J . Phys. Chem. 1990, 94. 8036-8040
8036
electronic states. If the photochemical reaction occurs from the triplet state T I , reached by intersystem crossing from the S, state, c, relates to the So SI transition but 7,relates to the forbidden So transition. Actual triplet lifetimes are usually much TI shorter than the radiative limit but even so, typical triplet lifetimes of 1 MUScombine with typical emax values of -lo4 dm3 mol-' cm-l for the So SI absorption to give quite favorable photostationary state concentrations. However, the longer lifetime is obtained at the price of the Sl-Tl energy loss.
-
-
- -
VI. Concluding Remarks We may summarize our overall conclusions as follows: The combination of rate constants required for ideal chemical potential from a reversible photochemical reaction is highly implausible. However, real photochemical reactions are invariably irreversible to the sense that pLp< wR*. Thus a drop in the lifetime, and hence in the chemical potential, of R* below the ideal limiting value is of no practical consequence as regards the energy stored in P. However, the maximum energy storage efficiency from irreversible photochemical reactions is generally modest. The Einstein relation, which sets a fundamental limit on the radiative lifetime of the excited state of a given absorber, predicts limiting lifetimes up to -200 times smaller than those required for attainment of the ideal photostationary state composition in the molecular and semiconductor systems we have chosen as typical examples. This causes a moderate derogation from ideality in the output voltage V'or chemical potential. However, the actual
lifetime T of an excited state is usually considerably shorter than the radiative lifetime determined by interband recombination; this produces a further deterioration in I/' according to eq 36 and renders even the Einstein limit unattainable. We have considered only the uniform photostationary state and have not addressed the efficiency of the transport of the excited states or their daughter products to a collection point or plane such as the junction of a photovoltaic cell. Here intensely absorbing substrates score heavily over weakly absorbing ones, and semiconductors, with electronic diffusion coefficients of 1 0-3 m2 s-I, score heavily over liquid systems, in which molecular diffusion coefficients are lo4 mz s-I. This renders the collection of short-lived molecular products by diffusion a very tricky process. Indeed, in the most successful energy storing photochemical reaction, photosynthesis, there is no need for molecule transport in the crucial early stages.
-
-
Acknowledgment. We thank Dr. John Connolly and Dr. Alan Haught for helpful discussions, and the referees for their perceptive comments, in particular on the effect of shorter wavelength absorption in molecular chromophores. J.R.B. thanks the Natural Sciences and Engineering Research Council of Canada for an Operating Grant to support this research. Registry No. Ge, 7440-56-4; Si, 7440-21-3;GaAs, 1303-00-0;CdS, 1306-23-6. chlorophyll a, 479-61 -8; rhodamine 6G, 989-38-8; norbornndicnc. I 2 1-46-0, water. 7732- 18-5; tetracene. 92-24-0. 2.j-dlphcn~lox~17olc. 92-7 1-7
ARTICLES FTIR Studies of Reactions between the Nitrate Radical and Chlorinated Butenes I. Wangberg,* E. Ljungstrom, Department of Inorganic Chemistry, University of Goteborg and Chalmers University of Technology, S-412 96 Goteborg, Sweden
J . Hjorth, and G . Ottobrini Commission of the European Communities Joint Research Center-Ispra Establishment, I-21020 Ispra (Va), Italy (Received: March 31, 1989: In Final Form: May 8 , 1990)
The products formed in the reactions between the nitrate radical and 2-chloro-l-butene, 3-chloro-l-butene, I-chloro-2-butene, and 2-chloro-2-butene were studied by FTIR spectroscopy. The experiments were performed in synthetic air in a glass/Teflon reactor and using N205as the NO3 source. The principal products formed with chlorobutenes, where the chlorine atom was substituted at a carbon participating in the double bond, were acid chlorides, aldehydes, and NO,. The products formed from chlorobutenes with the chlorine atom substituted at a carbon next to the double bond were aldehydes, chlorinated carbonyl nitrate compounds, and NO2. The reaction rate constant for the initial reaction step for 2-chloro-I-butene was determined cm3 molecule-' s-' at 299 K by using a fast-flow system. to be 1.73 (10.31) X
The ~0~radical reaction with unsaturated hydrocarbons has been suggested as an important loss process for naturally emitted organic compounds such as isoprene and monoterpenes, as well as for alkenes of anthropogenic origin,l-3 The distribution of
or less stable products from the N03/alkene reaction is therefore of considerable interest. If stable nitrogen-containing compounds are they may Serve as a reservoir for and a mean Of transport of nitrogen oxides. Nitrated ComPounds are also SUSpected of causing health effects." If the nitrate radical reaction
Winer. A. M.: Atkinson. R.: Pitts. J. N.. Jr. Science 1984. 224. 156. (2) Finlayson-Pitts, B. J.: Pitts, J. N.. Jr. Atmospheric Chemisrry; Wiley: New York, 1986.
