THERMAL DECOMPOSITION OF SOMEPOLYHITROALKANES Acknowledgment. The authors are very grateful to Professor E. Scrocco, who suggested the model, and to
1513
Professor S. Franchetti for his kind interest in the subject.
Thermal Decomposition of Some Polynitroalkanes’ H. P. Marshall, F. G. Borgardt, and P. Noble, Jr. Lockheed Palo Alto Research Laboratory, Palo Alto, California
94804 (Received August 3, 1967)
The rates of thermal decomposition of tetranitromethane, bromotrinitromethane, chlorotrinitromethane,and 3,2-dichlorotetranitroethanewere determined in a number of solvents. The decomposition of the methane series of compounds in CC14 proceeds by a first-order reaction; their specific rate constants are best repre1400)/RT). The products of reaction from the thermal decomposition sented by k = 1016Jexp((-38,400 of tetranitromethane are best represented by the equation C(N02)4= C 0 2 0.25Nz0 2.25N02 1.25NO. Based on the kinetic data, the rate-determining step for the decomposition of the polynitromethanes is the rupture of the C-N bond. The decomposition of 1,2-dichlorotetranitroethanein solution proceeds at a rate 1.8 x l o 5 times faster than that of tetranitromethane at 85”. The decomposition proceeds by a first-order reaction with k = 1016.6 exp(-31,00O/RT), the increased rate of decomposition relative to tetranitromethane being due to a much smaller energy of activation.
*
Introduction As part of the general study of the chemistry of polynitro compounds, the rates of thermal decomposition of tetranitromethane (TNRI), chlorotrinitromethane (ClTNM), bromotrinitromethane (BrTXRI), and 1,2dichlorotetranitroethane (DClTNE) were determined in selected solvents at, various temperatures. The results of this study are discussed and a mechanism for the decomposition is proposed.
Experimental Section and Results The solvents Freon 113 and Freon 114B2 (purchased from Du Pont) were distilled prior to use. CCl,, reagent grade, was used as received, and the cyclohexane was purified by the method of Wiberg2 T N M (Humme1 Chemical Co., Newark, N. J.) was purified by distillation at reduced pressure. The BrTK;”\’n!Iand C1TNR4 were prepared by halogenation of potassium trinitrometharie as described by YlacBeth3 for the preparation of the bromo compounds. Final purification of the compounds was achieved by distillation at reduced pressure. DClTNE was prepared by chlorination of symmetrical dipotassium tetranitroethane, as outlined by Borgardt, et ala4 The polynitro compounds gave characteristic infrared spectra and the elemental analyses were compatible for this class of compounds. The rate of decomposition of TNRf was determined by three techniques previously described for hexanitroethane (FINE).‘ The rates of the halonitromethanes were determined by only the infrared technique. The rates of decomposition of DClTNE were deter-
+
+
+
mined by following the rate of formation of 1,Bdichloro1,2-dinitroethylene, a product of decomposition. The molar extinction coefficient of the 1,2-dichloro-1,2dinitroethylene in Freon 113 and in cyclohexane at 265 mp is about 5400, whereas for DClTNE it is only about 146. The rates were determined by analysis of the solution at this wavelength. Corrections were made for the absorption due to the presence of DClT N E in calculating the rates. The dichlorodinitroethylene undergoes further reaction to other products, but the reaction in these dilute solutions is extremely slow and does not present any problems in obtaining good rate data. The specific rate constants listed in Table I were computed by the use of the first-order rate expression. The rate constants are the average of four runs, each run comprising a minimum of eight points. The decomposition reaction in each case was followed to about 70% completion. Typical results in terms of specific first-order rate constants are given in Tables IIV for the decomposition of TNM, ClTNN, BrTKM, and D ClTNE, respectively . The l,Zdichloro-l,2-dinitroethylene,a liquid, was (1) Portions of this work have been carried out as part of the Lockheed Independent Research Program. (2) A. K. Wiberg, “Laboratory Techniques in Organic Chemistry,” McGraw-Hill Book CO., Inc., New York, N. Y., 1960. (3) K. MacBeth and D. D. Pratt, J . Chem. Soc., 354 (1921). (4) F. G. Borgardt, A. K. Seeler, and P. Noble, Jr., J . O w . Chem., 31, 2806 (1968). ( 5 ) H. 1’. Marshall, F. G. Borgardt, and P. Noble, Jr., J . P h y s . Chem., 69, 25 (1965). Volume 72, Number 5
