J. Phys. Chem. 1992, 96, 272-281
272
exp(2610/T) M-' and 0.001 04 exp(2780/T) M-I, respectively, between 5 and 35 OC. These values are independent of pH between 4 and 8 and are unaffected by ionic strength up to 0.3. The kinetics of the formation of aqueous H M P from and its decomposition to H202and H2C0were studied also in the temperature range 5-35 OC. The temperature-dependent rate expressions of these reactions at pH 7.07 f 0.2 are kl = 6.0 X lOI3 exp(-9450/7') M-I s-l and kl = 1.0 X lOI5 exp(-11800/7') s-I, respectively. The Henry's law solubilities of H M P and BHMP in water were determined; the values at 22.0 and 10.0 OC, respectively, are 5.0 X lo5 and 6.2 X lo5 M atm-I for H M P and 6 X lo5 and 2 X lo6 M atm-l for BHMP. The rate constant of the reaction of H M P with S(IV) in aqueous solution was determined to be (2.2 X 107)[H+] s-l between p H 3 and 4 at 22.0 O C . Exhibiting a high solubility, HMP is expected to be efficiently scavenged by hydrometeors, much in the same fashion as H202. Upon dissolution, H M P undergoes decomposition to H 2 0 2and H2C0, the rate of which being p H dependent. The life-time of dissolved H M P ranges from less than a few minutes at pH -7
to hours a t pH below 5.5. As a result, H M P is expected to be detected only in acidic atmospheric liquid samples. Furthermore, because of the low H2CO concentrations found in atmospheric water which would result in a low aqueous HMP concentration at equilibrium, any HMP detected is most likely to have originated from the gas phase. Under typical conditions, HMP accounts for only a few percent of S(1V) oxidation in aqueous phase. However, when gaseous concentrations of H M P and H202are comparable, H M P may contribute up to 20% of the S(1V) oxidation. The possible damaging effect of H M P to plant tissues may be facilitated by its high solubility and its stability in acidic solutions as associated with the acid precipitation phenomenon. Acknowledgment. This work was conducted under Contract No. DE-AC02-76CH00016 with the U.S.Department of Energy under the Atmospheric Chemistry Program within the Office of Health and Environmental Research. Reeistry NO. HMP, 15932-89-5; BHMP, 17088-73-2; H202, 772284-1; H2C0, 50-00-0; Na2S03,7757-83-7.
Kinetics of Pyrolysis of the Isomeric Butenenitriles and Kinetic Modeling Alan Doughty and John C.Mackie* Department of Physical and Theoretical Chemistry, University of Sydney, NS W 2006, Australia (Received: July 1 , 1991)
Kinetics of pyrolysis of the butenenitrile isomers, cis- and trans-crotononitrile and allyl cyanide, have been studied dilute in argon in a single-pulse shock tube over the temperature range of 1200-1500 K at uniform gas residence times behind the reflected shock of between 650 and 750 ps and at pressures between 18 and 20 atm. Thermal decomposition was preceded by isomerization of the butenenitriles whose rates are coupled with the rates of thermal decomposition. The decomposition was found to follow a free-radical mechanism with the major chain involving propagation reactions of the cyanomethyl radical to produce acetonitrile and acetylene. Other routes important to the mechanism involve hydrogen atom addition to the butenenitriles. HCN principally arises from this route. A detailed kinetic reaction model is presented to model the experimental reactants and products profiles as a function of temperature. From the modeling and experiment, the following initiation CH3 + HC=CHCN (klI= exp(-96S/RT) s-I), rate constants have been obtained: trans-CH3CH=CHCN cis-CH$H=CHCN CH3 + HC-CHCN (kI2 = IO1'," exp(-98.5/RT) s-I), CH2=CHCH2CN H2C=CH + CH2CN (kI3 = 10'5.53exp(-82.6/RT) s-l), where the activation energies are in kcal mol-'.
-
Introduction The combustion of heavy fuels such as coal and coal-derived liquids can result in the evolution of environmentally detrimental levels of fuel-derived NO,.'-3 Generation of NO, from the fuel-bound nitrogen (FBN) is known to proceed through pyrolysis of the nitrogen-containing compounds to yield NO, precursors, followed by the reaction of the precursor compounds with oxygen to yield oxides of nitrogen. Study of the kinetics and mechanism of the pyrolysis of nitrogen-containing coal model compounds can further an understanding of the mechanism of NO, formation in combustion of low-rank fuels. The nitrogen originally bound in the fuel has been shown to consist largely of aromatic heterocyclic structures containing of the pyrolysis pyridine and pyrrole ring s t r ~ c t u r e s . ~Studies .~ of p y r r ~ l ehave ~ . ~ shown the major primary products of pyrolysis to be the straight chain isomers of pyrrole, i.e., allyl cyanide and (1) Pershing, D. W.; Wendt, J. 0. L. Proceedings ofthe Sixfeenth Symposium on Combustion;The Combustion Institute: Pittsburgh, PA, 1977; p 389. (2) Painter, P. C.; Coleman, M. M. Fuel 1979, 58, 301. (3) Turner, D. W.; Andrews, R. L.; Siegmund, C. W. AIChE Symp. Ser. 1972, 68, No. 126, 55. (4) Snyder, L. R.; Anal. Chem. 1969, 41, 314. (5) Brandenberg, C. F.; Latham, D. R. J . Chem. Eng. Data 1968,13, 391. (6) Lifshitz, A,; Tamburu, C.; Suslensky, A. J . Phys. Chem. 1989, 93, 5802. (7) Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, M. Int. J . Chem. Kinet. 1991, 23, 733.
