both H-atom abstraction from the -CHO group and OH addition to the conjugated double bond system. For crotonaldehyde and methacrolein, both pathways are known to occur with about equal importance whereas for acrolein the major pathway is H-atom abstraction (1). Abstraction and addition pathways are thought to be of approximately equal importance for the reaction with OH radicals with unsaturated 1,4-dicarbonyl compounds (18). There are a number of factors that can influence the effect of the carbonyl functional group on the reactivity of the double bond (i) the negative inductive and mesomeric effect of the CO group that decreases OH addition to the double bond, (ii)resonance stabilization of radicals formed either by abstraction or addition, and (iii) a contribution of H-atom abstraction from the aldehyde group to the rate coefficient. Generally, the addition of an aldehydic functional group adjacent to a double bond results in a substantial loss of reactivity of the bond compared to the unsubstituted analogue due to inductive effects, at least for monoalkenes. This can be seen in a comparison of the rate coefficient pairs propene/acrolein and E-2-butenel crotonaldehyde ( I ) . However, the addition of a second aldehydic group on the other side of the double bond does not further decrease the reactivity but rather increases the reactivity again, e.g., E-2-butene/crotonaldehydelZ-butenedial ( I , 18). This also appears to be the case when acetyl groups are attached to the double bond; compare, for example, the rate coefficient for E-2-butene with 40x02-pentenal and 3-hexene-2,5-dione ( I , 18). Although H-atom abstraction is certainly a contributory factor for the aldehydic groups, the observation of the same increase for the acetyl group implies that resonance stabilization of the intermediate radicals is also an important factor. As stated above, in the case of the reaction of 2,4-hexadienedials with OH radicals studied here, the measured rate coefficients are approaching the gas kinetic limit and are similar to the unsubstituted dienes. Since aldehydeH-atom abstraction reactions are usually of the order of 1 x lo-" cm3 molecule-' s-l, it can be estimated that this channel will contribute less than 10% to the overall reaction rate.
- 1 o,30
1
b) E,E-2,4-hexadienediaID
- @ O
c) E,E-2-methyl-2,4-hexadienedial
0.25
o
n del
.
***
;
a
5
n
.
e
z
:
$ 8 8 ,
!O
30 40 50 60 reaction time Cminl
70
FIGURE 5. Concentration-time profiles of the products identified in the OH initiated oxidation of (a) E,Z-2,4-hexedienediaI, (b) €,EfA-hexadienedial, and (c) €,€-2-methyl-2,4-hexadienedial: ( 0 ) E-butenedial, (+) glyoxal, (A)methylglyoxal, (0)HCHO, (0)maleic anhydride, ( 0 ) HCOOH; (v)CO. The concentration for E-butenedial has been estimated using a value of 2.3 x lW3ppm-' m-l (base e) for the absorption coefficient (see text).
The maintainment of reactivity of the conjugated double system in the presence of two aldehydic functional groups can probablybe attributed largely to resonance stabilization of the initially formed OH adduct. The observed product distribution also supports dominance of the addition pathway as is discussed below. Products from the Reaction of OH Radicals with 2,4Hexadienedials. The product analyses were carried out using the photolysis of CH30NO/N0/02mixtures with the VIS lamps as the OH radical source. Figure 5, panels a-c, shows the concentration-time profiles of the products observed for (a) E,Z-2,4-hexadienedia11,(b) E,E-2,4-hexadienedial, and (c)E,E-2-methyl-2,4-hexadienedial. Reliable IR absorption coefficients are not available for the product E-butenedial (18). The concentrations of this compound given in Figure 5 are estimations based on a value of 2.3 x ppm-' m-l (base e) for the absorption coefficient; this is the value determined in this laboratory for acrolein, a structurally similar compound. E-Butenedial, glyoxal, HCHO, and CO are major products of the OH-initiated oxidation of both E,Z-Z,4-hexadienedial and E,E-2,4-hexadienedial. Interestingly, for E,Z-2,4-hexadienedia11, only E-butenedial but not Z-butenedial was observed in the product spectrum. This indicates that the primary OH adduct, a delocalized radical, can readily isomerize. However, at later reaction times there will also be a significant contribution to the formation of E-butenedial from the E,Z VOL. 29. NO. 9, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY.
