PYROLYSIS O F
%NITROPROPANE
2427
Pyrolysis of 2-Nitropropane by D. J. Waddington* and M. Ann Warriss Department of Chemistry, University of York, Heslington, York YO1 6 0 0 , England
(Received February $3, 1.971)
Publication C05t5 borne completely by T h e Journal of Physical Chemistry
A study of pyrolysis of 2-nitropropane at 323 and 350' is compared with recent data obtained for the decomposition of nitromethane and nitromethane-da. Additional experiments with added oxides of nitrogen and with oxygen show that 2-nitropropane decomposes almost exclusively by an intramolecular mechanism to propylene. Minor products such as acetone and methyl cyanide, previously discussed in terms of a chain mechanism involving the nitroalkane, are probably formed by secondary oxidation of propylene. Independent experiments involving the oxidation of propylene and nitrogen dioxide are described.
Introduction
2-nitropropane is quantitative (Figure 1). The principal nitrogen-containing products are nitric oxide and Nitromethane decomposes via fission of the C-N nitrogen dioxide, the yield of nitrogen at the end of reacbond to yield methyl radicals which, in turn, react with tion being less than 3%. Small amounts of acetone, the nitroalkane by a short-chain radical mechanism. l t 2 It has been suggested that the higher n i t r o a l k a n e ~ ' ~ * ~ ~acrolein, ~ - ~ 1,2-epoxypropane1 methyl cyanide, and nitromethane were also detected, their presence being condecompose by intramolecular rearrangement to elimfirmed by mass spectral analyses (Table I). inate an alkene. However, some of the reaction prodPropylene may be formed from 2-nitropropane by ucts from the pyrolysis of these nitro compounds suggest intramolecular elimination of HNOz a radical m e c h a n i ~ m ,and ~ the present work is concerned with elucidating the importance of radical reactions in the pyrolysis of 2-nitropropane.
Experimental Section Materials. 2-Nitropropane (BDH) was purified by fractional distillation hsing a spinning-band column (Buchi). Methyl cyanide (Fisons Ltd.) and isopropyl nitrite (Koch-Light Laboratories Ltd.) were also purified by fractiond distillation, while the gases were commercially available at over 99.5% purity, impurities not being detectable by gas chromatography. Apparatus. The apparatus was similar to that already described, pressure measurements being made with a transducer (Consolidated Electrodynamics) connected to a recorder (Goertz R E 511). AnaZgsis. The reactant and most of its pyrolysis products were determined by gas chromatography,2 the chromatograph being linked to a mass spectrometer (AEI RiIS 12) in order to identify the peaks. Columns of silica gel (100-150 mesh) (for hydrocarbom), Celite (acid-washed, 100-120 mesh)-10% Carbowax 1500 (for nitromethane, acrolein, 2-nitropropane, acetone, methyl cyanide, 1,2-epoxypropane, and isopropyl nitrite), and Porapak Q (100-120 mesh) (for hydrogen, nitrogen, and nitric oxide)' were used. The total concentration of nitric oxide and nitrogen dioxide was determined by an adaptation of a method for nitrogen dioxide, using azulene and p-nitroaniline,* and the concentration of nitrogen dioxide was found by subtraction.
Results and Discussion At the start, of reaction, the yield of propylene from
or by fission of the C-N bond, followed by radical reactions
NO2
I
-
CH,-CH-CH, (1) (1)
+ (11)
(2)
(11)
NOz
I
C3H8
+ NO2
-+ CHa-CH-CH3
+ (CH,-C--CH,
NO2
I
or CH3-CH-(?H2) (111)
(3)
The activation energy has been determined in several previous studies. In a static system a value of (1) (a) C. Frejacques, C. R. Acad. Sci., 231, 1061 (1950); (b) T.L. Cottrell, T. E. Graham, and T. J. Reid, Trans. Faraday Soc., 47, 684 (1951); ( e ) P.Gray, A. D. Yoffe, and L. C. Roselaar, ibid., 51, 1489 (1955). (2) C. G. Crawforth and D. J. Waddington, ibid., 65, 1334 (1969). (3) T. L. Cottrell, T. E. Graham, and T. J. Reid, ibid., 47, 1089
(1951). (4) T. E.Smith and J. G. Calvert, J . Phys. Chem., 63, 1305 (1959). (5) K.A. Wilde, I n d . Ens. Chem., 48, 769 (1956). (6) G. N. Spokes and S. W. Benson, J . A m e r . Chem. SOC.,89, 6030 (1967). (7) C. G. Crawforth and D. J. Waddington, J . Gas Chromatogi-., 6 , 103 (1968). (8) E. E. Garcia, Anal. Chem., 39, 1605 (1967). T h e Journal of Physical Chemistry, Vol. 76, No. If3? 1.971
2428
D. J . WADDINGTON AND M. ANN WARRISS
Table I : Formation of Some Products (mm) at 350" from (a) the Pyrolysis of 2-Nitropropane, (b) the Reaction of 2-Nitropropane and Oxygen, (c) the Reaction of 2-Nitropropane and Nitrogen Dioxide, and (d) the Reaction of Propylene and Nitrogen Dioxide
-
Product
7
Acetone
Acrolein
1,2-Epoxypropane
(a) 2-Nitropropane, 20 mm
0.15
(b) 2-Nitropropane, 20 mm;
0.6
0.3 0.04
0.2 0.1
1.1 1.0
0.5 1.6
0.0 0.0
( e ) 2-Nitropropane, 20 mm;
0.4
0.3
0.2
1.4
1.2
0.0
nitrogen dioxide, 20 mm (d) Propylene, 50 mm; nitrogen dioxide, 50 mm
0.8
0.4
0.3
0.8
0.4
2.6
Reaction
Methyl cyanide
Nitromethane
2-Nitropropane
oxygen, 40 mm
___*----
_*----
401
1'O
6
E
El 30
30 d;
E
P
In"
c
c
22 20
20
$In
oxygen does not markedly alter the yield of propylene in the early stages of reaction (Table 11),although the yield of propylene is reduced towards the end of reaction and the yield of acetone increases (Table I). Under these conditions, oxygen would have reacted with the precursor, as occurs on its addition to decomposing nitromethane, the yield of methane being reduced from 30% to zero.2
F! a 10
IO
Table I1 : The Pyrolysis of 2-Nitropropane and the Formation of Propylene at 323"
100
Figure 1. Pyrolysis of 2-nitropropane. Formation of products a t 323": Initial pressure, 30 mm; 0 , hydrogen ( X 10); (3, nitrogen dioxide; 0 , nitric oxide; E?, propylene; -, pressure change.