0022-3654/92/2096-272$03.00/0
-
-
,~ included the develcrotononitrile. Our earlier s t ~ d i e swhich opment of a detailed kinetic mechanism of pyrolysis, showed that the major products observed from the pyrolysis of pyrrole arise from the decomposition of the primary products allyl cyanide and crotononitrile. An understanding of the thermal decomposition of allyl cyanide and crotononitrile is therefore essential to developing a detailed mechanism of the pyrolysis of pyrrole and to an improved understanding of the pyrolysis of heterocyclic fuel bound nitrogen. A detailed kinetic study of the pyrolysis of crotononitrile and allyl cyanide has not previously been attempted. Jeffers et aL8 and Butler et a1.9 studied the thermal isomerization of cis- to trans-crotononitrile, but no attempt has been made to study the thermal decomposition of either compound. King et al.I*l3 have studied the thermal decomposition of saturated nitriles but made only a brief exploratory study of pyrolysis products of allyl cyanide and cr0tononitri1e.l~~~~ The purpose of the present work has been to study the pyrolysis of allyl cyanide and crotononitrile using the single-pulse shock tube (SPST) technique with capillary gas (8) Marley, W. M.; Jeffers, P. M. J . Phys. Chem. 1975, 79, 2085. (9) Butler, N . B.; McAlpine, R. D. Con. J . Chem. 1963, 41, 2487. (10) King, K. D.; Goddard, R. D. Znt. J . Chem. Kinet. 1975, 7, 109. (11) King, K. D.; Goddard, R. D. J . Am. Chem. SOC.1975, 97, 4504. (12) King, K. D.; Goddard, R. D. J . Phys. Chem. 1976, 80, 546. (13) King, K. D.; Goddard, R. D. J . Phys. Chem. 1978,82, 1675. (14) King, K. D.; Goddard, R. D. Int. J . Chem. Kinet. 1975, 7 , 837. (15) King, K. D.; Goddard, R. D. In!. J. Chem. Kinel. 1981, 13, 7 5 5 .
0 1992 American Chemical Society
Pyrolysis of Isomeric Butenenitriles
The Journal of Physical Chemistry, Vol. 96, No. I , 1992 273
chromatography for product analysis and to develop a detailed kinetic model describing the process. As this work will demonstrate, isomerization of the butenenitriles occurred concurrently with decomposition. Thus decomposition of either crotononitrile or allyl cyanide cannot be considered in isolation from the other isomer, and therefore a single reaction mechanism can be developed to model the decomposition of both compounds. This mechanism considers important nitrogen-containing radicals whose rearrangements play an important part in the decomposition of the butenenitriles.
100;
Experimental Section The shock tube used in this study has been described previand was used in our earlier study of the pyrolysis of pyrrole.' Analysis of the reactants and nitrogen-containing components of the product mixtures was carried out using a Hewlett-Packard 5890/II gas chromatograph equipped with a HP1 capillary column. Detection was by nitrogen phosphorus detector (NPD). Hydrocarbon products were quantified using a G C fitted with an alumina column and FID detector. Hydrogen analyses were carried out for several runs using a Shimadzu GC-IA, with detection by TCD. Samples of allyl cyanide and crotononitrile were purchased from Aldrich (stated purity 98% in each case). The crotononitrile consisted of a mixture of cis and trans isomers and was further purified before use via three bulb to bulb distillations under vacuum. The same procedure could not be followed for allyl cyanide, due to its marked tendency to isomerize to crotononitrile during the distillation procedure. Allyl cyanide was therefore used without further purification. Before preparation of the reactant mixtures, the samples were thoroughly degassed by three freeze/pump/thaw cycles. After distillation crotononitrile (cis trans) was of purity >99.9%. The only significant impurity present in allyl cyanide was crotononitrile (cis and trans). The only other detectable impurities in the reactant mixtures were acetonitrile and H C N (at less than 0.001% by GC). In this work, the decomposition of both crotononitrile and allyl cyanide was studied in the concentration range of 0.25-0.50% in argon. Crotononitrile pyrolysis was also studied in a lower concentration region of 0.084.10%. Pressure and temperature behind the reflected shock were calculated from the measured incident and reflected shock velocities. Residence times were obtained from pressure profiles recorded using Kistler pressure transducers. The experiments were conducted over the temperature range of 1200-1500 K. Typical residence times were 650-750 ws, with pressures between 18 and 20 atm. Product identification was achieved using the Finnigan GCMS facility at the CSIRO Division of Coal and Energy Technology. Where commercial samples were available, product assignments were confirmed through matching of G C retention times. Calibration for hydrocarbon analysis was carried out using standard mixtures in N2 obtained from Scott Specialty Gases. Nitrogen-containing products were calibrated, where possible, using commercial samples to obtain the response of each nitrogen compound relative to acetonitrile. The relative response for H C N was obtained using a standard 1% mixture of HCN in N2, obtained from Matheson Gases. The NPD was found to respond almost equally to the majority of compounds studied, the only exceptions being HCN and acetonitrile. Thii allowed estimates of the relative response to be made for products which could not be obtained commercially. Such compounds included cyanoacetylene and cyanopropyne. Calibration of the FID was carried out daily. Due to the NPD being prone to sensitivity drift, the detector was calibrated prior to each run. The analysis of the nitrile reactants and products proved to be difficult due to the adsorption of these compounds on to the walls of the gas collection system. This coupled with
10
90-1 80-1
no
0
*
Allyl cyanide
P
8
30 .a
4
40g. D O
8200
Y o n
*
ljoo
12'50
DOC
=a
~
Tempera-
1350
no--
no
lh
1450
l h
(K)
Figure 1. Temperature dependenceof designated species in the pyrolysis of allyl cyanide: (0)allyl cyanide remaining (0.4% mixture in Ar); (0) crotononitrile (0.4% mixture), (A)crotononitrile (0.09%);(--- and ---) model predictions (0.4%), .) model prediction (0.09%). (.a
100
ea
3
F
+
(16) Cathro, W. S.;Mackie, J. C. J. Chem. Soc., Faraday Trans. 1972, 68, 150. (17) Doolan, K. R.;Mackie, J. C. Combusr. Flame 1983, 49, 221.
0
Do
c
*
Crotononitrile
Allylcyanide 404
?%€I
1250
lhl
14bo
ld50
1450
15kl
Temperature (K)
Figure 2. Temperature dependence of designated species in the pyrolysis of crotononitrile: (0 and A) crotononitrile remaining (0.4% and 0.09% mixtures, respectively);(0)allyl cyanide (0.4%mixture); and ---) model predictions (0.4%), model prediction (0.09%). (-a-
(..e)
301 25
i
Figure 3. Temperature dependence of the yield of HCN from pyrolysis of (0)allyl cyanide (0.4% mixture), (0and A) crotononitrile (0.04% and 0.09% mixtures, respectively); (-.-and ---) model predictions (0.4%), .) model prediction (0.09%). (.e
a "memory effect" which was found to occur with the analysis of these compounds resulted in sample gas injections being reproducible to *lo%. Nitrogen mass balances of reactants and recovered products were generally 100 10%. Relative to nitrogen, carbon and hydrogen balances started to decrease above 1400 K, being about 90% at 1500 K,suggesting the progressive production of some heavy hydrocarbon products at elevated temperatures.
