2555
PYROLYSIS KINETICSOF ACETONITRILE
Pyrolysis Kinetics of Acetonitrile by Thomas W. Asmus1 and Thomas J. Houser Chemistry Depavtment, Western Michigan University, Kalamazoo, Michigan
(Received November 4, 19B8)
The pyrolysis kinetics of acetonitrilewas studied in the temperature range of 880-960" at 1 atm total pressure, using a stirred-flow reactor and helium as the carrier gas. Under these conditions a fractional reaction order greater than 1 was found. Plots of reaction rate/reactant concentration us. reactant concentration produced straight lines, the intercepts and slopes of which were equal to first-order, kl, and second-order, kl,rate constants, respectively. From Arrhenius plots of these rate constants the following expressions were obtained : k , = 1011.8exp(-72,000A 4000/RT) sec-', and k z = 1020.5 exp(-120,000 =t6000/RT) 1. mmol-l sec-l. The major volatile products of the reaction were methane and hydrogen cyanide, the relative amounts of which shifted by about a factor of 2 in going from 7 to 60y0 reaction. Small amounts of ethylene and vinyl cyanide were also observed. A brown, nonvolatile polymeric residue was formed ii, the reactor's exit which, from elemental analysis and infrared data, is believed to be of the cyano-substituted ethylenic type. The pyrolysis rate of acetonitrile-& was found to be about 40% slower than that of normal acetonitrile. To explain the rate and product data a dual mechanism was proposed consisting of first- and second-order contributions.
This work involved the study of the kinetics and products of the gas-phase, thermal decomposition of acetonitrile in the temperature range of 880-960" using a stirred-flow reactor. The literature has revealed no recent work on the thermal decomposition kinetics of acetonitrile. I n 1942, Rabinovitch and Winkler2 studied the products of the thermal decomposition of acetonitrile at 865" and atmospheric pressure; however, no mechanism was proposed. These investigators reported hydrogen, methane, and hydrogen cyanide as the major volatile products. Acetylene and/or ethylene were reported as minor volatile products. McElcheran, et ~ l . in, ~1958, reported results of the photolysis of acetonitrile at 1849 8 and proposed a mechanism to account for the observed products. They suggest that initiation may occur either by C-H or C-C bond breakage where the former process is more probable. These authors also suggest that HCN is formed predominantly by an H atom abstraction of the CN group which is believed to occur through addition to the multiple bond with the formation of an unstable imine radical.
Experimental Section Apparatus and Procedure. Experiments were carried out in a conventional flow system using a motordriven syringe to inject the liquid reactant as described p r e v i ~ u s l y . ~The flow system was equipped with a stirred-flow reactor, constructed of Vycor, the design of which had been previously tested for stirring effi~iency.~After an experiment the volatile products and unreacted acetonitrile in the traps were transferred under vacuum a t room temperature to a vial which contained a weighed amount of p-xylene to be used as as an internal standard for quantitative gas chromatographic analysis.
Materials. Eastman spectrograde acetonitrile (stored above Linde Type 3A molecular sieve for adsorption of water) was used without further purification. The reactant was checked for impurities by mass spectrometric analysis (all mass spectra were run on an Atlas Model CH 4) and found to be better than 99% pure. hlatheson Coleman and Bell analytical grade p-xylene was used as the internal standard for the quantitative gas chromatographic determination of acetonitrile. Analytical Techniques. For the quantitative determination of unreacted acetonitrile, diethylene glycol succinate was used as the liquid phase on a Chromosorb W (acid and DRICS treated) column in an F & M Model 720 gas chromatograph. A calibration curve of known acetonitrile to p-xylene weight ratios vs. peak area ratios was constructed. The selection of p-xylene as an internal standard was based on its resolvability with acetonitrile and hydrogen cyanide and on its ability to produce moderately broad peaks. The mass spectra of volatile products from several experiments covering a broad range of conditions were obtained also. The ratio of HCN/CH4 in the reactor's exit stream was determined with a gas chromatograph which was linked to the flow system by a Beckman gas sampling valve. The column had a solid phase of Chromosorb W (acid and DMCS treated); dinonyl phthlate was used as the liquid phase. The (1) This research is in partial fulfillment of requirements for an M.S. degree. (2) B. S. Rabinovitch and C. H. Winkler, Can. J . Res., 20B, 69 (1942). (3) D. E. McElcheran, M. H. J. Wijnen, and E. W. It. Steacie, Can. J. Chem., 36, 321 (1958). (4) T. J. Houser and B. M. H. Lee, J . Phys. Chem., 71, 3422 (1967). (5) J. M. Sullivan and T. J. Houser, Chem. Ind. (London), 1057 (1965).