(3) Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Pitts, J. N., Jr. Enuiron. Sei. Technol. 1985. 19. 159. (4) Akimoto, H:;Hoshino, M.; Inoue, G.; Sakamaki, F.; Bandow, H.; Okuda. M. J. Enciron. Sei. Health 1978, A13(9), 677.
Introduction
(1)
0022-3654/90/2094-8036$02.50/0G 1990 American Chemical Society
Nitrate Radical-Chlorinated Butenes Reactions results in the re-formation of NO2 and the formation of reactive organic decomposition products, this may have an influence on regional ozone formation. To estimate the impact of various hydrocarbons on atmospheric processes it is necessary to know their reactivity. Experimental information about reaction rates and product distributions for nitrate radical reactions has become more and more available over the years. It would, however, also be desirable to predict such properties with greater confidence. Basic information leading toward this goal may be obtained, e.g., by studying series of compounds where some property is varied in a systematic way. No information on reaction products from butene/N03 is available. Previous work on propene7v8has shown that the NO3 radical preferentially adds to the double bond. Formaldehyde, acetaldehyde, nitroperoxypropyl nitrate, propylene glycol dinitrate, and other nitrated species have been identified among the reaction product^.^*^.^" Bandow et aLl0 proposed a mechanism based on product studies using long-path FTIR and GC-MS that accounted for the observed product distribution. The main stable end products were formaldehyde, acetaldehyde, propylene glycol dinitrate, and NO2. In a later study of the same system, using GC, GC-MS, and HPLC techniques, Shepson et aL5 confirmed the earlier work. In addition, they also observed considerable amounts of 1-nitroxy-2-propanone and nitrated propyl alcohols among the products. The observed products were accounted for by the detailed reaction mechanism proposed by these authors. Thus, from the literature it appears that the main features of the propene-NO, system are well understood.
Experimental Section The experiments were performed in a Teflon-coated glass reactor 1.5 m long and 60 cm in diameter. The reactor was equipped with a multiple reflection, White type mirror system, adjusted to give a total optical path of 84 m. This optical system was connected to a Bruker IFS 1 13 V FTIR spectrometer. Spectra were recorded at an instrumental resolution of 1 cm-l. All chlorobutenes were of reagent grade and distilled before use. The cis/trans ratios from the I-chloro-2-butene and the 2-chloro-2-butene samples were 94/6 and 48/52, respectively, as determined by gas chromatography. The simple aldehydes and acid chlorides found as products were identified by comparison with IR spectra of the pure compounds. The concentrations of chlorobutenes, NO2, NzOs, H N 0 3 , HCHO, CH3CH0, CH2CICH0, CH,CH2COCI, and CH3COCI were determined from their infrared spectral absorptions. The absorption cross sections were evaluated by in-house calibrations with the exception of formaldehyde, for which a literature value was used.I2 The chemicals used for calibration were of reagent grade and obtained from commercial sources. 3-Nitroxy-Zbutanol and 3-nitroxy-2-butanone were prepared by C. Lohse and H. Skov at the Department of Chemistry, University of Odense, Denmark. The experiments were made in synthetic air at 99 kPa. To minimize the water content in the reactor, the air was passed through a P 2 0 s scrubber. The NO, radical was prepared in the reactor by mixing ozone with excess nitrogen dioxide from a 1% commercial gas mixture in air, according to reactions 1 and 2. O3+ N O 2 NO3 + O2 (1)
+
-
NO3 NO2 (+M) N 2 0 5 (+M) (2) The ozone was formed in air by a silent discharge ozone generator. (5) Shepson, P. B.; Edney, E. 0.;Kleindienst, T. E.; Pittman, J. H.; Namie, G.R.; Cupitt, L. T. Environ. Sei. Technol. 1985, 19, 849. (6) Shepson, P. B.; Kleindienst, T. E.; Nero, C. M.; Hodges, D. N.; Cupitt, L. T.; Claxton, L. D. Environ. Sei. Technol. 1987, 21, 568. (7) Morris, E. D., Jr.; Niki, H.J . Phys. Chem. 1974, 78, 1337. (8) Japar, S.M.; Niki, H . J . Phys. Chem. 1975, 79, 1629. (9) Hoshino, M.; Ogata, T.; Akimoto, H.; Inoue, G.; Sakamaki, F.; Okuda,
M.Chem. Left. 1978, 1367.