*
R€!SultS Figures 1-10 illustrate the temperature profiles of all products of significance for both allyl cyanide and crotononitrile pyrolysis. The decomposition profile of each starting isomer is also included in the respective plot.
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
274
Doughty and Mackie
30-
201
?h Figure 4. Temperature dependence of the yield of acrylonitrile from pyrolysis of (0)allyl cyanide (0.4% mixture); ( 0 and A) crotononitrile (0.4% and 0.09% mixtures, respectively); and ---) model predictions (0.4%), model prediction (0.09%). (-a-
(..a)
.@e=W+-
ml&50
13bo
1350
Temperahlre 6)
1d00
I&
lsbo
Figure 7. Temperature dependence of the yield of methane from pyrolysis of (0)allyl cyanide (0.4% mixture), (0and A) crotononitrile (0.4% and 0.09% mixtures, respectively); (-*- and ---) model predictions (0.4%), model prediction (0.09%). (.-a)
30-
201
"1
0
15j
0
I
20-
Figure 5. Temperature dependence of the yield of acetonitrile from pyrolysis of (0)allyl cyanide (0.4% mixture), (0 and A) crotononitrile (0.4% and 0.09% mixtures, respectively); and ---) model predicmodel prediction (0.09%). tions (0.4%), (-a-
(-e.)
F i 8. Temperaturedependence of the yield of ethylene from pyrolysis of (0)allyl cyanide (0.4% mixture), ( 0 and A) crotononitrile (0.4% and 0.09% mixtures, respectively); (--- and ---) model predictions (0.4%), .) model prediction (0.09%). (e.
201
70 i
15 i
Figure 6. Temperature dependence of the yield of acetylene from pyrolysis of (0)allyl cyanide (0.4% mixture), (0 and A) crotononitrile (0.4% and 0.09% mixtures, respectively); and ---) model predictions (0.4%), model prediction (0.09%).
Figure 9.
The same set of products was observed for both allyl cyanide and crotononitrile, with only the relative yields of the products changing with starting isomer. HCN,acetylene, acetonitrile, and acrylonitrile were all observed at very low extents of decomposition, and remained the major products at high extents of decomposition. Minor nitrogen-containing products were ethyl cyanide, cyanoacetylene, and cyanopropyne/cyanoallene. Methane was also present in significant concentrations, along with smaller quantities of ethylene, propene, and propyne. Traces of the following compounds could also be detected: cis- and trans-l-cyano-l,3-butadiene, bemnitrile, pyridine, methacrylonitrile, pyrrole, diacetylene, 1,3-butadiene, I-butene, benzene, and toluene. The major difference in product distributions between the reactants crotononitrile and allyl cyanide was that allyl cyanide
yielded significantly higher proportions of HCN, acetonitrile, acetylene and ethylene. C3hydrocarbons were also generated in higher concentrations for allyl cyanide when compared with crotononitrile. The studies of crotononitrile carried out at two different concentrations showed no observable dependence of the rate of decomposition of the reactant upon initial concentration. However, some concentration dependence was observed for the yield of acetonitrile. The relative yield of this product was seen to decrease significantly with decrease in initial reactant concentration. The products observed were generally in agreement with those found by King et al. during their brief exploratory study of the pyrolysis of the butene nitrile^.^^^^^ However, King et al. did not observe acrylonitrile as a reaction product. This inconsistency
(-a-
(a
e.)
Temperature dependence of the yield of cyanoacetylene from pyrolysis of (0)allyl cyanide (0.4% mixture), (0 and A) crotononitrile (0.4% and 0.09% mixtures, respectively); (-.- and ---) model predicmodel prediction (0.09%). tions (0.4%), (am.)
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 215
Pyrolysis of Isomeric Butenenitriles
SCHEME I:
10 1
Free-Radical C h i n Mechanisms in Butenenitrile
Pyrolysis4 H
- c'
'C
'H
H
H
'.c
,H
,c=c
H
CN
I
CN
H + CHzCCHCN
- c/
\
H
\cH,CN
I
Figure 10. Temperature dependence of the yield of ethyl cyanide from pyrolysis of (0)allyl cyanide (0.4% mixture), (0,and A) crotononitrile (0.4% and 0.09% mixtures, respectively);(--- and ---) model predicmodel prediction (0.09%). tions (0.4%),
H3C,
HSC,
c- c'
'
CHICN
H'
\CN
(as.)
could be explained by the complications caused by surface reaction in the very low pressure pyrolysis (VLPP) technique. The possibility of surface reaction was supported by the observation of King et al. of a tarry material in their pyrolysate.