M a y 1968
1514
H. P. MARSHALL, F. G. BORGARDT, AND P. NOBLE,JR.
Table I : Kinetic Data and Activation Parametera for the Decomposition of Some Polynitro Compounds Temp, Solvent
Compounda
C(NOz)4E
CCl4
Freon 114B-2 Freon 113
ClC(NOz)3
BrC (NO&
ClC(NOz)zC(NOz)&l
CCl4
CCla
Freon 113
Cyclohexane C(NOz)aC(NO2)a
CCla
k, sea-1
OC
85 100 120 135 150 120 135 100 i20 135 85 120 135 146.1 85 100 120 134.4 45 55 70 85 4.5 55 70 86
2 . 6 x 10-Ed 3.08 i 0.14 X lo-? 4.56 f 0.19 X 10-6 3.24 f 0.22 X 10-6 1.65 i 0.09 x 10-4 4.71 f 0.22 X lod7 3.42 i 0.13 X 3.30 i 0.16 X lo-? 4.58 f 0.21 x 10-3 3.32 f 0.15 X 10-6 2 . 2 x 10-7 2.56 X 0.05 X 1.46 i 0.06 X lo-' 5.34 X 0.09 x 10-4 4 . 3 x 10-7 3.46 f 0.13 X 4.68 f 0.12 X 2.53 f 0.11 X lo-' 1.96 f 0.06 X 8.21 =t0.15 x 10-6 6.82 f 0.19 x 10-4 4.6 f 10-3 1,84 f 0.03 X 7.49 0.17 x 10-5 6.26 i 0.24 X 10-4 2.2 x
Relative rete at
AE+
AS+^^^
Log A
39.7
15.7
16.7
41.3
20.0
17.7
39.8
16.1
16.8
38.0
14.7
16.5
37.6
15.3
16.6
31.0
15.0
16.6
30.7
13.8
16.3
850
1.0
8.5
17
1 . 8 X los
860
Concentration of polynitro compounds was -6 X IOm2mol/l. for TNM, ClTNM, and BrTNM; for IIClTNE, the concentration was -2 X 10-3 mol/l. k's are average of four runs, average deviation given for each k. J. M. Sullivan and A. E. Axworthy, J . Phys. Chem., 70, 3366 (1966), give k = 10'7.63exp(-40875/RT) sec-1 for gas-phase pyrolysis. Extrapolated from data a t other temperatures. e Taken from ref 5, k = exp(-37,800/RT).
Table 11: Decomposition of TNM a t 135' in CCla"
120 180 243 300 375 465 555 675 735
0.244 0.313 0.368 0.468 0.542 0.602 0.642 0.726 0.769
2.33 2.09 1.88 2.10 2.08 1.98 1.85 1.92 1.99 Av 2 . 0 2 f 0 . 1 1
Analysis of TNM by the infrared method. decomposed. a
Table 111: Decomposition of ClTNM a t 135' in CChO
Fraction
prepared in large quantities from the decomposition of DClTNE for ten half-lives in Freon 113 at 47'. During the preparation a stream of argon was passed through the solution to sweep out the NOZ. The solution was concentrated by removal of solvent at reduced pressure, followed by distillation of the 1,2-dichloro-1,2-dinitroethylene product: bp 34-35' at 0.03 mm, 12% = 1.5172. Anal. Calcd for CzC1~Nz04: C, 12.85;C1, The Jozirnal of Physical Chemistry
16 45 60 77 90 110 130 150
0.149 0.320 0.415 0.496 0.563 0.633 0.698 0.756
10.08 8.57 8.94 8.89 9.20 8.87 9.21 9.40 Av 9 . 1 5 f 0 . 3 3
'Analysis of ClTNM by the infrared method. decomposed.
Fraction
37.93;N, 14.35. Found: C , 12.67;C1,36.98;N, 14.25. Attempted chlorination of 1,2-dichloro-1,2-dinitroethylene by standard techniques was unsuccessful; the starting material was recovered unchanged in essentially quantitative yield. The stoichiometry of the decomposition reaction of DClTNE was determined as follows. DClTNE (0.5 g) was decomposed in vacuo in a modified U tube, with the center part of the tube plugged with glass wool. One
THERMAL DECOMPOSITION OF SOME POLYNITROALKANES Table IV: Decomposition of BrTNM a t 135" in CCla"
1.19 1.42 1.34 1.49 1.57 1.49 1.53 1.48 1.38 Av 1 . 4 3 f . 0 . 0 9
0.114 0.248 0.331 0.450 0.544 0.591 0.658 0.695 0.732
10 20 30 40 50 60 70 80 95
Analysis of BrTNM by the infrared method. decomposed. -
'Fraction
~~
Table V: Decomposition of DClTNE at 55" in Freon 113" X/a
t,
min
1
- ,-kt
IO%, b
0.0842 0.130 0.170 0.200 0.271 0.366 0.449 0.645 0.752
20 30 40 50 70 100 130 225 300
Analysis by the ultraviolet method. posed.