2327
to E,E photoisomerization of the E,Z-2,4-hexadienedia11. In the initial stage of the reaction, the concentrations of glyoxal and E-butenedial are approximatelyequal; the concentration of E-butenedialfalls off rapidlywith increasing reaction time due to photolysis and reaction with OH radicals. As the reaction time progresses, HCOOH and maleic anhydride (2,5-furandione) are formed. The maleic anhydride is a product of the further reactions of E-butenedial(18,211,as evidenced by the concentration-time profiles of these two species. Two mechanisms have been proposed for the formation of maleic anhydride from butenedial, details of which are given in Bierbach et al. (181. For 2-methyl-2,4-hexadienedial, the OH-initiated oxidation resulted in the formation of E-butenedial, glyoxal, methylglyoxal, HCHO, and CO as major products. In the initial stages of the reaction, the concentrations of methylglyoxal and E-butenedial were similar, but that of Ebutenedial fell off with time and the formation of HCOOH and maleic anhydride was observed. Formation of formaldehyde, formic acid, and CO was observed in all of the systems; these are all products of the photolysis of the OH source methyl nitrite. Formic acid and, to a lesser degree, formaldehyde can also result from desorption from the chamber walls. The formaldehyde and CO concentrations given in Figure 5, panels a-c, have been corrected for contributions from the methyl nitrite photolysis and should be considered as only approximate. The correction factor was determined by running an experiment under the same conditions employed in the product studies of the hexadienedials, but without the hexadienedial itself. The concentrations of formaldehyde and CO formed from methyl nitrite as a function of time in this experiment was subtracted from the amounts formed in the OH product studies also as a function of time. Carbon dioxide could not be quantified due to the fact that the spectrometer was flushed with dry air containing COZ,resulting in saturation of the COz absorption bands. For E,Z-2,4-hexadienedia11, a somewhat steeper rise and subsequent sharper falloff of the concentrations of glyoxal and E-butenedial can be observed in Figure 5, panel a, compared to the other two muconaldehydes investigated. For these aldehydes, a similar but not so pronounced behavior is observed at later reaction times. The reasons for the apparently higher reactivity in the E,Z-2,4-hexadienedial experiment are not clear. After the subtraction of all the identified compounds from the product spectra, residual absorptions remained that could not be attributed to any known compound. The residual absorption spectra obtained for E,Z-2,4-hexadienedial, E,E-2,4-hexadienedia11, and E,E-2-methyl-2,4-hexadienedial in the range 1900-700 cm-' are shown in Figure 6, panels a-c, respectively. No significant absorptionswere visible in the spectral range associated with C-H valencebond vibrations (3200-2600 cm-'). AU the residual absorptions are very weak, suggestingthat the concentrations of products giving rise to the absorptions must be low and that they can only play a minor role in the atmospheric oxidation of hexadienedials. The wavenumbers displayed in Figure 6, panels a-c, represent the maximum intensity of the corresponding absorption bands. Due to the very low intensities in the residual spectra, the signal-to-noise ratio is low, and the uncertainty in the position of the maxima is relatively high, about f 5 cm-l. The carbonyl range of the residual spectrum of E,Z2,4-hexadienedial is identical to that of E,E-2,4-hexadi2328 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9. 1995
a) E,Z-2,4- hexadienedial IHCHO
b) E,E-2,4-hexadienediaI
1103
c) E,E-2-methyl-2,4-hexadienedial
IHCHO,
1900
1700
1500,
1300
1100
900
700
Wavenumber (cm')
FIGURE 6. Residual product spectrum obtainedfor the OH-initiated oxidation of (a) €,Z-2,4-hexadienedial, (b) €,E-2,4-hexedienedial, and (c) €,€-2-methyl-2,4-hexadienedial after 75 min reaction time and subtraction of all identified products.