+
Discussion General Features of the Reaction Mechanism. The product distribution observed in the pyrolysis of the butenenitriles can best be described in terms of a free-radical mechanism. The observation of a mixture of C3HSCNisomers in the product mixture shows that isomerization of the starting isomers occurs simultaneously with decomposition, as suggested earlier. The most likely initiation reaction for the free-radical mechanism would occur through C - C single bond fission. For crotononitrile, C3-C4bond fission would occur, whereas with allyl cyanide C2-C3 fission would be most favorable. The resulting initiation reactions are illustrated below. (Reaction numbering refers the mechanism given in Table I.) CH3CHaCHCN CH3 + HC-CHCN (-1 1, -12) +
HzCxCHCHzCN
+
HzC=CH
+ CH2CN
(-1 3)
Alternatively, initiation through C-H bond fission is possible; however, the significantly higher C-H bond energy would make this only a minor initiation pathway. Although HCN, acetylene, acetonitrile, and acrylonitrile were all major products, it would appear that a simple chain mechanism can only be used to explain the formation of acetylene and acetonitrile. This chain mechanism involves the formation of C3H4CN radicals from the loss of H atoms from the reactants through abstraction by CH2CN, CH3, or H. The major chain involves the isomerization of the allyllic C3H4CN radical, cyanoallyl, AC3H4CN, to a nonallylic C3H4CN radical which can readily fission into C2H2and regenerate the propagating radical, CH2CN. See Scheme I. Major propagation steps would therefore be CHZCN + C3HSCN AC3H4CN + CH3CN (26, 27, 28) 4
AC3H4CN C3H4CN C3H4CN C2H2 + CHZCN +
+
(38, 39) (48)
Minor chains involve the formation of cyanoallene with regeneration of H and isomerization of the allyllic to an nonallylic C3H4CNradical which can either lose H and form cyanopropyne or fission into CH3 radicals and cyanoacetylene. The detailed reaction mechanism given in Table I incorporates this scheme and can model the above products satisfactorily. The cyanoallyl radical is a key radical in this reaction scheme. By analogy with its hydrocarbon counterpart, allyl, this radical would be expected to possess resonance stabilization. Thermochemical data are not available in the literature for cyanoallyl, although there have been studies of the effects of substitution of halogens and OH upon the resonance stabilization energy of ally1.'8.'9 These studies show that the effect of halogen substitution
j/
CH3 + HCCCN
HCCH CHzCN
H
+
CH3CCCN
*Large arrows indicate the major chain mechanism. is to reduce the resonance stabilization energy of allyl to approximately 7.5 kcal mol-'. If the added stabilization energy of the C%N group is included we would estimate the resonance stabilization energy of cyanoallyl to be about 12 kcal mol-'. Although the resonance stabilization energy of cyanoallyl has not been measured directly, kinetic measurementsZoof the rate of isomerization of cyanoallyl in solution from its syn to gauche conformations give some indication of the likely resonance stabilization energy. Since the transition state for syn gauche transformation would not be expected to be resonance stabilized, the activation energy for transformation should correspond to the resonance stabilization energy of cyanoallyl. ESR experimentsZ0 yields an activation energy of 10 kcal mol-' in reasonable agreement with the above estimate. In the kinetic modeling described below, the value of 10 kcal mol-' was used for the cyanoallyl resonance stabilization energy. Thermochemical data for cyanoallyl and other nitrogen-containing species of importance in the present work are given in Table 11. Where possible, Third Law methods are used to evaluate entropies and heat capacities in this table. Where not possible, group thermochemical methods have been employed.2' The expected stability of the cyanoallyl radical would therefore make it unlikely that it would undergo C-C fssion to give a stable product and propagating radical. More likely is the rearrangement to one of the non-allyllic C3H4CNradicals depicted in reactions 38 and 39. In the reaction model of Table I, activation energies of (38) and (39) were initially estimated by assuming the isomerization in, for example, (39), to take place by a 1,3-H shift which should proceed via a four-membered cyclic transition state. The resulting estimate of E j g = 44 kcal mol-'. As discussed below, modeled products and reactants profiles exhibit sensitivity to the rate constants k38and k39,and hence the activation energies in this table have been optimized by modeling to be E38 = 36 kcal mol-' and Ej9 = 40 kcal mol-'. Both of the postulated initiation reactions, (-1 1, -12) for cisand frunr-crotononitrile and (-1 3) for allyl cyanide, yield radicals which are capable of undergoing quite rapid unimolecular H fission, Le., the vinyl radical and its cyano- analogue, cyanovinyl,
-
(18) Alfassi, Z. V.; Golden, D. M.; Benson, S. W. Inr. J . Chem. Kinet. 1973, 5, 155. (19) Alfassi, Z. V.; Golden, D. M. Inr. J . Chem. Kinet. 1973, 5, 295. (20) Sustmann, R.; Trill, H.; Vahrenholt, V.; Brandes, D. Chem. Ber. 1977, 110, 255. (21) Benson, S. W. Thermochemical Kinerics; John Wiley: New York, 1976.
276
Doughty and Mackie
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
SCHEME II: Reaction Pathways Proceeding via H Addition across a Double Bond H
H
‘c H3C’
=
c’
H
+ H
7 A
SCHEME I V Mechanism for Isomerization Reactions Occurring in the Pyrolysis of tbe Butenenitriles H
H
\c
/
H,Cjc-C--CN
HsC/
H
‘CN
- c’
H3C,
H
,,c= H
‘CN
,H
c
\
CN
1
I; H
H
c=c
, \
/
/’
H
\
CN
CH3
+
\
H
”’‘
CHCN
H
\c
=
/
‘
.>\
0.85 -
0.8 0.71 7
..\\ “\
‘.. .
D ‘
0.41 0.3
+ \d l
Figure 11. Variation with temperature of sensitivity coefficients for the
0.4
Figure 12. Variation with temperature of sensitivity coefficients for the designated species. Only the most sensitive reactions are shown.
designated species. Only the most sensitive reactions are shown.
take place via an energized succinonitrile intermediate, this reaction would not exhibit Arrhenius temperature dependence. The given rate parameters would then not have any physical significance. Kinetic Modeling. Kinetic modeling of the detailed reaction model given in Table I was carried out using the Sandia code C H E M K I N ~together ~ with a shock tube code24modified to take into account the effects of quenching, and the LSODE ordinary differential equation solver.25 Rate of production and sensitivity analyses were carried out using the SENKIN code.26 Reactions 1-6 in the model comprise the major isomerization reactions of cis- and tram-crotononitrile and allyl cyanide. Because product yields of methacrylonitrile and cyclopropyl cyanide are so low, these species have been omitted from the model. Although the heat of formation of allyl cyanide has been reported*’ and spectral data for the statistical calculation of entropies and heat capacities have been difficulties still exist in the evaluation of the equilibrium ratio of the two conformers, syn and gauche, as a function of temperature. A treatment similar to that (23) Kee, R. J.; Miller, J. A.; Jefferson, T. H. ‘CHEMKIN; A General Purpose, Problem Independent,Transportable FORTRAN Chemical Kinetics Code Package”;SANDIA National LaboratoriesReport SAN80-003, March, 1980. (24) Mitchell, R. E.; Kee, R. J. ‘A General-Purpose Computer Code for Predicting Chemical Kinetic Behavior Behind Incident and Reflected Shocks”; SANDIA National Laboratories, SAND82-8205, March 1982. (25) Hindmarsh, A. C. LSODE AND LSODI; Two New Initial Value Differential Equation Solvers. ACM Signum Newsletter 1980, 15 (4). (26) Lutz, A. E.; Kee, R. J.; Miller, J. A. ‘SENKIN: A FORTRAN Program Predicting Homogeneous Gas Phase Chemical Kinetics with Sensitivity Analysis”; SANDIA National Laboratories Report SAND87-8248, February, 1988. (27) Konekek. J.; Prcchazaka, M.; Krestanova, V.;Smisek, M. Collect. Czech. Chem. Commun. 1969, 34, 2249. (28) Sanstry, K. V. L. N.; Rao, V. M.; Dass, S . C. Can. J. Phys. 1968,46, 959. (29) Silvi, B.; Sourisseau, C. J . Chim. Phys. 1976, 73, 100. (30) Ziesburger, B.; Botskor, I. Z . Naturforsch. 1983, 97, 323.