min-1
4.39 4.62 4.66 4.46 4.52 4.56 4.58 4.60 4.62 Av 4 . 5 6 f 0 . 0 6
' Fraction
decom-
arm of the tube contained the sample which was placed in a bath at about 70" and the other arm was cooled in liquid nitrogen. The center part of the U tube containing the glass wool was heated with an air gun. First, one arm was placed in the heated temperature bath and then the other, but a t all times keeping the nonheated arm at liquid-nitrogen temperature. This procedure was used because the DClTNE sublimes readily and may be transported into the cold arm of the U tube. Also, cooling with liquid nitrogen was employed to mainttiin an extremely low partial pressure of NOz in the system. From the weights and analysis of the reaction products in each arm of the U tube, the over-all reaction can be represented by l.ODC1TNE
==
0.981Cl(NOz)C=C(NOz) C1+ 2.18NO2
(1)
I n the arm of the U tube containing the X02,a small amount of "chloride" was shown to be present, equivalent to 0.049 mol as AgCl. The mass balance for the reaction was determined as 102%. Other attempts were made to determine the number of moles of NO2 formed per mole of DClTiSE. Samples of
1515
DClTNE in glass ampoules, which were sealed in oucuo, were heated a t 78' for various times. The NOz was determined tritrimetrically after separating it from the 1,2-dichloro-1,2-dinitroethylene. The NO2 determined was 2.28 and 2.44 f 0.10 mol for heating times of 2.8 and 4.0 hr, respectively. These experiments show the continued formation of NOZupon prolonged heating of DClTNE. Pressure-measurement techniques5 indicated the formation of 3.58 d= 0.12 mol of gaseous products for a sample of DClTNE heated overnight (-16 hr) at 85". Attempts to obtain the rates of decomposition of solid DClTNE were unsuccessful. The decomposition of solid DClTNE is complicated by other reactions taking place. They probably are: (a) reaction of the dichlorodinitroethylene with the NOz formed and/or with the starting material, DClTNE; and (b) the change of phase of DClTNE due to the dichlorodinitroethylene formed during the reaction, resulting in a change in the decomposition rates as described by Garner.B Freon 114B2 appears to react with either TNRl or its decomposition products. The formation of Brz in small amounts was observed. However, this side reaction did not appear to have a significant effect on the measured rates of reaction. The only reaction products detected from the thermal decomposition of T N M were NO, NOz, NzO, and C02,as also reported by Sullivan.' Using procedures previously described by US,^ 3.52 (f1%) mol of NO and NOz and 4.75 (d=l%) mol of total gaseous products were formed per mole of decomposed TNn4. Thus the over-all reaction for the decomposition of T N M is best represented by C(11'02)4 +coz
+ 0.25NzO + 2.2511'02
+ 1.25NO
(2)
Discussion The rate-determining step for the decomposition of the polynitromethanes is the rupture of the C-N bond (NOz),XC-NOz +(KO2)zXC.
+ XOz
(3) (where X = C1, Br, or NO,) as proposed by Sullivan7 from gns-phase pyrolysis studies and by BielskP from photolysis studies of the decomposition of TNM. The observed activation energies (35-39 kcal/mol) and the preexporiential factors (-lo1') are essentially the same for the three polynitromethanes in the present studies, indicating that the initial step (rate-controlling step) is probably the same. Within experimental error, our observed values of the activation parameters for T N M solution are the same as reported by Sullivan' from gas-phase pyrolysis studies. (6) W.E.Garner, Ed., "The Chemistry of the Solid State," Butterworth and Co., Ltd., London, 1955, Chapter 10. (7) J. M. Sullivan and A. E. Axworthy, J. Phys. Chem., 70, 3366 (1966). (8) B. H.J. Bielski and R. B. Tirnmons, ibid., 68, 347 (1964). Volume 78. Number 6 May 1968
H. P. MARSHALL, F. G. BORGARDT, AND P. NOBLE,JR.
1516
0-0 02N-C-d--C1
I I+
0 2
N-C-C-C
/
\
NO2
1
KO2
IIa
\
c