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THOMAS W. ASMUSAND THOMAS J. HOUSER
detector cell (Gowmac Model No. 9677) was equipped with a pair of matched thermistors operating at room temperature which formed part of a Wheatstone bridge. This system was also used to determine the isotope effect using deuterated acetonitrile.
Results Products. All peaks which appeared in the mass spectra of the exit gases with a height of at least 1% of the base peak height, along with the probable species to which they were attributed, are shown in Table I. The major volatile products of the pyrolysis of acetonitrile were hydrogen cyanide and methane. Unlike previously reported results from the study of this reaction at 865°,2 hydrogen did not appear in detectable quantities. Ethane also was not observed as a reaction product. Compared to the major volatile
Table I : Relative Intensities of Key Ions from Mass Spectrometer Analysis of Products
m/e
15 16 26 27 28 38 39 40 41 51 52 53
,-Relative peak height-? At50% At 10% decomp decomp
5 5 2 10 1 9 18 50 100 1 1 1
36 47 15 100 7 2 7 22 47 2 2 2
Probable species
CHa CHI CN, CzHz HCN, C2Ha CZH4 CCN CHCN CHzCN CHaCN CCHCN, CHCCN CHCHCN, CHzCCN CHiCHCN
products, ethylene (28) and vinyl cyanide (53) appeared only in relatively small amounts. There is a greater degree of uncertainty in the height of the 28 peak because of a relatively high nitrogen background in the instrument at the time the spectra were run. A shift in the relative amounts of the major products was observed and, within the precision of the experiments, appears to be a function of the extent of reaction. The data from the gas chromatographic measurements are presented in Table 11. Because it was not possible to calibrate the chromatograph for HCN and because the the response factor was not product mole ratios are not known with any degree of certainty.6 However, it can be Seen ,.hat the relative amounts of HCN and methane changed by nearly a factor of 2, from about 1.0 to 1.9. A brown, nonvolatile polymeric product, which was readily soluble in acetone and dimethyl sulfoxide, coated the Of the reactor Outlet and the traps. Quantitative elemental analysis of a sample The Journal of Physical ChemistTy
Table I1 : Relative Concentrations of Major Products Contact time, 80C
Reactant concentration, mol %
2.2 2.15 4.6 4.45 5.9 5.9 10.5 10.1
1.85 4.8 1.90 4.6 1.85 4 6 1.90 4.6
1.35 1.3 2.75 2.65 3.7 3.6 9.9 9.5
1.80 4.35 1.90 4.6 1.85 4.5 1.85 4.5
T
Peak area ratios (HCN/CHd
% reaction
= 910°
1.20 0.90 1.25 1.10 1.45 1.50 1.60 1.55
8 7 14 15 18 21 28 30
1.25 1.15 1.60 1.45 1.75 1.70 1.90 1.90
14 17 26 23 42 37 60 65
T = 960’
(physically removed from the apparatus near the reactor exit) suggested the following empirical formula, (CllH,N&. Quantitative analysis of the HCN in the exit stream (via cyanide ion determination) coupled with the amount of acetonitrile consumed indicated that roughly 25-3070 of the CN produced by the pyrolysis reaction remained in the apparatus in a polymeric residue. A network cyclic structure (paracyanogen) has been suggested as a possible product resulting from the polymerization of cyanogen ;7 however, these authors suggested that paracyanogen is unstable at temperatures above 850”. Consequently, it is felt that this polymeric residue is of the cyanosubstituted ethylenic type. This argument is supported by the infrared spectrum of this material which shows major absorption at 2200 cm-l. This roughly corresponds to the CN triple bond stretch which suggests that the CN groups are intact. Kinetics. The kinetic data are shown in Table 111. With a stirred-flow reactor, explicit values for the rate can be determined according to eq l8 rate =
(cg
- c)/t
=
f(c)
(1)
where co = intial concentration of acetonitrile and c = (6) An estimate of the relative response factor for HCN of 50 was obtained by extrapolation, to a molecular weight of 27, of a graph of molecular weight us. relative response factors for the alkylnitrile homologous series. Using this value and 36 for methane (relative to 100 for benzene) indicates that the rate of production of methane probably is higher than that of HCN at low extents of reaction. However, the rate of production of HCN is definitely higher than that of methane at higher extents of reaction. All relative response factors were obtained from A. E. Messner, D. M. Rosie, and P. A. Argabright, Anal. Chem., 31, 230 (1959). (7) C. F. Cullis and J. C. Yates, J . Chem. SOC.,2833 (1964). (8) K. J. Laidler, “Chemical Kinetics,” 2nd ed, McGraw-Hill Book co., Inc., New York, N. Y . , 1965, p 24.