1.1
-
Auylcyanide S d t i v i t y for HCN 1.4
Figure 13. Variation with temperature of sensitivity coefficients for the designated species. Only the most sensitive reactions are shown.
used to calculate the entropy of the 1,3-butadiene conformers3’ has been used in the present work to calculate the thermodynamic
The Journal of Physical Chemistry, Vol. 96, No. 1. 1992
278
Doughty and Mackie
TABLE I: Reaction Model for CJHSCNPyrolysis" forward reaction 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
-----
reactions* cis-C3H5CN tr-CIH5CN rr-C,H,CN cis-CJHSCN cis-C3H5CN ALLYLCN cis-C3HSCN ALLYLCN ALLYLCN tr-C,HSCN ~ T - C ~ H ~ C ALLYLCN N H AC3H4CN = ALLYLCN H AC3H4CN = tr-CjH5CN H AC3H4CN = cis-C3H,CN H + C-CjHdCN = ALLYLCN CH3 HCCHCN = tr-C,H5CN CH3 HCCHCN = cis-C3HSCN C2H3 CH2CN = ALLYLCN H tr-C3H5CN = AC3H4CN + H2 H cis-C3H5CN = AC3H4CN + H2 H +tr-C,H,CN = C3H4CN+ H2 H cis-ClH5CN = C3H4CN+ H2 H ALLYLCN = AC3H4CN + H2 H ALLYLCN = C-CIHICN + H2 CH, tr-C3HsCN = AC3H4CN CH4 CH3 c ~ s - C ~ H ~ = CN AC3H4CN CH4 CH3 tr-C3H5CN = C3H4CN+ CH4 CH3 cis-C3HSCN= C3H4CN+ CH4 CH3 ALLYLCN = AC3H4CN + CH4 CH3 ALLYLCN = C-C3H4CN + CH4 CHZCN ALLYLCN = AC3H4CN + CH4 CH2CN tr-C3H5CN= AC3H4CN CH3CN CH2CN cis-C3H5CN = AC3H4CN + CH3CN HCCHCN Cr-CSHSCN = AC3H4CN + H2CCHCN CN AC3H4CN + HZCCHCN HCCHCN c ~ s - C ~ H ~ = HCCHCN ALLYLCN = AC3H4CN + HZCCHCN C3H5 + tr-CIH5CN = AC3H4CN + C3H6 C3H5 cis-C3H5CN = AC3H4CN + C3H6 C3H5 ALLYLCN = AC3H4CN + CpH6 C2H3 ALLYLCN = AC3H4CN + C2H4 C2H3 W-C,H,CN = AC3H4CN + C2H4 C2H3 cis-C3HSCN= AC3H4CN + C2H4 ACSH4CN = C3H4CN AC3H4CN = C-C,HICN C3H4CN = CH3CCCN H C3H4CN = HCCCN + CH, AC3H4CN = CH3CCCN + H H CH3CCCN = CH3 HCCCN HCCHCN=HCCCN+H 2AC3H4CN = ALLYLCN CH3CCCN 2AC3H4CH = tr-CSH5CN + CH3CCCN 2AC3H4CN = cis-C3H5CN CH3CCCN C-C3H4CN = C2Hz CH2CN H + ALLYLCN = C2H4 + CHZCN H ALLYLCN = C3H5 + HCN H + tr-C3HSCN = CHj + H2CCHCN H cis-C3H5CN= CH, + H2CCHCN H + tr-C3HSCN = [C4H6N] H + cis-C3H5CN = [C,H6N] HCN + CHACHCH = [C4H6N] H + Ir-CIHSCN = C2H4 + CHZCN H + cis-C3HSCN= C2H4 CH2CN tr-C3HSCN = C3H4P+ HCN cis-C3HSCN= C3H4P+ HCN ALLYLCN = C3H4P+ HCN H + HCCHCN = HZCCHCN CHZCN HZCCHCN = CH3CN + HCCHCN C3H5 + HZCCHCN = C3H6 + HCCHCN CH3 + HZCCHCN = HCCHCN CH4 H + HICCHCN = HCCHCN H2 H + H2CCHCN = HCN + C2H3 H + CH2CN = CH3CN H + CH3CN = CH3 + HCN CH3CN + C2H3 = CH2CN + C2H4 CHI + CH,CN = CH2CN + CH4 C2HSCN = CH3 CHZCN C2HSCN + H = C2H5 + HCN C2HsCN + H H2 + CH3CHCN CZHSCN + CH3 = C H I + CH3CHCN CIHSCN + CH2CN = CH3CHCN + CH,CN
+ + +
+ + + + +
+ + +
+ + + + +
+
+ +
+ + +
+
+ + +
+ + + + +
+
+
+
+
+ +
+ +
+
+
+
+
+
A
n
0.160E+14 0.1 30E+ 14 O.l50E+ 14 0.3508+14 0.1 50E+ 14 0.250E+14 0.500E+13 0.500E+ 13 0.500E+13 0.500E+ 13 0.250E+14 0.250E+14 0.150E+13 O.l50E+ 13 0.150E+13 0.100E+13 O.lOOE+ 13 0.100E+ 13 0.100E+13 0.150E+13 0.150E+13 O.lOOE+ 13 0.100E+13 0.1 50E+ 13 0.150E+13 0.800E+12 0.100E+13 0.100E+13 0.100E+ 13 0.1OOE+ 13 0.700E+ 12 0.400E+ 13 0.400E+13 0.400E+13 0.500E+ 12 0.500E+12 0.500E+ 12 0.400E+ 14 0.500E+14 0.200E+14 0.600E+15 0.300E+14 0.500E+ 13 0.100E+13 0.100E+13 0.1OOE+ 13 0.100E+13 0.500E+15 0.50OE+13 0.350E+13 0.500E+13 0.500E+13 0.200E+13 0.200E+13 0.100E+ 14 0.400E+13 0.400E+13 0.200E+ 14 0.200E+14 0.200E+14 0.200E+14 0.400E+ 12 0.100E+13 0.500E+12 0.500E+14 0.100E+14 0.500E+13 0.204E+13 0.1OOE+ 13 0.500E+12 0.250E+16 0.500E+11 0.630E+ 1 1 0.300E+ 12 0.200E+12
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
reverse reaction
E 58100.0 56800.0 65100.0 69000.0 65200.0 67800.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8000.0
8000.0 9000.0 9000.0 8000.0 8000.0 9000.0 9000.0 11000.0 11000.0 9000.0 10000.0 12000.0 14500.0 16500.0 8000.0 8000.0 8000.0 18000.0 20000.0 12000.0 8000.0 10000.0 10000.0 36000.0 40000.0 39000.0 42000.0 46000.0 4000.0 47000.0 8000.0 8000.0 8000.0 27000.0 3000.0 3000.0 2000.0 2000.0 1500.0 1500.0 200.0 2000.0 2000.0 92000.0 92000.0 95000.0
0.00
0.0
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
25000.0 28000.0 10000.0 8000.0 4000.0 0.0 7860.0 3000.0 7000.0 8 1000.0 4200.0 6600.0 9000.0 15000.0
A
n
E
ref
O.OOOE+OO 0.000E+00 O.OOOE+OO O.OOOE+OO O.OOOE+OO O.OOOE+OO 0.565E+14 0.677E+ 15 0.809E+15 0.149E+16 0.280E+ 18 0.335E+18 0.339E+16 0.452E+ 11 0.378E+11 0.469E+11 0.393E+11 0.361E+12 0.137E+11 0.146E+13 0.123E+13 0.1528+13 0.127E+13 0.176E+14 0.664E+12 0.513E+13 0.535E+12 0.448E+ 12 0.580E+12 0.485E+12 0.487E+ 13 0.595E+12 0.498E+12 0.714E+ 13 0.139E+14 0.1 15E+13 0.967E+12 0.623E+14 O.l89E+ 13 O.l28E+ 14 0.274E+ 14 0.300E+14 0.356E+ 12 0.590E+ 13 0.113E+14 0.135E+15 0.162E+15 0.124E+15 0.693E+12 0.492E+12 0.350E+11 0.293E+11 0.1 13E+ 13 0.948E+ 12 0.212E+ 16 0.462E+11 0.387E+11 0.568E+ 12 0.476E+ 12 0.682E+13 0.157E+16 0.369E+12 0.257E+12 0.842E+12 0.260E+ 13 0.108E+12 0.363E+ 15 0.575E+ll 0.432E+ 13 0.912E+ 12 0.383E+13 0.156E+11 0.390E+11 0.602E+ 13 0.220E+13
0.00