enedial (panels a and b), whereas that of E,E-2-methyl2,4-hexadienedial (panel c) shows a similar residual spectrum but with slight shifts of the maxima. The three spectra are also similar in the fingerprint region, visible differences can be attributed to slight over- or underestimations of the subtraction factors of the various products. Bands around 1725,1290,and 790 cm-' are characteristic for peroxynitrate-typecompounds and bands around 1660, 1290,and 845 cm-' are characteristic for pitrates (22).Bands in these regions are present in the residual spectra, and it is probable that low concentrations of these types of compounds are being formed in the reaction system. The absorptions around 1830cm-1 may be due to the CO stretch from a PAN-type compound; however, absorptions in this region are also observed in the ozonolysis of the hexadienedials (see below), which suggests that another compound is probably responsible for this absorption. As possible candidates, multifunctional compounds of the type H-CO- CH=CH -CH (OH)-CO- CHO and H- CO-CH= CH-C(OH)2-CO-CH0 may be considered (23). Because of the complexity and reactivity of the hexadienedials,many other reactions will be possible, and much effort will be required to identify the compounds giving rise to the unassigned IR absorptions. Reaction Mechanism for Reaction with OH. The formation of E-butenedial, methylglyoxal, and glyoxal can be accounted for by the simple mechanism outlined in
account for a high proportion of the gas-phase products. Under the conditions employed, the formation of aerosols is not expected to be very important. There were no indications either visually in the reactor or in the IR spectra for aerosol formation. A noteworthy feature of the OH oxidation of E,Z-2,4hexadienedial is the fact that only E-butenedial, and not Z-butenedial, was observed in the product spectrum. This indicates that the primary OH adduct, a delocalized radical, might readily isomerize. Another important feature is the formation of only one methylated product, methylglyoxal, from E,E-2-rnethyl-2,4-hexadienediale Methylglyoxal is 02’$N ’O2 formed through the addition of the OH radical to the 02”+N02 methylated double bond of E,E-2-methyl-2,4-hexadienedial. An OH addition at the other double bond would result in the formation of glyoxal and E-2-methylbutenedial. No evidence could be found for the presence of E-2-methylbutenedial in the product spectra. In analogy to the formation of maleic anhydride from E-butenedial (It?), reactions of 2-methylbutenedial can lead to the formation of 2-methyl maleic anhydride (citraconic anhydride). An enrichment of citraconic anhydride would, therefore, be an indicator for the formation of 2-methylbutenedial, even if the concentration of the latter compound was below the detection limit because of its possible high reactivity. However, in the experiments no citraconic anhydride was found. Further, the concentration-time profiles of glyoxal R I formation always showed an induction period, which is o”c\c//o more consistent with the further reactions of E-butenedial I being its major source rather than direct formation in the H reaction of OH with E,E-2-methyl-2,4-hexadienedial, All FIGURE 7. Possible reaction mechanism for the OH-initiated of this evidence supports that the addition of OH to the oxidation of muconaldehydes. methylated double bond at the C-3 position is the dominant Figure 7. For the unsubstituted hexadienedials, the addition site of attack in the reaction of OH with E,E-2-methyl-2,4of the OH radical and subsequent addition of molecular hexadienedial. The addition at this position can be oxygen can result in the formation of three different ROz attributed to the strong positive inductive effect of the methyl group, which increases the electron density in the radicals. In the case of E,E- and E,Z-2,4-hexadienedia11, the position of addition and resulting products will probably adjacent double