THERMAL DECOMPOSITION OF HEXANITROETHANE
25
Thermal Decomposition of Hexanitroethane1"vb
by Henry P. Marshall, Frank G. Borgardt, and Paul Noble, Jr. Lockheed iMissiks and Space Company, Palo Alto, California
(Received Februury 81, 1964)'
The rates of deconiposition of hexanitroethane were determined in a number of solvents and as the pure solid in the temperature range of 60 to 100". The products of reaction from the thermal decomposition of the solid are best represented by the equation: (NOz)3CC(XO2)3 + 3?rTo2 SO IC'ZO 2C02. Decomposition of hexanitroethane ( H K E ) for the pure solid and in CC14 proceeds by a first-order reaction; the specific first-order rate constants are, for solid HNE, k = 1018.6exp(-38,900/RT) set.-', and for H N E in CCl,, IC = 1018.5exp( - 37,80O/RT) sec. - I . The rate of decomposition of hexanitroetha ne in n-heptane is 11.5 times faster, and in cyclohexane it is 23 times faster, than that of the pure solid a t 85". Based on the kinetic data and products of reaction, a mechanism for the decomposition process is proposed.
+
+
I. Introduction Hexanitroethanee was first prepared in 1914 by Will,3 but its chemistry has been neglected until recent years. Preliminary data from these laboratories4 have appeared in the literature describing some of the properties of HNE. We have undertaken an' investigation of the kinetic behavior of H N E , because of our interest in this material as a potential oxidizer for propellants and because of our interest in the mechanism of decomposition of solids.
11. Experimental Materials. Hexanitroethane was prepared by the method of Will3 or was obtained from American Cyanamide5 Prior to use in kinetic studies, the H N E was recrystallized from methylene chloride and then sublimed; the m.p. was 140-150' with decomposition. A saturated solution of H N E in CCl, showed no proton spectra by r1.ni.r. although it exhibits a weak absorption band a t 2060 cni.-'. I t has been observed that many polynitro-substituted compounds which contain no protons exhibit an absorption band in the 2860cm. region6 Solvents. Carbon tetrachloride and CHC13, ACS reagent grade, were used as received. The hydrocarbons, n-pentane and cyclohexane, were purified by the method described by Wiberg.' The perfluorokerosine (PFK, Halocarbon C O . )was ~ distilled prior to use; the fraction boiling between 70 and 90" was used in the kinetic studies.
+
Analytical Methods. Three methods of analysis were used for the rate measurements: (1) the infrared method, ( 2 ) the titrimetric method, and (3) the nianometric method. Infrared Method. H N E shows two intense absorption bands occurring a t 6.10 and 6.17 p . Comparison of the sample intensity, using the average of the two absorption bands, with that of a standard calibration curve permitted the determination of the amount of H N E a t various stages of reaction. For these studies, NaCl windowed cells were used, which readily become converted to N a N 0 3 by the NOn formed in the samples. The N a N 0 3 shows an iritense absorption band a t about 7.5 p , and this may overlap with the absorption band used for analysis. Therefore, new cells were employed as required to obtain good kinetic data. (1) (a) This paper was presented before the Division of Physical Chemistry at the 145th National Meeting of the American Chemical Society, New Tork, N. T.,Sept. 1963; (b) This work has been carried out as part of the Lockheed Independent Research Program, (2) For the sake of brevity, hexanitroethane is abbreviated to HNE in the remainder of this paper. (3) W. Will, Ber., 47, 961 (1914). (4) F. G. Borgardt, J. A. Gallaghan, C. J. Hoffman, P. Noble, and W. Reed, A I A A . 1, 395 (1963). ( 5 ) American Cyanamid Co., Bound Brook, N.3 (6) Unpublished results, this laboratory. (7) K. Wibery, "Laboratory Techniques in Organic Chemistry," McGraw-Hill Book Co., New York, N . T., 1960. (8) Halocarbon Co.. Hackensack, N. J.
Volume 69, Number 1 January 1966
26
H. P. MARSHALL, F. 0. BORGARDT, A 3 D P. SOBLE, JR.
Titrimetric Method. The method of Johnsong was used to follow the amount of NO plus NOz produced during the course of reaction. This method was used only on the runs for the pure solid. Samples of H K E decomposed ten half-lives gave equivalent weights of 73.2, 74.5, 76.3, and 73.5, yielding an average value (of 74.4 (theoretical value is 75.0; see eq. 1). Manometric Method. For solid decomposition, the pressure developed due to the gases produced was measured continuously using a pressure transducer as the sensing device. The output of the transducer was read on a digital voltmeter after suitable amplification of the output signal of the transducer. The input voltage to the transducer was from a stabilized voltage supply. The H N E was decomposed in a Pyrex glass container fitted with a Teflon stopcock with a Viton O-ring seat. Coupling of the transducer with the glass was accomplished through a stainless steel-to-glass seal. Decomposition of known weights of H K E for ten half-lives gave a final pressure reading, which showed that seven moles (&3%) of gases were produced in the decompo sition. Rate Procedures. Known weights of solid H K E or a standard solution of H N E in a solvent were transferred .nto glass ampoules, which were sealed in uacuo, using liquid nitrogen as the coolant. The rate studies were carried out in a constant temperature bath controlled to better than ltO.3'. For solution runs the samples after quenching were opened, and the solution was placed in the infrared cell. Then the intensity of the absorption band a t 6.10 and 6.17 p was determined and compared with the standard calibration curve. [n the case of decomposition of solid H K E , after opening the ampoule, a known volume of the desired solvent was added followed by determination of the infrared spectra in the desired wave length region. For the rates determined by the titrimetric method, the ampoules after quenching were cooled to liquid nitrogen temperature to ensure complete condensation of the gaseous products. Then the ampoules were opened rapidly, immediately placed in 25 ml. of 3% hydrogen peroxide, and allowed to stand 1 hr. The nitric acid formed was titrated with standard base. Rates determined by the manometric method were carried out using the equipment and procedure previously described. Pressures a t various stages of reaction and a final infinity pressure value were obtained. From the data rate constants were calculated.