0.0 0.0 0.0 0.0 0.0 0.0 90710.0 96654.1 98460.7 104297 96506.7 98313.3 82640.0 15770.3 13963.8 9398.5 7592.0 21714.3 8127.6 17614.0 15807.5 12242.3 10435.8 23557.9 10971.3 11377.5 7933.8 8127.4 17225.7 15419.2 23169.7 8946.8 9140.4 8890.2 23210.6 19266.8 17460.2 28631.2 26416.7 1989.0 10512.7 1617.7 9523.8 7993.9 54324.6 60268.6 62075.1 8285.5 26280.3 24569.9 11372.6 9566.0 4723.9 2917.3 9966.7 19336.3 17529.7 52156.6 50350.0 61100.8 105879 9209.6 9722.6 9389.1 6545.3 3847.8 90086.6 15577.9 18833.7 22181.3 2678.2 13145.9 18488.4 21732.2 12552.0
8 PW PW PW PW PW est est est
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.00
est
PW PW PW est est est est
est est
PW PW est
est PW est
PW PW PW PW est est
est est est est est
est est
PW est est est est est est est est
PW PW PW PW PW PW PW PW PW PW est est est
7 est
est est 7 7 est 43 est est
13 13 13 13 13
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 279
Pyrolysis of Isomeric Butenenitriles
TABLE I (Continued) forward reaction reactionsb CHXHCN = H
76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
+ HXCHCN
A 0.630E+14 0.600E+ 12 0.200E+ 13 0.200E+13 0.150E+13 0.977E+13 0.900E+ 13 0.960E+ 13 0.100E+13 0.490E+12 0.100E+14 0.200E+13 0.300E+14 0.500E+14 0.500E+13 0.100E+ 13 0.440E+ 13 0.500E+14 0.900E+13 0.100E+13 0.420E+12 0.500E+12 0.200E+ 14 O.l78E+ 16 0.200E+13 0.100E+13 0.650E+03 0.700E+13 0.260E+18 0.200E+16 0.500E+13 0.100E+13
reverse reaction
n
E
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
46000.0 7000.0 7000.0 6000.0 3000.0 43100.0 33000.0 43500.0 25200.0 22400.0 58000.0 2000.0 0.0 3000.0 0.0 0.0 2400.0 8000.0 4oooO.O 8000.0 11000.0 4000.0 2400.0 89110.0 7700.0 1500.0 7700.0 0.0 79300.0 82500.0 0.0 33000.0
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.00 0.00 0.00 0.00 0.00 0.00
A 0.608E+13 0.1 17E+ 14 0.351E+15 0.382E+14 0.204E+ 15 0.518E+12 0.396E+14 0.102E+15 0.494E+ 13 0.747E+ 1 1 0.242E+14 0.977E+14 0.605E+15 0.495E+15 0.144E+18 0.587E+15 0.235E+13 0.652E+12 0.142E+13 0.248E+ 15 0.1 78E+ 12 0.462E+14 0.996E+ 1 1 0.461E+ 13 0.685E+12 0.106E+11 0.102E+16 0.166E+17 0.636E+ 16 0.406E+14 O.l30E+ 16 0.679E+17
n
E
ref
0.00 0.00 0.00
3956.8 48550.1 41610.3 45565.2 52713.2 18492.8 55 171.5 10195.4 8377.8 4733.9 2528.0 34122.6 87599.4 5 1947.1 102377 65546.3 42773.9 6504.4 2242.3 72894.3 10348.0 457.7 11097.6 -224.1 23625.7 16581.6 16534.1 87676.9 37436.8 45676.0 58830.8 115752
13 PW
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
est PW
est 32 33 33 34 34
est est 7 7 35 36 37c 38 39 39 39 36 40 41 42 42 39 39 39 35 41 41
"Units for A, cm3mol-' s-I or s-I as appropriate. Units for E, kcal mol-l. PW indicates rate constant evaluated in present work. est indicates rate constant was estimated in present work. E+n X 10". Species identification: cis-, tr-C3HSCN,cis-, trans-crotononitrile; ALLYLCN, allyl cyanide; AC3H4CN, allyllic C3H4CN radical; C3H4CN and C-C3H4CN, non-allyllic C3H4CN radicals; for structures, see Table 11; CH3CCCN, cyanopropyne; C3HS,allyl; [C4H6N],H adduct, see text; C3H4P,propyne; C4H6, 1,3-butadiene; C3H3,propargyl; L-C&,, open-chain species. 'Falloff value. Pressure dependence as given in reference. parameters of allyl cyanide in Table 11. Nevertheless, because the differences in enthalpies of formation of the C3HSCNisomers are less than 5 kcal mol-', the residual uncertainties in calculated thermochemical parameters for allyl cyanide and, indeed, the cis and trans isomers of crotononitrile are such that use of the equilibrium constants to obtain the reverse rate constants for cis-, trans-crotononitrile P allyl cyanide leads to incorrect predictions of the species concentration profiles. Therefore, each isomerization reaction has been uncoupled from its reverse and individually optimized to the experimental concentrations' profiles of the isomers at very low or negligible extents of decomposition. We would expect the activation energies for C3H5CN isomerization to be similar to that foundz2for rearrangement of cy~~~
~~
-
cis-CH,CH=CHCN CHz4HCHzCN
-
CH3
+
+ HC=CHCN
HzC=CH
kI2 = exp(-98S/RT) s-I
+ CHzCN
k-13 = lOI5 5 3 exp(-82.6/RT) s-I
~
(31) Compton, D. A. C.; George, W. 0. J . Chem. SOC.,Perkin Trans. 2 1976, 1666. (32) Ondruschka, B.; Ziegler, U.; Zimmermann, G. 2.Phys. Chem. (hipzig) 1986, 267, 1127. I331 Naroznik. M.: Niedzielski, J. J . Photochem. 1986, 32. 281. (34) Loser, U.; Scherzer, K.: Weber, K. 2.Phys. Chem. ( h i p z i g ) 1989, 270, 237. (35) Kiefer, J. H.; Mitchell, K. I.; Wei, H. C. Inr. J . Chem. Kinet. 1988, 20, 187. (36) Tsang, W.; Hampson, R.F. J. Phys. Chem. Re$ Data 1986, 25, 1087. (37) Payne, W. A. and Stief, J. J . Chem. Phys. 1976, 64, 11.50. (38) Weissman, M. A.; Benson, S . W. J . Phys. Chem. 1988, 92, 4080. (39) Warnatz, J. In Combustion Chemistry; Gardiner, W. C. Jr., Ed.; Springer: New York, 1984; p 197. (40) Warnatz, J.; Bockhorn, H.; Moser, A.; Wenz, H. W. Nineteenth Symposium (International) on Combustion: The Combustion Institute: Pittsburgh, PA, 1982; p 197. (41) Organ, P. P.; Mackie, J. C. J . Chem. Soc.,Faraday Trans. 1991,87, 815.