bond, thereby increasing the probability of OH addition to this bond. be indistinguishable; however, for 2-methyl-2,4-hexadienedial the different addition possibilities should result in Another reaction channel open to the hexadienedials is different end products. In any case, under the present the abstraction of an aldehydic H-atom; however, as experimental conditions, the resulting peroxy radicals are discussed above, the measured rate coefficients suggest expected to react with NO to yield the corresponding oxy that this channel is probably not very significant. This is radicals (1). The other possible channel that would result also supported by the observed concentration-time bein the formation of a nitrate is expected to be very minor havior of the products. An abstraction pathway would not (1).The oxyradicalscan undergo cleavage of the C-C single lead directly to the formation of glyoxal (methylglyoxal)as bond, followed by an 0 2 initiated H-atom abstraction to a primary product and thus could not explain the observed yieldE-butenedialand glyoxal or methylglyoxal. The further parallel “prompt”rises in the concentrationsof E-butenedial reaction of E-butenedial will also lead to the formation of and glyoxal (methylglyoxal in the case of E,E-2-methyl2,4-hexadienedial) in the first about 10 min ofthe reaction. glyoxal (18). The oxyradicalformed from the peroxyradical might also undergo H-atom abstraction with molecular Furthermore, an H-atom abstraction channel would proboxygen, which would result in the formation of 5-hydroxyably lead to the formation of a ketene-type product, as has 1,4,6-trioxo-2-hexene(R = H) or 5-hydroxy-5-methyl-1,4,6- been observed in the 254-nm photolysis of the hexaditrioxo-2-hexene (R = CH3). This variant, however, would enedials. Since ketene formation was not observed, this appear to play only a minor role in the OH-initiated also supports the idea that H-atom abstraction is of minor oxidation of hexadienedials. First, the absorptions of the importance. products remaining after subtraction of all identified Products from the Reaction of O3 with 2,4-Hexadicompounds (Figure 6, panels a-c) are, as discussed above, enedials. Figure 8, panels a-c, shows the concentrationextremelyweak, indicating low concentrations. Secondly, time profiles for the products observed in the reaction of no absorptions due to an OH vibration were discernible in 0 3 with E,Z-2,4-hexadienedia11, E,E-2,4-hexadienedia11, and the spectra even though the compounds 5-hydroxy-1,4,6E,E-2-methyl-2,4-hexadienedial. The products identified trioxo-2-hexene and 5-hydroxy-5-methyl-1,4,6-trioxo-2in the ozonolysis of the hexadienedials are the same as hexene are expected to be comparablystable. Theweakness those observed in the OH radical reaction studies. In all of the residual absorptions in Figure 6 , panels a-c, is a cases CO, HCHO, HCOOH, E-butenedial, glyoxal, and strong indication that the compounds shown in Figure 5 maleic anhydride are observed: and in the case of E,E-2VOL. 29. NO. 9 . 1 9 9 5 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2329
,
r
_
.
a) E3Z-2,4-hexadienedia1
a) E.Z-2.4-hexadienedial
~
I
0 61
1
0,5t 041
0.31 0.;
0.45 m
E 0.
Y Q
'F
1
h) E.€-2.4-hexadienedial
0,401
1
b) E,E-2,4- hexadienedial
0.35t
E.E-?-methyl-2,4-hexadienediaJ
c)
c) E,E-2-methyl-2,4-hexadienedial
p
I
i
t 0 4-
v
n
t
0 3r
01---
\&qn:-
I Yl, *-
* * o O0 . . - . ' b o . ~
g:o88i
20
2
g
ti D
0
f
.
, , , , , , , , , 40 60 80 100 120 140 reaction time Cminl ,
I
h,
=
3,bD
2960
2920
2880
2840 D O::
1' 1
,J'1
I
FIGURE 8. Concentration-time profiles of the products identified in the ozonolysis of (a) €,2-2,4-hexadienediaI, (b) €,&2,4-hexadimedial, and (c) €,€-2-methyl-2,4-hexadienedial: ( 0 )E-butenedial, (4) glyoxal; (A)methylglyoxal, (0)HCHO; (0) maleic anhydride, (0) HCOOH; (v)CO. The concentration for E-butenedial has been estimated using a value of 2.3 x ppm-' m-l (base e) for the absorption coefficient (see text).