(IO) Bandow, H.; Okuda, M.; Akimoto, H. J . Phys. Chem. 1980,84,3604. ( I 1) Akimoto, H . ; Bandow, H.; Sakamaki, F.; Inoue, G.;Hoshino, M.; Okuda, M. Environ. Sci. Technol. 1980, 14, 172. (12) Cantrell, C. A,; Stockwell, W. R.; Anderson, L. G.; Busarow, K. L.; Perner, D.;Schmeltekopf, A,; Calvert, J. G.;Johnston, H. S.J . Phys. Chem. 1985, 89, 139.
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8037 After the N 2 0 5preparation, the organic compound was introduced into the reactor with a stream of synthetic air. The typical initial concentrations of N 2 0 5and chlorobutene were 10-20 ppm, with one of the two in excess. In experiments with 3-chloro-l-butene, however, concentrations of up to 50 ppm N2O5 and 90 ppm chlorobutene were used due to the low reactivity of this compound. The concentration of the NO3 radical was also varied by varying the starting concentration of NO2 according to equilibrium 2. Some model calculations were made for 2-chloro- I-butene to check if the experimental data could be explained by the proposed reaction scheme. The model calculations required the 2-chloro1-butene/nitrate radical reaction rate constant, which was determined by two methods. Initially, the rate constant was determined in nitrogen at 100 kPa and 296 f 1 K by an indirect method, with N205 as the NO3 radical source. N205 was prepared in the reactor as described above. After addition of the organic, the concentrations of NO2, N205, and 2-chloro- 1-butene were followed by FTIR spectroscopy. In the evaluation of the rate constant k 3 , reactions 2-6 were
- -+ + + -
NO3 + 2-chloro-I-butene
products
N,OS (wall)
NO3 + NO2 NO3
NO2
NO
NO
2N02
(3) (4)
O2
(5)
(6)
accounted for. Reaction 4 is the heterogeneous decomposition of N2Os on the reactor walls. This reaction is close to first order in N205. The decomposition rate is dependent on the reactor wall condition and was determined before each experiment. In practice this was made by following the decrease in N 2 0 5concentration for 30 min before the addition of 2-chloro-1-butene. The range of variation of k4 between experiments was less than f 1 5 % of the mean value. The fraction of N 2 0 5disappearing via reaction 4 was typically 0.35. After addition of chlorobutene, the reaction was followed by collecting spectra every 4-10 min during a period of 50-110 min depending on the initial concentrations of the reactants. The rate constant k , was evaluated through numerical integration of the rate expressions for reactions 2-6 and least-squares fitting of k 3 to the observed concentrations of N20S, N02, and 2-chloro- I-butene, by using the computer program FACSIMILE.'^ The rate constant k3 was the only adjustable parameter during this procedure. K 2 = 4.18 X 1O-Il cm3 molecule-l at 296 K was taken from Kircher et aI.,l4 while Graham and Johnston's values of 4.0 X and 1.9 X IO-" cm3 molecules-' s-I for k S and k6 were used.I5 Doubts concerning the validity of the indirectly determined rate constant, mainly caused by the uncertainty associated with K2, made it necessary to do an absolute rate determination as well. The determination was made using a fast-flow discharge system. The apparatus and experimental technique are described in detail in ref 16. Briefly, the nitrate radicals were generated by reacting atomic fluorine, produced in a microwave plasma, with nitric acid. Helium was used as the carrier and dilution gas. The flow tube was equipped with a fixed optical detection system, measuring the NO3 absorbance at 662 nm. Time resolution was obtained by adding the organic compound through an axially movable injector inside the flow tube. Flow velocities of around 4 m s-' were used. Due to the limited volatility of the 2-chloro-l-butene, it was not possible to use pseudo-first-order conditions. The experiments were evaluated by using the second-order treatment described in ref 16, assuming a 1:l reaction between NO3 and 2-chloro-l(13) Chance, E. M.; Curtis, A. R.; Jones, I . P.; Kirby, C. R. FACSIMILE, Report R 8775, United Kingdom Atom Energy Authority, Harwell, 1977. (14) Kircher, C. C.; Margitan, J. J.; Sander, S. P. J . Phys. Chem. 1984, 88, 4370. (15) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978, 82, 254. (16) Canosa-Mas, C.; Smith, S. J.; Toby, S.; Wayne, R. P. J . Chem. SOC., Faraday Trans. 2 1988, 84(3), 247.