clopropyl cyanide into the open-chain isomers since the latter rearrangement must proceed through similar biradical intermediates. Our optimized values of El-& are indeed similar to those activation energies reported for cyclopropyl cyanidezz and for cis/trans isomerization in crotononitrile.* The major initiation reactions in the model comprise the reactions -1 1, -1 2, and -1 3 discussed above. Rate constants derived from the model for these reactions are trans-CH,CH=CHCN CH3 HC=CHCN k-', = exp(-96.5/RT) s-l
(42) Kiefer, J. H.; AI-Alami, M.Z.; Budach, K. H. J. Phys. Chem. 1982, 86, 808.
and where the activation energies are in kcal mol-'. The Arrhenius parameters obtained for these initiations have A factors in the expected range for loose transition states and the activation energies correspond well with estimated bond enthalpies. At a total pressure of 20 atm we would expect that these reactions are at or near the high-pressure limit. Rate constants for abstraction reactions in the model have been estimated from analogous reactions of hydrocarbons. Methyl, vinyl, allyl, cyanomethyl, and cyanovinyl radicals were all assumed to abstract a t a similar rate to that of the methyl radical in hydrocarbon systems. Starting estimates of A factors for these abstraction reactions were therefore 10" cm3 mol-' s-I and activation energies from 8 to 11 kcal mol-' plus the endotherm of the reaction, although, for sensitive abstraction reactions, some optimization of rate constants has been possible. Where the abstractions are exothermic, abstractions from reactants to yield both allyllic and non-allyllic C3H,CN radicals were included in
Doughty and Mackie
280 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 TABLE 11: Thermochemical Parameters for the C&CN
structure
System
worn,
Cop, cal K-I mol-’
s03001
kcal mol-’
cal K-I mol-’
300
500
1000
I500
2000
ALLY L C N ~
31.96
74.43
22.17
30.27
42.1 1
47.89
50.71
tr-C3H5CNC
33.63
71.58
19.74
28.28
40.88
46.82
49.70
cis-C3H5CNC
32.03
71.58
19.44
27.96
40.77
46.74
49.68
HCCHCN
97.90
67.15
15.92
19.84
25.28
27.71
29.33
AC3HdCN
77.00
72.96
19.13
28.03
39.20
43.55
46.10
C-C 3 H&N
9 1.60
80.30
2 1.08
27.97
37.26
42.75
45.36
CJH4CN
85.00
72.25
21.49
28.48
37.81
41.98
44.73
CH3CHCH
62.80
65.96
16.46
21.37
30.33
35.71
39.08
[C4H6NI
80.60
76.50
22.27
32.09
45.53
50.98
54.38
CH2CNd CH3CCCN
59.00 7 1 .OO
59.79 68.24
12.34 19.49
15.49 25.28
20.21 33.99
22.59 38.19
23.78 40.25
name’
’Name as shown in Table 1. bParameters calculated from data of refs 28-30
the mechanism. If the reaction is endothermic, the stability of the resulting radical has a large influence on the rate of the abstraction. Thus abstractions which yield the cyanoallyl radical should be very much faster than those which produce the nonallyllic radical. In such cases, only abstractions which yield the cyanoallyl radical have been included in the mechanism. Abstractions comprise reactions 14-37 of Table I. As discussed above, hydrogen addition reactions play an important role in the mechanism. The approach taken in estimating rates of H additions was to compare the estimated rate of fission of the H adduct with the rate at which the adduct could undergo the reverse H fission. If these rates were fairly similar, then the intermediate H adduct was specifically considered in the mechanism. Where adduct fission was very much faster than the rate of H fission, the H adduct was not specifically included in the mechanism and the reaction path was represented by a single step whose rate was taken to be that of H addition to the reactant. Thus, for the reactions H + cis-, trans-CH$H=CHCN HCN CH,CH=CH
-
+
the intermediate adduct [C4HsN] would be expected to fission into HCN and CH3C=CH at a comparable rate with the reverse H fission. Thus the mechanism includes the following reactions: H + CH,CH=CHCN [C,HbN] (53, 54) [C4H6N] s HCN
+ CH,CH=CH
(-55)
Hydrogen addition reactions comprise reactions 49-57 of the model. The detailed mechanism also includes submechanisms for the decomposition of primary products from the C3H5CNisomers. These products include acetonitrile, acrylonitrile, ethyl cyanide, and several hydrocarbons (reactions 61-76). Comparison between reactant and products profiles predicted by the model and experiment is given as a function of temperature in Figures 1-10. In general, the model predictions are in satis-
CDatafrom ref 27. dData from refs IO and 14