--
164.4 kJ mol-l was found (between 250 and 337°),4 while, in flow systems, values of 163.1 (between 367 and 397°)5 and 167.3 2.1 kJ mol-' (between 500 and 705°)6 have been reported. Computation for the enthalpies of reactions 1 and 2 may be compared with these experimental activation energies. Standard enthalpies of formation of 2-nitropropane, propylene, isopropyl, nitrogen dioxide, and "02 of -143.9, 20.1, 73.6, 33.0, and -77.9 kJ mol-' were used.g On considering reaction 1 first, and attempting to estimate Arrhenius parameters, one may apply the generalization that the activation energy of such an endothermic reaction is at least (AH 34) kJ As both the NH group and the oxygen atom appear to be sterically equivalent to the methylene group and, in general, the same strain conditions will prevail in heterocyclic compounds as in carbocyclic systems,l1 one may allow for a strain energy of approximately 44 kJ mo1-1,12 giving a lower limit for the activation energy of 164 k J mol-', compared with an endothermicity of 251 kJ mol-l for reaction 2. Moreover, significant amounts of propane are not found during the reaction, and the addition of
*
+
The Journal of Physical Chemistry, Vol. 76, No. 16,1971
Time, min
1.0 2.0 2.5 4.0 5.0 6.0 7.0 8.0 9.0 9.5 10.0 11.0 12.0
---.
Propylene, mm
7
50 Time, min.
2-Nitropropane, 2-Nitropropane, 20 mm; 20 mm oxygen, 40 mm
0.8 1.2 1.3 3.0
0.8 1.4 3.4
3.2 4.0 4.2 4.0 4.4
2-Nitropropane, 20 mm; nitrogen dioxide, 20 mm
1.2 1.3, 1 . 4 1.5, 2.0 2.7 2.8 2.8 3.3 4.1, 4 . 5
5.6 7.0
To test further whether radicals are formed during t'he pyrolysis of 2-nitropropane, nitric oxide was added. The rate of pressure increase was not altered (Table 111),but this is not, in itself, a diagnostic test for the absence of radicals in the system. For example, addi(9) S. W . Benson, F. R. Cruickshank, D. M. Golden, G . R . Haugen, H. E. O'Neal, A. S.Rodgers, R. Shaw, and R. Walsh, Chem. Rev., 69, 279 (1969). (10) S.W. Benson, J . Amer. Chem. SOC.,87, 972 (1965). (11) For example, h l . Hanack, "Conformational Theory," Academic Press, New York, N. Y., 1965, p 20. (12) For example, E. L. Eliel, N. L. Allinger, S. J. Angyal, and G. A .
Morrison, "Conformational Analysis," Interscience, New York, N. Y., 1965, p 200.
PYROLYSIS O F
2429
!&NITROPROPANE
tion of nitric oxide does not affect the rate of decomposition of nitromethane, yet the pattern of reaction products is changed.13 Methyl radicals react with excess nitric oxide to form nitrogen, the radicals being regenerated. The overall reaction can be represented as 4N0
-%N2 + 2N02
Table 111: The Pyrolysis of 2-Nitropropane; the Addition of Nitric Oxide a t 323" Pressure of nitric oxide added, mm
(4)
,----Time 5 mm
(min) for change of pressure--16 mm 20 mm 25 mm
10 mm
Final pressure (mm) of nitrogen formed
2-Nitropropane, 30 mm
Similar reactions have been reported involving ethyl' and iert-butyl15 radicals. Thus, if radicals are taking part in the pyrolysis of 2-nitropropane, one would expect, the yield of nitrogen to increase in the presence of excess of nitric oxide, but this does not occur. Experiments with nitrogen dioxide confirm that radical reactions do not contribute extensively to the
0.0 20.0 20.0 20.0 20.0 57.0 100.0
5.0 4.5 4.5 4.0 4.0 5.0 5.0
10.0 9.0 9.0 9.0 7.5 11.0 9.0
15.0 14.0
21.0 19.0
14.0 12.0 14.0 14.0
17.0 20.0
28.0
30.0
