PYROLYSIS O F
NITROMETHANE-&
2793
agreement with Posner’s value for the interaction of the OH group. The activation energy for positively charged head groups of the trimethylammonium type are comparable with the residual energy found here. This is surprising in view of the gaseous-like character of these monolayers. However, it may be due to a soft “icelike” structure at the water surface formed by the orientation of the water molecules around the charged head groups leading to a small value of the free surface area. Thus the results presented here are
consistent with the transportation of the C 0 2 molecules through the monolayer in a single-step process, rather than a series of jumps as required by the Fickean diffusion mechanism. Acknowledgment. This project has been supported and financed, in part, by the Federal Water Pollution Control Administration, U. S. Department of the Interior, pursuant to the Federal Water Pollution Control Act.
Pyrolysis of Nitromethane-d, by C. G.Crawforth and D. J. Waddington Department of Chemistry, University of York, York YO1 6 0 0 , England
(Received February 1% 1970)
The pyrolysis of nitromethane-& between 300 and 400’ has been studied and results are compared with the pyrolysis of nitromethane. It is shown that there is a deuterium isotope effect which may be ascribed to the reactions: CDa. CD3N02+ aCDzN02 CDd (3a), CDaO. CD3NO2-+.CD2N02 CDaOD (7a).
+
+
Introduction Although there is considerable evidence that the initiation reaction in the pyrolysis of nitromethane involves fission of the C-N bond1+ CHaN02 +CH3.
+ NO2
(1)
it is not clear what part nitrogen dioxide plays in subsequent reactions, for a t these temperatures it is able not only to react with radicals such as methyl,4 but it may also abstract hydrogen from saturated aliphatic molecules,6for example, nitromethane to form nitromethyl. 2n CHsNOz
+ NO2 + eCH2N02 + HNOz
(2)
This paper is concerned with the pyrolysis of nitromethane&, both in the presence of added nitrogen dioxide and alone, in order to elucidate whether reaction 2 is important in the pyrolysis of nitromethane. Experimental Section Materials. Nitromethane (BDH Ltd.) was purified by fractional distillation using a spinning band column (Buchi). It and nitromethane-da (Merck) were over 99% pure as determined by gas chromatography and by pmr (Perkin-Elmer R6O). Methyl cyanide and methanol (BDH Ltd.) were also purified by fractional distillation. Methane, ethylene, carbon dioxide, nitrous oxide, nitrogen dioxide (Cambrian Chemicals
+
+
Ltd.) and methane-& (Merck Sharp and Dohme of Canada Ltd.) were stated to be over 99% pure, and hydrogen and nitrogen (British Oxygen Co. Ltd.) over 99.5% pure, and were used without further purification, impurities not being detectable by gas chromatography. Methyl nitritee and methyl nitrate7 were prepared from methanol. Apparatus. A static vacuum apparatus was used. The reactants were introduced into a cylindrical borosilicate glass reaction vessel (250-ml capacity, surfa,ce: volume ratio 0.95 cm-l), suspended in an electrical furnace, maintained at a temperature to within k0.2’. Pressure measurements were made with a transducer (Consolidated Electrodynamics) linked to a recorder (Goertz RE 5 11). (1) (a) C. Frejacques, C.R. Acad. Sci., 231, 1061 (1950); (b) T. L. Cottrell and T. J. Reid, J. Chem. Phys., 18, 1306 (1950); (c) T. L. Cottrell, T. E. Graham, and T. J. Reid, Trans. Faraday Soc., 47, 584 (1951). (2) (a) L. J. Hillenbrand and M. L. Kilpatrick, J. Chem. Phys., 19, 381 (1951); 21, 525 (1953); (b) P. Gray, A. D. Yoffe, and L. C. Roselaar, Trans. Faraday Soc., 51, 1489 (1955). (3) C. G. Crawforth and D. J. Waddington, Trans. Faraday Soc., 65, 1334 (1969). (4) L. Phillips and R. Shaw, “Tenth Symposium on Combustion,” Cambridge, England, 1965,p 453. (5) J. H. Thomas, Oxid. Combust. Rev., 1, 137 (1965). (6) J. I. McGarvey and W. D. McGrath, Trans. Faraday SOC.,60, 2196 (1964). (7) A. H. Blatt, Ed., Organic Syntheses, Vol. 11,.J. Wiley, New York, N. Y.,1943,p 412. T h e Journal of Physical Chemistry, Vol. 7 4 , N o . 14, 1070
2794
C. G. CRAWFORTH AND D. J. WADDINGTON