factory agreement with experiment. The predicted &/trans ratio of crotononitrile is also in good agreement with experiment. A significant achievement of the model is its ability to predict that the formation of acetonitrile is not first order with respect to initial crotononitrile concentration. Sensitivity and Rate of Production Aanlyses. Reaction flux analyses indicate that isomerization is a more important reaction for allyl cyanide than for crotononitrile. When the starting isomer is allyl cyanide, a significant proportion of the decomposition products arises from crotononitrile formed by initial isomerization of the allyl cyanide. H-addition pathways are also more important for allyl cyanide than for crotononitrile. Sensitivity analysis of the model has revealed the m a t important rate-determining steps. The validity of a sensitivity analysis is dependent not only upon the choice of an appropriate detailed reaction mechanism but also on the accuracy of the specific rate constants. Since there are many reactions involving nitrogencontaining radicals in the model whose reactions have not previously been studied, the present analysis should be considered to be qualitative rather than quantitative. Sensitivity coefficients for formation of the major nitrogencontaining products and acetylene are shown at various temperatures in Figures 11-14 for the important reactions in the mechanism. The variation of sensitivity coefficients with temperature is typical of what might be expected for free-radical reactions. At low levels of decomposition most products exhibit highest sensitivity to the initiation reactions. As the temperature increases, sensitivity to the initiations decreases and sensitivity to the propagation reactions increases as propagation becomes more important. The reason for the failure of acetonitrile to follow kinetics first order in initial reactant concentration can be identified from sensitivity analysis. As may be seen from Figures 11 and 14, yields of acrylonitrile and acetylene, for example, are most sensitive to the initiation reactions which are first order. Acetonitrile, however, is most sensitive at all temperatures to the rate of the biomolecular
J. Phys. Chem. 1992, 96, 281-290 crotononitrik
281
group of allyl cyanide. Rate of production analysis indicates that a significant reaction flux to HCN flows through reaction 77, the disproportionation reaction of two cyanomethyl radicals to form HCN and acrylonitrile. From Figure 11 it may be seen that the sensitivity of acrylonitrile toward the rate of the allyl cyanide initiation reaction H 2 C 4 H C H 2 C N -w H,C=CH + CH2CN (-1 3)
Sensitivity for Acetylene 1.1
11
exceeds unity. This implies that the reaction is slightly branching in nature. This can arise if the vinyl radical initially produced in (-13) decomposes sufficiently rapidly to C2H2 H.
+
Allylcyanide Sensitivity for Acetylene 11
0.0
8 Ja
1
--. CUfN
0.6
Figure 14. Variation with temperature of sensitivity coefficients for the
designated species. Only the most sensitive reactions are shown. abstraction reaction of cyanomethyl radicals with crotononitrile, reaction 27. From Figure 13 it may be seen that two reactions have high sensitivities for HCN production. These are the unimolecular fission of allyl cyanide into cyanomethyl and vinyl radicals (reaction -1 3) and reaction 50, the addition of H atoms to the nitrile
Conclusions The butenenitriles undergo thermal decomposition between 1200 and 1500 K at about 20 atm pressure and at residence times from 650 to 750 NS. Isomerization precedes thermal decomposition. The decomposition is free radical in nature, with a major chain mechanism involving the cyanomethyl radical, leading to the production of acetonitrile and acetylene. Other reaction pathways have been identified involving H-atom additions to the reactant isomers. Several nitrogen-containing radicals including cyanomethyl, cyanovinyl, and cyanoallyl, together with their hydrocarbon counterparts, methyl, vinyl, and allyl, play important roles in the pyrolysis mechanism. v o l y s i s of the butenenitriles can be satisfactorily modeled by a detailed kinetic mechanism involving the aforementioned radicals and hydrogen atoms. Yields of most major and minor decomposition products exhibit kinetics of formation approximately first order in the initial concentration of the starting butenenitrile isomer. The outstanding exception is acetonitrile, whose kinetics are largely influenced by cyanomethyl radical abstraction. Arrhenius parameters for initiation reactions of the butenenitriles can be obtained from experiment and kinetic modeling. Acknowledgment. We thank Drs Robin Walsh and Peter Nelson for helpful discussions. Dr. Nelson also assisted with GCMS analyses. Mr. M. Esler is thanked for research assistance. Finanical support of the ARC and CSIRO/hiversity of Sydney Collaborative Funds are gratefully acknowledged. Regis@ NO.(Z)-H$CH=CHCN, 1190-76-7; (E)-H$H