8038 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 butene and employing a value of the NO3 absorption cross-section of 2.23 X lo-'' cm2 molecule-l at 662 n117.I' Reaction Scheme
The following reaction scheme has been proposed for propene5*I0 and may also be applied to chlorobutenes, as will be shown later. The first step in the NO3 radical/alkene reaction is an addition of NO3 to the double bond (eq 3). Although it is likely that the RlR2C=CR3R4
+ NO3
-
R,R2C(ON02)-CR3R4 (3)
addition occurs preferentially at the less substituted carbon, the other alternative must also be considered. An alkyl radical from reaction 3 will react rapidly with oxygen to form a peroxy radical, which in turn reacts with NO2 to give a nitroperoxy nitrate compound. The equilibrium reaction 8 serves RIR2C(ON02)-CR3R4 + 0 2 RIR2C(ON02)-C(02)R3R4 (7) RIR2C(ON02)-C(O,)R,R4 + NO2 F= R IR2C(ONO2)-C(02NO2)R,R4 ( 8 )
Wangberg et al. TABLE I: Intermediate Nitroxy and Nitroperoxy IR Absorption Bands Observed in the Reaction between NO3 and Chlorinated Butenes at 99 kPa and 1-an-' Resolution" organic compd nitroxv. cm" nitroDeroxv, cm-' 2-chloro-I-butene 840, 1283, 1677 786, 1300, 1744 3-chloro- I-butene 840, 1285, 1676 789, 1296 I-chloro-2-butene 839, 1281, 1675 788, 1299 2-chloro-2-butene 837, 1284, 1675 787, 1298, 1744 "The nitroxy and nitroperoxy bands found from each chlorobutene correspond to the @NO2, the symmetric NO3, and the antisymmetric NO, stretching vibrations, respectively. The nitroperoxy band near 1740 cm-l from 3-chloro-I-butene and I-chloro-2-butene could not be resolved from the strong N,O, band in that region.
-
-
RIR2C(02)-C(ON02)R3R4 + NO3 R,R2C(O)-C(ON02)R,R,
-
+ NO, + 0 2
2Rl R2C(02)-C(ON02)R,R, 2RIR2C(O)-C(ON02)R,R,
+
RIR2C(02)-C(ON02)R,R4 NO ---* RIR2C(O)-C(ON02)R3R,
(9)
+0 2
(IO)
+ NO2
( 1 1)
as a reservoir for the peroxy radical, for which there are several possible reactions. The alkoxy radical formed in reaction 9, 10, or 1 I will react further. In the case in which R, or R2 is a hydrogen atom, the radical can undergo hydrogen abstraction with
-
02:
+
RIHC(O)-C(ON02)R,R, 0 2 R,C(O)-C(ON02)RjR,
+ HO2
(12)
The products are a carbonyl nitrate compound and a hydroperoxy radical. The alkoxy radical may also decompose according to reaction 13. A peroxy radical self-reaction has been proposed5 RIR2C(O)-C(ON02)R3& ---* RlR2CO + R3RdCO + NO2 (13) that also gives a carbonyl nitrate compound and at the same time, the homologue alcohol. As reaction 14 involves a hydrogen 2 R I H C( 02)-C( ON02) R3R4 R 1 C (0)-C( ON 02) R3R4 + R,HC(OH)-C(ONO2)R,R.q + 0 2 (14) -+
transfer, it requires that either R, or R, is a hydrogen atom. Still another reaction that the alkoxy radical can undergo is one with NO2 to form a dinitrate compound according to (1 5 ) . A dinitrate compound may also be formed directly from the peroxy radical via reaction 16. RlR2C(O)-C(ON02)R,R, NO2 ---* R,R,C(ON02)-C(ONO2)R3R4 (15)
+
-
R,R2C(02)-C(ON02)R3R4 + N O R I RzC(ON02)-C(ONOz)R3R, ( 16) Results and Discussion A 1 :1 disappearance of the organic starting material and N 2 0 5 ,
when corrected for reactions 4-6,was observed in all experiments. This indicates that reaction 3 is the only reaction involving the organic starting material that is of any significance in our experiments. Strong bands appeared in the 1R spectra during the first part of a typical run. This observation was common to all experiments, the only difference being that the rate of appearance varied between the investigated compounds. These bands are caused by (17) Canosa-Mas, C. E.; Fowles, M.; Houghton, P. J.; Wayne, R. P. J Chem. Soc., Faraday Trans. 2 1987. 83, 1465.
d
I!