PYROLYSIS
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KINETICSOF A C E T O N I T R I L E
the intercept equal to kl and slope equal to kz. The exclusion of other possible treatments of the kinetic data and the selection of this approach as best representing all the data will be discussed in the next section. Table IV summarizes the values of kl and kz obtained by least-squares treatments of the data. Arrhenius plots of the rate constants yield the following expressions.
Table I11 : Pyrolysis of Acetonitrile Kinetic Data Fraction reacted
Rate, mmol/l.-' see -1
2 8 5 5 5 5 8 2 8
0.824 0.219 0.206 0.500 0.825 0.206 0.516 0.832 0.206 0.822 0.515 0.516 0,516 0.516 0.820 0.205 0.820
Data a t 880" 6.2 0.103 6.7 0.116 13.4 0.171 12.9 0.182 12.4 0.215 20.2 0,285 19.2 0.250 18.5 0.290 26.5 0.300 24.5 0.308 25.5 0.325 9.5 0.136 16.2 0.203 21.7 0.325 16.2 0.217 17.6 0.188 15.8 0.246
0.0137 0,0031 0.0026 0.0071 0.0143 0.0029 0.0067 0.0131 0.0023 0.0103 0.0066 0.0073 0,0065 0.0077 0,0109 0.0022 0.0127
5 2 2 5 8 8 5 2 5
0,500 0,200 0,201 0.505 0.808 0.807 0,506 0.202 0.B03
Data at 910' 6.3 0.188 6.5 0.174 13.0 0.330 12.3 0.361 11.9 0.345 17.9 0.399 18.7 0.434 19.2 0.352 24.5 0.526
0.0150 0.0054 0.0051 0.0148 0.0234 0.0180 0.0117 0.0037 0.0108
2
2
0.196 0.785 0.196 0.782 0.196 0.790 0.177
Data at 940' 6.3 0.333 5.7 0.404 12.5 0.504 11.4 0.561 18.7 0.574 17.1 0.664 2.6 0.177
0.0106 0.0556 0.0079 0.0385 0.0061 0.0306 0.0119
8 2 8 8 2
0.770 0.192 0.778 0.776 0. I92
Data at 960" 5.6 0.527 12.3 0.676 11.1 0.700 16.6 0.765 19.0 0.710
0.0732 0.0106 0.0489 0.0358 0.0072
Initial concentratione rnmol/l. mol %
8 2 2 5 8 2 5
8
8 2 8 2
8
Contact time, see
kl =
kz = 1020.5 exp( - 120,000 f SOOO/RT) 1. mmol-* sec-l
= rate = klc
-/- kzcz
(3)
(4)
Table IV : Pyrolysis of Acetonitrile Kipetic Results
concentration of acetonitrile at contact time, t (reactor volume/volume rate of flow). Thus, it was possible to plot values of the rate against various functions of the concentration to obtain the best rate expression. Plots of log rate vs. log c yielded straight lines having slopes of about 1.2. Since the apparent order of the reaction was greater than 1, a rate equation involving a first- and secondorder term was tried and found to fit the kinetic data reasonably well. Thus, if -dc/dt
exp(-72,000 f 4000/RT) sec-l
Temp, OC
ki X 109, see-1
1. mmol-1 8ec-1
880 910 940 960
1.5 3.3 6.7 14.6
0.64 1.9 12.6 19.8
k2 X 102,
Several experiments at 910" and about 5 mol % reactant were made to determine the effect of additives on the rate. Two experiments conducted with HCN added at a concentration of one-fifth that of the reactant produced no measurable effect on the rate of disappearance of reactant, indicating no autocatalysis by this product. On the basis of pyrolysis data for methane,g it was assumed that methane was chemically inert (about 0.1% decomposed) at reaction conditions. Cyanogen bromide, a source of radicals at reaction temperatures, was also used as an additive in two experiments at a concentration of about one-eighth that of the reactant. With this additional concentration of radicals the rate would be expected to increase significantly if a radical-chain mechanism were operating. However, within the reproducibility of the experiments (about f10%) no increase in the rate was observed (the data for the additive experiments were subject to a greater experimental uncertainty). Three experiments were made using deuterated acetonitrile (CDSCN) at a concentration of about 5 mol %, one a t 910" and two at 940" at different contact times. These experiments were run immediately before or after an experiment with normal acetonitrile, with all experimental conditions unchanged for each pair of experiments to reduce experimental scatter. The normal acetonitrile reacted about 40 f 10% ( k ~ / k=~ 1.4) more rapidly than the deuterated reactant based on either reactant consumption or product formation.
(2)
then a plot of rate/c os. c should be a straight line with
(9) H. B. Palmer and T.