Analysis. Nitromethane and its decomposition products were determined by gas chromatography. Injections of gas samples were made using heated glass sampling tubes (20 ml capacity), equipped with a bypass. Pye 104 Rlodel chromatographs were used, with flame ionization and with thermal conductivity detectors. Columns of molecular sieve 5A, 60-85 mesh (methane), Porapak Q, 100-120 mesh (carbon dioxide, ethylene, and nitrous oxide),s and Celite (AW), 100-120 mesh 10% Carbowax 1000 (methanol, methyl cyanide, methyl nitrite, methyl nitrate, nitromethane) were used. The chromatograph was linked to an AEI R4S 12 mass spectrometer and was used to identify the peaks. Concentrations of reactant and products were obtained from a knowledge of the response of standard amounts, following a series of experiments to ensure that there was no interaction between products during sampling and analysis. Results and Discussion Nitr~methane-d~ was allowed to decompose under 20 40 60 80 Time, min. similar conditions to n i t r ~ m e t h a n e . ~While the overall pressure increase is similar, the rate of decomposition Figure 1. Pyrolysis of nitromethane and nitromethane-da; of nitromethane-d3, as determined by analysis, and consumption of reactants and formation of methane at 368’; the rate of formation of methane-& are reduced (Figinitial pressure, 50 mm; 8 , methane; 0 , methane-&; ure 1). As in the decomposition of n i t r ~ m e t h a n e , ~ 0, nitromethane; Ornitromethane-ds; - - - -, pressure change . pressure change (nitromethane-da). (nitromethane); the concentration of methane-& can be used as a measure of the rate of decomposition, for the fraction of methane formed, in the early stages of reaction, is a constant function of the amount of nitroalkane decomposed (Figure 2), and for at least the first 20% of the reaction the yields of methane and methane-& from the two nitroalkanes are similar. This yield, as would be expected, is lower than that obtained at 424” from n i t r ~ m e t h a n e . ~Figure 2 also shows that the change in pressure is a true reflection of the decomposition of the nitroalkanes. The reaction order for the decomposition of nitromethane-d3 varies with pressure, being first order above 5 10 15 20 25 ca. 100 mm pressure (Figure 3). On lowering the Pressure change, rnrn. pressure, the rate constant decreases. At 50 mm initial pressure, the order of reaction for nitromethane-d3, Figure 2. Pyrolysis of nitromethane and nitromethane-da; consumption of reactants and formation of methane as determined by the rate of formation of methane-&, as a function of pressure change at 365’; initial is 1.3 (Figure 4), the results being similar to those pressure, 50 mm: 0 , methane; @, methane-&; found for nitromethane.3 Under these conditions the 0, nitromethane; 0 , nitromethane-&. energy of activation is 49.3 f 0.5 and 53.7 f 0.5 kcal/mol for nitromethanes and nitromethane-da, reUnder the conditions of these experiments, it can spectively. be calculated8 that 27.5 mm of the 50 mm of nitrogen The principal source of methane appears to be dioxide added decomposes ( K 4 (368) = 0.16 atm”*), from reaction of methyl with the nitroalkane2t3 the calculation being confirmed by experiment, the CH3. CH3NOz +CH, mCHzN02 pressure change being 14.0 mm. The equilibrium mix(3) ture retards the consumption of both nitromethane CD3. CD3NOZ CD, :CD~NOZ (3a) and nitromethane-d3, the results in Figure 5 being To investigate the role of reaction 2, nitrogen dioxide was added to the reaction vessel and allowed to de(8) C. G. Crawforth and D. J. Waddington, J . Gas Chrornatogr., 6 , 103 (1968). compose to equilibrium
-
+ +
a ,
+ +
NOz
NO
+ ‘/zOz
The Journal of Physical Chemistry, Vol. 74, N o . 1.1, 1970
(4)
(9) “Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards, Technioal Note 270-1,1965.
2795
PYROLYSISOF NITROMETHANE-&
I
SL
I
I
50 100 Initial Pressure, mrn.
I 150
Figure 3. Pyrolysis of nitromethane and nitromethaneds; variation of rate constant with initial pressure of reactant at 365" : 0 , nitromethane; 0, nitromethane-&.
log [reactant].
Figure 4. Pyrolysis of nitromethane and nitromethane-dr; rate of formation of methane as a function of pressure of reactant at 365"; initial pressure, 50 mm: 0, methane; 0, methane-&.
I
60
Time, min.
Figure 5 . Pyrolysis of nitromethane and nitromethane-da; rate of consumption of reactant as determined by analysis in presence of added nitrogen dioxide a t 365": 0, nitromethane, 50 mm; 0, nitromethane-dr, 50 mm; 0, nitromethane, 50 mm; nitrogen dioxide, 50 mm; 6,nitromethane-ds, 50 mm; nitrogen dioxide, 50 mm.