,edl- ;k( 600 700 800 900 1000 1100 1200 1300 1400 llcm Figure 1. Product spectrum from (a) 3-chloro-l-butene, (b) l-chloro2-butene, (c) 2-chloro-2-butene, and (d) 2-chloro-I-butene. Chlorobutenes and H N 0 3 have been subtracted. Spectra a and b were taken at the end of the experiment when all nitroperoxy bands had disappeared. Spectra c and d show the respective intermediate nitroperoxybutyl nitrate. Some N 2 0 5remaining from the subtraction can be seen at 740 and 1245 c d . Spectrum e is from an authentic sample of 3-nitroxy-2-butanone.
nitroxy and nitroperoxy groups attached to the same molecule, a chlorinated nitroperoxybutyl nitrate compound. This type of compound has been observed earlier in the corresponding reactions with p r ~ p e n e . ~The J ~ positions of the observed bands are given in Table I. The fairly well-established route to these products is given by reactions 3, 7, and 8. Depending on the position of the chlorine substitution, two different sets of end products could be seen, in addition to the NO2 always formed. Chlorine Substitution next to the Double Bond. For the two investigated compounds with the chlorine atom substituted next to the double bond, the intensity of the nitroperoxy bands passed through a maximum while the nitroxy bands continued to increase during the course of the experiments. A carbonyl band at approximately 1750 cm-' was seen at the same time. This indicates that either a carbonyl nitrate compound or a nitrate and a carbonyl compound were formed through decomposition of the nitroperoxy nitrate intermediary. Small amounts of acetaldehyde and chloroacetaldehyde were also found among the products formed from the reaction with I-chloro-2-butene. In the reaction with 3chloro- I-butene, significant amounts of formaldehyde were formed. These observations may be understood in terms of the above reaction scheme, in which reactions 12 and 14 both yield carbonyl nitrate compounds. In the vapor phase, an alcohol 0 - H stretching vibration gives a sharp and quite strong infrared band around 3600 cm-l. This is the case for example in the spectrum of an authentic sample of 3-nitroxy-2-butanol. In the product spectrum, however, there is no support for the formation of the homologue alcohol. Thus, reaction 14 seems to be of minor importance in this case. In Figures 1 and 2, the product spectra are compared with a spectrum of an authentic sample of 3-nitroxy-2-butanone. The resemblance between the product spectra and that of 3-nitroxy2-butanone shows that nitroxybutanones are the main products for both reactants. The likely products formed from I-chloro-2-butene are 1chloro-3-nitroxy-2-butanoneand some 1 -chloro-2-nitroxy-3-bu-
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8039
Nitrate Radical-Chlorinated Butenes Reactions
TABLE 11: Initial Concentrations and Rate Constants at 296 K for Individual Runs in the Indirect Determination of the NO,/Z-Chloro- 1-butene Reaction NO,, N205, 2-chloroiOi4k3, cm3 run uum upm 1-butene, pum molecule-I s-I 1 100 46 84 1.85 2 3
1400
1500
1600
1700
1800
1900
2000 l/cm
Figure 2. Product spectrum from (a) 1-chloro-2-butenecompared with the spectrum from an authentic sample of (b) 3-nitroxy-2-butanone. The strong bands at 1670 and 1750 cm-I are due to NO3antisymmetric and C=O stretching vibrations, respectively. Bands due to NO2 at 1600 cm-I and to added NO at 1876 cm-' are also seen in spectrum a.