J. Hirt, J. Phys. Chem., 67, 709 (1963). Volume 78,Number 8 August 1969
THOMAS W. ASMUSAND THOMAS J. HOUSEH
2558 Discussion and Conclusions The magnitude of the reaction order, i.e., about 1.2, and the observation that there is a large shift (about a factor of 2) in the relative amounts of the major products indicate that the pyrolysis mechanism is complex. A radical-chain mechanism was ruled out because additives had relatively small effects on the rate, and the observed activation energies (72 and 120 kcal/mol for the first,- and second-order rate constants, respectively) were as large or larger than the weakest bond in the molecule, i.e., the carbon-hydrogen bond which is reported to have a bond energy of 72 kcal/ mol.lo It is believed that no simple mechanism could lead to the large shift in the relative quantities of the major products that was observed. Thus, it is reasonable to propose that a t least two mechanisms are operating simultaneously to correspond to the two terms in the rate equation. The first-order term could be accounted for by the following radical mechanism.
+H H + CH3CK --%CH3 + HCN CH3 + CH3CN -% CH4 + CHzCN CH3CY -% CH2CK
(5)
(6)
(7)
nCHzCN + products (polymer, CZH~CN, C2€Id,HCX)
(8)
Steps 6 and 7 are assumed to be rapid enough to make step 5 rate controlling; Le., steady state is assumed for the H and CH3 radicals. Thus, the apparent firstorder rate constant for the disappearance of acetonitrile, kl, is equal to 3k5. The observation that the activation energy for the first-order rate constant and the reported C-H bond energy are the same supports a mechanism with C-H bond rupture as the rate-controlling step. In addition, an isotope effect as large as 40% also indicates that the C-I-I bond rupture must play an important role in a rate-controlling step of the reaction. An estimate of the isotope effect to be expected for a simple bond rupture (as in reaction 5 ) can be made using the reported infrared spectral data for acetonitrile and acetonitrile-cis.11 Using as a model the “rigid” activated complex, Le., all fundamental vibrational frequencies except that leading to bond rupture are assumed the same in both the complex and reactant a maximum isotope effect of about 70%
The Journal of Physical Chemistry
1.7) was calculated at a temperature of 1200°K. If the C-C bond rupture were the ratecontrolling step in the mechanism, then a maximum isotope effect of approximately 12% would be predicted from the infrared data and the above model. Thus, the observed and calculated isotopes effects appear consistent with the above mechanism being a major contributor to the over-all pyrolysis of acetonitrile. For the above mechanism, the HCN/CH4 molar ratio must be at least 1, with some increase occurring as the polymeric residue, and/or radicals leading to polymers, contribute additional HCN at longer reaction times. Therefore, to account for lower HCN/CH4 product ratios it is necessary to assume that the secondorder term represents a mechanism which leads to the production of higher concentrations of methane than HCN, at least initially. The more rapid decrease in the rate of reaction involving a second-order term than that involving a first-order term, as the reactant is consumed, would also contribute to the observed over-all shift in the product ratio, The large activation energy for the second-order term may be due to the C-C bond rupture playing an important role in the rate-controlling step. The C-C bond dissociation energy was reported to be 110 and 122 kcallmol from electron impact and photodissociation studies, respectively.13 It was found that an alternate rate equation consisting of first- and three-halves-order terms fit the data equally well. However, this equation was discarded because a radical-chain mechanism was required for the three-halves-order term which would have had an activation energy much lower than that observed experimentally. ( ~ s H / ~ E , D=
Acknowledgment. We wish to express appreciation for the mass spectrometer analyses run by Dr. M. Grostic and Mr. R. Wnuk of the analytical laboratory of The Upjohn Company. (10) Ti. H. Dibeler and S. K. Liston, J . Chem. Phys., 48, 4765 (1968). An earlier value of 5 7 9 kcal/mol was reported by R. F. Pottie and I?. P. Lossing, J . Amer. Chem. Soc., 83, 4737 (1961). ( 1 1 ) E. L. Pace and L. J. Noe, J . Chem. Phys., 49, 5317 (1968). (12) The equation used for this model is k d k D = sinh ( h v a / 2 k T ) / sinh ( h v ~ / % T ) where , k is the Boltzmann constant, h is the Planck constant, T is the absolute temperature, and Y is the fundamental vibrational frequency of the normal mode leading to reaction. J. Bigeleisen, ibid., 17, 345 (1949). (13) D. D. Davis and H. Okabe, ibid., 49, 5526 (1968). The data for the 110 value were obtained from C. A. McDowell and J. W. Warren, Trans. Faraday Soc., 48, 1084 (1952),who reported a value of 102 kcal/mol.