methane, a major product in the absence of added nitrogen dioxide, is not detectable. Similarly, for nitromethane-ds, chromatography peaks (confirmed by mass spectronietric analyses) corresponding to methyl nitrite-da, methanol-de, and methyl cyanided3 are much reduced, in the presence of nitrogen dioxide, while methane-d4 is not detectable. As the addition of nitrogen dioxide decreases the rate of decomposition of nitromethane and nitromethane-da, it appears that reaction 2 does not play a significant role under these conditions. The reduction in rate is presumably due to reaction between radicals, formed during the decomposition of the nitroalkanes and nitrogen dioxide, for example CHI*
obtained by analysis of residual nitromethane and nitromethane-d3. Separate experiments show that nitric oxide does not retard the consumption of nitromethane, the oxide reacting with methyl to form nitrosomethane which subsequently reacts further with nitric oxide to regenerate methyl, while forming nitrogen.av10 Nor does oxygen retard the early stages of reaction,1° the nitrogen dioxide formed by reaction 1 reacting preferentially with methyl. Only in the later stages of reaction when the concentration of nitrogen dioxide is very low does oxygen play a significant part. Thus the observed retardation of nitromethane and nitromethane-d3, under the conditions of the experiments discussed in this paper, in the presence of the equilibrium mixture of oxides of nitrogen and oxygen, is due to nitrogen dioxide. It has been shown that the addition of nitrogen dioxide to nitromethane reduces the concentrations of methyl nitrite, methanol, and methyl cyanide, while
6
40
10
CDa.
+ NO2 (+M)
+ NOz (+M)
--j
CHaNOz (+AI)
----+
CD3NOz (+M)
(5) (5a)
and the rate of secondary attack by methyl on nitromethane (reactions 3 and 3a) is reduced. This is confirmed by analysis for methane, which is not detected in presence of nitrogen dioxide. If methane had been formed it would have been found on analysis as it is stable under the conditions of these experiments. In the presence of nitrogen dioxide, methyl reacts not only as in reactions 3 and 3a to form nitromethane but also to yield methoxy13** l l z
+ NO2 +CHaO. + NO CD3. + NO2 --+ CD30. + NO CH3.
(6)
(64
(10) C. G.Crawforth and D. J. Waddington, to be published. (11) For example, A. B. Gagarina and N. M. Emanuel, Russ. J. Phgs. Chem., 33, 90, 197 (1959). (12) C.G. Crawforth, Ph.D. Thesis, University of York, 1968.
The Journal of Physical Chentistry, Vol. 74, No. 1.6, 1970
C. G. CRAWFORTH AND D. J. WADDINGTON
2796 Methanol and methanol-d4 formed early in the decomposition are produced by disproportionation reactions of methoxyl13or by hydrogen abstraction.
+ CHal\’Oz CD30. + CDaNOz -+ CHsO*
+ .CHzNOz (7) CDsOD + eCDtN02 ( 7 4
4CHaOH
Methanol is not detectable in larger quantities for it is rapidly oxidized by nitrogen dioxide under these conditions.14 Moreover, reactions 7 and 7a will have to compete with reactions 8 and Sa, respectively, particularly when excess nitrogen dioxide is added. 1b CHsO *
CDaO *
+ NO2 +CHaONOz + NOz +CDaONOz
(8) (84
The stabilization of radicals by reactions 5, 5a, 8 and 8a accounts for the reduction in rate of decomposition of the nitroalkanes in the presence of added nitrogen dioxide and for the reduced yield of methanol and of methyl nitrite, also formed from methoxyl. If there is no interaction between methyl and nitrogen dioxide ~ c H ~ N o ~=/ ~2 ,
This ratio will decrease if reaction 5 plays an important part in the reaction. Thus, in order to show that reaction 5 occurs to a significant extent, the ratio Of kCHsNOa:kCH8NOa+NOz should tend to 2. In fact, the ratio, as calculated from Figure 5 is almost
The Journal of Physical Chemhtry, Vol. 7.4, No. 14,1970
X min-’; ~ C H ~ N O ~ + = N O 5.6 ~ min-l). The addition of a similar amount of nitrogen dioxide only reduces the rate constant for the decomposition O ~ 6.2 X of nitromethane-da by a factor of 1.3 ( ~ C D ~ N = min-I) implying min-I; k m 8 N o l + N o z = 4.7 X abstraction reactions 3a and 7a are slower than the corresponding reactions 3 and 7. The difference in activation energy for the pyrolysis of nitromethane and nitromethane-da can thus be explained in terms of attack by methyl and methoxyl radicals on the parent nitroallrane. Although the role of nitrogen dioxide in the system is to reduce the rate of attack by methyl, reactions 5 and 5a must still be occurring, for even when excess nitrogen dioxide is added, the rate of consumption of nitromethane-d3 is still slower than that of nitromethane. ~ ( ~ c H ~ = N o 1.1 ~
X
Acknowledgments. Our thanks are due to Shell Research Limited for financial assistance for apparatus and to the Kingston upon Hull Education Department for a studentship (C. G. C.). Our thanks are also due to Miss M. A. Warriss for technical assistance, to Dr. C. B. Thomas for mass spectrometric analyses, and to Dr. D. M. Goodall for helpful discussions. (13) For example, P. Gray, R. Shaw, and J. C. J. Thynne, Progr. React. Kinet., 4, 63 (1967). (14) R. Silverwood and J. H. Thomas, Trans. Faraday Soc., 63, 2476 (1987). (15) G.Baker and R. Shaw, J. Chem. Soc., 6965 (1965).