tanone, which will be the corresponding product if the NO, radical is initially added to the second carbon atom in 1-chloro-2-butene. From 3-chloro- 1 -butene, 3-chloro- 1 -nitroxy-2-butanone is expected to be the main product together with some 3-chloro-2-nitroxybutanal formed for the same reason. As can be seen in Table I the IR shifts between different nitroxy bands are very small. The possibility of distinguishing between two isomers of carbonyl nitrates from their nitroxy bands is therefore limited. This also means that it is difficult to detect small amounts of other nitroxy compounds such as dinitrates with the method used. In product studies of the NO, radical reaction with propene, both dinitrate^^.^*^-" and I-nitroxy-2-propanolS have been found, which indicates that other side reactions mav take dace. The formation of aldehydes is likelyio occir through decomposition of the alkoxy radical via reaction 13. This is consistent with the Observation Of amounts Of and chloroacetaldehyde when I-ch10ro-2-butene is wed' amounts of formaldehyde were observed from 3-chloro-l-butene, but the expected formation of 2-chloropropionaldehyde has, for lack of a reference spectrum, not been proved. A rough estimate of the yield of carbonyl nitrates was made by the I R absorption cross section from 3-nitroxy-2butanone.I8 This procedure gave 65% and 80% carbonylnitrates from I-chloro-2-butene and 3-chloro-1-butene, respectively. Acetaldehyde and chloroacetaldehyde, corresponding to about 10% of the initial butene, were found-from l-chloro-2~butene,while 3-chloro- 1-butene yielded approximately 10% formaldehyde. Chlorine Substitution at the Double Bond. In the experiments with 2-chloro-I-butene and 2-chloro-2-butene, the initial increase in absorptions due to nitroxy and nitroperoxy groups were followed by a decrease and by a simultaneous formation of aldehydes and acid chlorides. The strong nitroxy and nitroperoxy bands that appeared were tentatively assigned to 2-chloro-2-(nitroperoxy)-1-butyl nitrate and 2-chloro-2-(nitroperoxy)-3-butylnitrate, respectively. Both compounds are formed through reactions 3, 7, and 8. The spectra are shown in Figure 1. Formaldehyde and propionyl chloride were formed in equal amounts in the reaction with 2-chloro-1-butene. The corresponding products from the reaction with 2-chloro-2-butene were acetaldehyde and acetyl chloride, which were also seen in equal amounts. The decomposition of the nitroperoxy nitrate compound is likely to occur via the formation of an alkoxy radical in a way similar to that discussed above, followed by reaction 13. Reactions I2 and 14 require an abstractable hydrogen atom to form carbonyl nitrate compounds. Whether an abstractable hydrogen atom is present or not depends on which carbon in the double bond the NO3 radical initially added to. N O was added at the end of the experiments to see whether nitrates, other than (18) IBI for 3-nitroxy-2-butanone between 800 and 882 cm-' (nitrate band) is 7.0 X IO-'* molecule-' cm. Integrated band intensity ( I B I ) = I / (c/)J%g~~ C l o / f ) du, c = concentration (molecules cm"), I = path length (cm). Hjorth, J; Lohse. C.: Skov. H. Unpublished work.
200
7.1
63 10
186
6.3
2.20 2.21
nitroperoxy nitrates were formed. All nitroperoxy nitrate compounds immediately disappeared when N O was added, and equal amounts of aldehydes and acid chlorides were formed. The likely course of events is a rapid formation of an alkoxy radical through reaction 11 followed by a direct decomposition through reaction 13. No nitrated compound remained when 2-chloro-2-butene was used. This indicates that the NO, radical added mainly to the third carbon atom, leaving no abstractable hydrogen on the second carbon atom. Instead, 60-93% of the butene reacted was found as acetaldehyde and acetyl chloride. In the experiments with 2-chloro-l-butene, 52-75% of the reacted butene was found as formaldehyde and propionyl chloride. A nitrated compound remained after the addition of NO. The remaining nitrated compound could not be identified from its infrared spectrum. One possible candidate is a carbonyl nitrate compound formed from an initial addition of NO, on the second carbon of 1-chloro-2-butene. In a study of photooxidation of allyl chloride in presence of NO,, Edney et aI.l9 proposed that the alkoxy radical CHCI(O)CH=CH, will decompose to give a chlorine radical and acrolein. The corresponding reaction could to some extent also occur in our case: CH 2(ON02)-C (0)(CI ) CH2CH3 CH,(ON02)-C(O)CH2CHJ + CI (17) +