methyl-2,4-hexadienedial also methylglyoxaland acetic acid are observed. The concentration-time pattern of the products is similar in all three cases. The very pronounced formation of CO is probably due to reactions of the various possible Criegee biradicals that can be formed in the system (1, 24). As in the OH reaction with E,E-2-methyl-2,4hexadienedial, no formation of E-2-methylbutenedial could be observed, indicating probable preferential addition of 0 3 to the methylated double bond. Maleic anhydride is known to be a product of the photolysis or OH reaction of E-butenedial. Since maleic anhydride is unlikely to be a product of the ozonolysis and it is known from other studies that OH radicals can be formed in considerable yields in 03/alkene reaction systems (25-28), it is concluded that the formation of maleic anhydride in the present systems is due to the reaction of OH radicals with E-butenedial. From a comparison of the yields of E-butenedial measured in the OH and O3 reaction systems, it can be estimated that reaction with OH radicals accounts for approximately 30-40% of the observed decay of the hexadienedials and that a significant fraction of the identified products stem from the OH oxidation. Rate coefficients have not yet been determined for the reactions of 0 3 with the hexadienedials due to difficultiesin analyzing these compounds in the presence of large concentrations of an OH scavenger. Preliminary investigations, however, allow an upper limit of 5 1 x lo-" cm3 molecule-1 s-' to 2330 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 9, 1995
1900
1800
1700
1600
1500
1400
1300
1200
1100
Wavenumber (cm')
FIGURE 9. Residual product spectrum obtained for the 03-initiated oxidation of (a) €,Z-2,4-hexadienediaI, (b) €,&2,4-hexadienediaI, and (c) €,€-2-methyl-2,4-hexadienedial after 140 min reaction time and subtraction of all identified products.
be put on the rate coefficients for these reactions. This limit is not unreasonable when the reactivityof dienes such as butadiene and isoprene (1, 24) are taken into consideration along with the deactivating nature of the -CHO groups toward O3 attack (28). Figure 9, panels a-c, shows the residual spectra obtained for the ozonolysis of E,Z-2,4-hexadienedia11, E,E-2,4-hexadienedial,and E,E-2-methyl-2,4-hexadienedial, respectively, after 140 min reaction time and subtraction of all identified products. It is evident from these spectra that one or more major products have not been identified. As for the OH reaction, the residual spectra for E,Z- and E,E-2,4-hexadienedial (panels a and b) are virtually identical. In each spectrum, three major bands with PQR structure are visible; aweakC-H vibration at -2956 cm-I and two strong carbonyl absorptions with band centers at -1822 and 1768 cm-l. The steep rise in absorption at -1000 cm-' is due to 03. Between 1000 and 700 cm-', no significant absorptions were observed, and this region is therefore not displayed in Figure 9. In the region between 1760 and 1700 cm-l, there are also absorptions that can be attributed to carbonyl vibrations. In the residual spectra obtained for the 03/ E,E-2-methyl-2,4-hexadienedial reaction system (Figure9, panel c), there is a weak absorption at -2956 cm-l, which can be attributed to a C-H vibration. In the carbonyl region only one band with a center at 1809 cm-' is clearly discernible. In the region from 1760 to 1700 cm-l, there
TABLE 3
Comparison of Rate Coefficients k (Units, cm3 molecule-l s-l) for Reactions of Hexadienedials with 03 and OH and NO3 Radicals and Estimations of Their Tropospheric Lifetimes Za OHd
03‘
ko,
compound €,Z-2,4-hexadienediaI €,€-2,4-hexadienediaI
€,€-2-methyI-2,4-hexadienedial
1x 1x 2 x
ro,M 1.7 1.7 1.7
NO^ (4
kNoa (5.26 f 0.18) x (5.34 i 0.12) x (10.2 0.3) x
*
4.4 4.3 2.3
kon (10.9 i 1.1) x (8.8 f 0.3) x I O - ” (11.8 f 0.2) x lo-”
photolysisa OH (h)
rphotolyais=l/j(h)
1.6 2.0 1.5
1.9 0.7 0.7
Estimations are also presented for the atmospheric noontime photolytic lifetimes of the compounds for clear sky conditions. The kvalues are estimates based on preliminary experiments using the relative rate technique with propene and ethene as reference compounds. The starting concentrations were in the same range as for the product studies. Because of formation of OH radicals in the ozonolysis, the values listed represent upper limits. The lifetimes were calculated using a 24-h average O3 concentration of 7 x 10” molecules ~ m (30 - ppb) ~ (37). CThek values were determined using the relative rate technique with propene as reference compound (32).The thermal decompositionof N205was used as the source of NO3 radicals. The lifetimes have been calculated using a 12-h average nighttime NO3 radical concentration of 5 x 108 molecules cm-3 (20 ppt) (33).dThe kvalues are from this study (Table 1). The lifetimes have been calculated using a 12-h average OH radical concentration of 1.6 x lo6 molecules ~ m (29, - 30). ~ e The photolysis lifetimes have been calculated for 40” N latitude for noon July 1 using the photolysis frequencies j determined in this work; see text for details. a