The chlorine atom formed in reaction 1 7 could then react with 2-chloro-1-buteneto give a greater disappearance of 2-chloro-lbutene than of N 2 0 S . This could, however, not be an important channel for removal of the chlorobutene as discussed above. In addition, the nitrate band was not accompanied by any carbonyl absorption at 1750 c m - ~ . The Occurrence of these routes is therefore considered to be limited. Another possibility is a nitrate compound formed through a 1,4 hydrogen shift (reaction 18), of the alkoxy radical from reCH,(oNo2)-C(o)(CI)CH2CH, CH,(ONO,)-C(OH)(Cl)CH~CH2 (18) -+
actions 9-1 1. This alkyl radical can enter the above reaction scheme at reaction 7 to form a peroxy radical, which may further react to form a stable nitrate compound. The nitrate band could also be due to a dinitrate compound formed in reaction 15 or 16. Dinitrates have been observedsJOin varying amounts as products from the propene-NO, reaction. The amount of the unidentified nitrated product was estimated to be 19-20% of the reacted starting material by applying the absorption cross section of 3-nitroxy-2-b~tan0ne.l~ The products found by this procedure, together with the formaldehyde and propionyl chloride, accounted for 72-94% of the reacted 2chloro-1-butene. In this case as well as with experiments with 2-chloro-2-butene, higher N 2 0 s / N 0 2 ratios gave an improved mass balance. In the experiments with good mass balance there was a fast formation of acid chlorides and simple aldehydes and probably less loss of heavier intermediates to the walls of the reactor. Rate Constant Determinations. The rate constant for the N03/2-chloro- 1-butene reaction, obtained by the indirect FTIR cm3 molecule-I s-'. method, was found to be 2.1 (f0.5)X The value is a mean of three experiments (cf. Table 11). The estimated error corresponds to two standard deviations and does not include the uncertainty of the equilibrium constant K,. In our determinations k , is linearly dependent on K,. Since the values (19) Edney, E. 0.;Shepson, P. B.; Kleindienst, T. E.; Corse, E. W. h i . J . Chem. Kine!. 1986, 18, 597.
8040
TABLE 111: Summary of Flow Tube Experiments To Determine the 2-Chloro-l-butene/NO? Rate ConstanP [NO,], [CH2=CH(CI)pressure, IOi4k, cm3 run oDm CH,CH21, DDm mbar molecule-' s-I 1 2 3 4 5
Wangberg et al.
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
4.23 4.23 4.23 4.23 4.23
16.8
1.54 1.47 1.45 1.43 I .46
10.6 6.05 3.27 6.53
1.52 1.61 1.74 I .90 1.81
"The table gives initial reactant concentrations, pressure, and rate constant determined. The temperature for all experiments was 299 K.
TABLE IV: Reactions and Equilibrium and Rate Constants Used in Computer Simulations of the Reactions in CH2=C(CI)CH,CH3, N20s,and NO2 Air Mixtures at 99 kPa and 296 K O reaction constant K, = 4.18 X IO-" NO3 + NO2 (+M) == N205 (+M) k , = 1.73 X CH2=C(CI)CH2CH3 + NO, CH,(ON02)-C(CI)CH2CH, N 2 0 S(wall) k , = 1.0 X IO4 k S = 4.0 X NO3 + NO2 NO2 + NO + 0 2 NO, + NO --. 2 N O 2 k , = 1.9 X IO-" k7 = 1.0 X IO-', CH,(ON02)-C(CI)CH,CH3 + 02 CH,(ON02)-C(02)(Cl)CH7CH, CH2(0NO2)-C(OZ)(CI)CH2CH3 + NO2 F= K, = 1.0 X IO-" CH2(ON02)-C(OZN02)(CI)CH2CH, k9 = 2.0 X CH2(ON02)-C(02)(CI)CH2CH3+ NO, CH2(ON02)-C(O)(Cl)CH,CH, + NO2 + 0 2 k,,, = 5.0 X 2CH2(ON02)-C(02)(CI)CH2CH3 2CH2(ONO2)-C(O)(Cl)CH,cH, + 0 2 k , , = 1.0 x lo-" CH2(0N02)-C(O2)(Cl)CH2CH, + NO CH2(ONO2)-C(O)(CI)CH,CH, + NO2 k , , = 2.0 x 103 CH,(ONO2)-C(O)(CI)CH,CH, CH2O + CH3CH2COCI + NO,
-
-
+
+
-
__
4'3
E':
:me
(r-:r.!
8;'
1::'
Figure 3. Measured time-concentration profiles for an experiment with an excess of 2-chloro-I-butene (2-chloro-l-butene, 0;NO2, +; N205, 0 ; 2-chloro-2-(nitroperoxy)-I-butylnitrate, 0 ;propionyl chloride, W). Solid lines show the corresponding time-concentration profile from a kinetic simulation with the same initial concentrations. N O was added after 9 4 min.
reported for this constant vary by a factor of more than 2,20our to 2.9 X cm3 value of k 3 may range from 1.2 X molecule-' s-' depending on the choice of equilibrium constant. The constant used is that of Kircher et aI.,l4 4.18 X IO-'' cm3 molecule-' at 296 K. The reason for choosing this particular value is that it is close to the mean of the available values. It has also been shown to give good agreement between absolute rate determinations and indirect determinations made via K 2 . 2 Table 111 contains some information about the flow tube experiments. The absolute method gives a value of 1.73 (f0.31) X cm3 molecule-' s-l, which is in good agreement with the indirect value discussed above. Since this method avoids the main source of error, that introduced by K2, we consider the value obtained by the absolute method as the most reliable. The rate constant2'*22 for the corresponding reaction with 1butene, (1.23 f 0.17) X cm3 molecule-' s-', gives a k2.chloro-l.butene/kI-butcne ratio of 1.4. For chlorine substitution on ethene the ratio kvinylchlde/~hene is reported2, to be 2.08. However, the same ratio for allyl c h l o r i d e / p r ~ p e n e ~is~ only < * ~ 0.06. The limited data suggest that the reactivity is slightly enhanced by a chlorine substitution on the carbon-carbon double bond but substantially decreased by a chlorine substitution next to the double bond. Rearfion Modeling. Simulations of the 2-chloro-1-butene/NO, reaction were made using the computer program FACSIMILE." The reaction schemeSJ0used to describe the process is given in Table IV. With the exception of k3 and k4. which were determined in this work, the rate constants were taken from the literat ~ r e . ~ ~When ' ~ Jno~ data ~ ~ for ~ the J ~chlorinated compounds were (20) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr; Kerr, J . A.; Troe, J. J . Phys. Chem. Ref. Data 1989, 18, 881. (21) Atkinson, R.; Plum, C . N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J . Phys. Chem. 1984, 88, 1210. (22) Atkinson, R.: Aschmann. S. M.: Pitts, J. N., Jr. J . Phys. Chem. 1988,
92. ~. 3454. (23) Atkinson, R.; Aschmann, S. M.; Goodman, M. A. Int. J . Chem. Kinet.
1987, 19. 299
-
-
+
"The equilibrium and rate constants K2,I4 k S , l 5k6,I5 k7.2 k9,I8 k l l , 2 and k132were taken from the literature. The constants involving hydrocarbons are valid for similar unchlorinated compounds. KsIoand k l o ' owere adjusted from 6.7 X IO-" to 1 X IO-" cm3 molecule-I and cm3 molecule-I s-l, respectively. All from 1.2 X IO-', to 5 X second-order rate constants are expressed in cm3 moleculed SKI. Firstorder constants are expressed in s-I, and the equilibrium constants are expressed in cm3 molecule-I.
available, values typical for similar unchlorinated compounds were chosen. The literature values of K, and k l oare estimates based on propene-NO, work.I0 During the model work it was found that a shift in K , and k , , improved the fit considerably. K8 was shifted from its suggested value of 6.7 X lo-', to 1.0 X lo-'' cm3 to 5.0 x cm3 molecule-' molecule-' and k l o from 1.2 x s-I. These shifted values of K8 and k i o were used in our model calculations. Figure 3 shows the measured time-concentration behavior in an experiment with 2-chloro-1-butene and N205together with the corresponding calculated values. The agreement implies that the mechanism shown in Table IV is consistent with the experimental data. Conclusions
From the work presented above it is concluded that the product pattern in the reaction between chlorinated butenes and NO, may be understood in terms of the reaction scheme presented for propene.sJO A butene with a chlorine substituted on a carbon next to a double-bond carbon will give products analogous to the products from propene. When the chlorine is attached to a double-bond carbon, the channel for formation of carbonyl nitrates is less important and the yield of aldehydes, and in this case acid chlorides, is increased. The rate constant for the reaction between NO, and 2chloro- 1-butene determined by an indirect method, via the (NO, NO, * N 2 0 5 )equilibrium constant of Kircher et aI.,l4agrees well with that determined by the absolute method.
+
Acknowledgment. We are indebted to the staff a t the Physical Chemistry Laboratory, University of Oxford, for help with the absolute rate determination and to C. Lohse and H. Skov, University of Odense, for putting samples of 3-nitroxy-2-butanone and 3-nitroxy-2-butanol at our disposal. The financial support of the National Swedish Environmental Protection Board (SNV) is acknowledged.