are again residual absorptions that are probably due to carbonyl compounds. One prominent feature that is not present in the nonmethylated hexadienedials is a strong absorption with a band center at 1200 cm-l. Interestingly, this absorption is not observed in the OH radical reaction, implying that it is purely an ozonolysis reaction product. The compound giving rise to this absorption has not yet been identified, but the region and form of the absorption are typical for acetyl groups and esters. It is obvious from the absence of the 1200 cm-1 absorption in the nonmethylated hexadienedials product spectra that the presence of the methyl group has led to some change in the ozonolysis reaction mechanism. More detailed product investigations, which adequately take into account the contribution from the OH radical reaction, are required for all of the hexadienedials before any reaction mechanisms can be written for their reactions with 03. Atmospheric Lifetimes of Muconaldehydes. Atmospheric lifetimeshave been calculated for the hexadienedials due to reaction with OH radicals using the rate coefficients determined in this study. The values have been calculated using a 12-h average OH radical concentration of 1.6 x lo6 molecules ~ m (29,301. - ~ These lifetimes are compared in Table 3 with those estimated for other atmospheric sinks, photolysis, and reactions with O3 and NO3 radicals. The rate coefficients for the reactions with O3 are the limits determined in this study. The rate coefficients for the NO3 reactions were determined in this laboratory (32). For the estimates, an O3 24-h average concentration of 7 x 10” molecules ~ m (30 - ~ppb) (28) and a NO3 12-h average nighttime radical concentration of 5 x lo8 molecules cm-3 (20 ppt) (33)have been used. The photolysis lifetimes have been estimated from the UVspectra measured in this study. The calculations indicate that reaction with OH radicals and photolysis will be the major atmospheric sinks for the hexadienedials. However, the present studies suggest that photolysis will probably only result in photoisomerization. Therefore, reaction with OH will be the dominant sink for the compounds. Because of the very high rate coefficients for the reaction with OH radicals, loss due to photolysis and reactions with 0 3 and NO3 will play only very minor roles.
Conclusions The major atmospheric sink for the hexadienedials will be the reaction with OH radicals. Apart from photoisomer-
ization, photolysis and also reactions with NOs and O3 are not expected to play a significant role. Major products of the reactions of the hexadienedials with both OH radicals and 0 3 are E-butenedial and glyoxal and also methylglyoxal in the case of E,E-2-methyl-2,4-hexadienedial. All of these compounds have been observed in oxidation systems of aromatic hydrocarbons. The high reactivity of the hexadienedials toward OH radicals has several important consequences for their atmospheric chemistry. First, because of this reactivity it will be extremely difficult,using the currently applied smog chamber techniques, to establish whether or not hexadienedials are primary products of the oxidation of the simple alkylated aromatics. This will be particularly difficult in systems in which the 254-nm photolysis of H202is used as the OH source since in such systems apart from fast oxidation by OH radicals the hexadienedials will rapidly photolyze. Secondly, the high reactivity also implies that their steady-state concentrations will be extremely low and thus difficult to detect in the atmosphere. However, if formed, their oxidation will lead principallyto the formation of unsaturated 1,4-dicarbonyls,which have been detected in the atmospheric oxidation of aromatics.
Acknowledgments This workwas supported by the Bundesminister fiir Bildung, Wissenschaft, ForschungundTechnologie (BMBF),and the EC (EuropeanCommission) within the LACTOZ subproject of EUROTRAC. The authors thank Prof. B. T. Golding for helpful discussions and his assistance with the synthesis of the compounds investigated. B.K. thanks the Landesamt f i r Graduiertenforderung NRW for a scholarship.
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Received for review January 4, 1995. Revised manuscript received May 25, 1995. Accepted June 2, 1995.@ ES9500015 Abstract published in Advance ACS Abstracts, July 15, 1995.