phase chromatography, by the method of Szulozewski and Higuchi, la of the decomposition products showed peaks corresponding to the gases indicated in eq. 1; their relative amounts were estimated as being
111. Results The products of reaction from the thermal decomposition of HNE were established as follows. Gas The Journal of Physical Chemistry
KO2
> c02 >> 3
2 0
z KO
(1)
Infrared spectra of the gaseous products showed absorption bands for the same gases as given in eq. 1. Although quantitative analysis of gas mixtures containing NO2 has been performed with success mass spectrometrically, special conditioning of the system is required to obtain reproducible results." Mass spectrometric analysis of the H S E decomposition products was carried out without preconditioning the system and, therefore, our analysis is considered only qualitative. The analysis does indicate that types of the gaseous products are represented by eq. 1. A particularly significant result of this analysis was that no other gaseous products were observed. Tritrimetric analysis of the gaseous decomposition products showed the presence of 4 moles of NO2 plus S O gases per mole of H N E deconiposzd. Also, it was demonstrated by manometric measurement that 7.0 moles (&3%) of gases are produced per mole of HIVE decomposed. The preceding observations are best represented for the gaseous products and their mole ratios as indicated by H N E + 3N02
+ NO + N2O + 2C02
(2)
Rate Constants. The rate constants for the thermal decomposition of H S E were determined for the pure solid and also in a series of solvents at various temperature. The data are summarized in Table I. Typical results in terms of the specific first-order rate constants for the thermal decomposition of H S E are given in Tables 11, 111, and IV. The manometric and titrimetric method of analysis, restricted to runs for decomposition of solid HSE, consistently gave rate constants with a smaller average deviation than the infrared method. The studies in the hydrocarbon solvents were carried out by procedures similar to that described for CCl,. I n the latter stages of the reaction in the hydrocarbon runs, the solution became turbid. The turbidity for the runs in cyclohexane appears to be the result of adipic acid formation as indicatea by the similarity (9) C. L. Johnson, A n a l . Chem., 24, 1572 (1952) (10) D. H. Szulosewski and T. Higuchi, ibid., 29, 1541 (1957). (11) I . M . Kolthoff, P. J. Elving, and E. B. Sandell, "Treatise on Analytic Chemistry," Part 11, Vol. 5, Interscience Publishers, Inc., New York, N. T., 1961, p. 253.
THERMAL DECOMPOSITION OF HEXANITROETHANE
27
Table IV:
Table I : Specific First-Order Rate Constants for H N E Decomposition
1,
HNE
T,O C .
concn. X IO', moles/l.
Solvent
60 70 85c,d 100 60 70 85" 100 85 100 85 100 85 100
None None None None
k, aec.-ln
b
h h h
cc14
cc14
cc14
cc1,
CHCla
PFKf n-Heptan e n-Heptane Cyclohexane Cyclohexane
2.8 3.0 3.0 2 . 2 to 3.3 2 . 7 to 3 . 8 to 3 . 0 to 3.1 3.7
3.2 4.1 4.2 4.1
1 4 6 6 4 2 2 2 2 2 7 3 1 5
56 f 0 84 f 0 41 f 0 79 f 0 70 f 0 41 o 24 f 0 22 f o 43 f 0 03 f 0 35 f o 46 f 0 53 f o 68 f 0
16 X 80 X 55 X 48 X 30 X io x 15 x 13 x 18 x 16 X 45 x 31 X 06 x 35 X
lo-'
10-5 10-4 10-6 10-5 10-4
a The k values are an average of at least two runs. The term following the f sign represents the average deviation. Sample size was about 7-15 mg. of H N E solid. The specific rate constant for H N E in the presence of ground glass (surface) was sec.-l a t 85'. The specific found to be 5.82 f 0.78 X rate constant for a 100-mg. run was 5.90 f 0.30 X 10-Esec.-l a t 85". e Rate with added CFBCOOH (3.0 X lo-* moles/l.) is unchanged; k is 2.17 f 0.05 X lo-' see.-'. P F K = perfluorokerosene.
Decomposition of HNE' a t looob
min.
40 63 76 95 118 157 180 217 237 256 266 287 300 324 390 421.6 m
Pressure, atm.'
0.148 ,213 250 294 350 430 472 ,535 565 ,591 606 ,631 ,648 ,673 ,701 ,728 . 868d
k
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
X 103, min
-1
71 47 47 35 38 36 36 42 44 46 50 52 58 61 23 33
4 45 f 0 09 Least-square treatment of thew data gives k = 4.48 X mim-1 with a standard deviation of 0.21 x 10-3 min.-l and an infinity pressure value of 0.876 atm. * Manometric method. Pressures in atm. were computed from the output voltage of the transducer as read on the digital voltmeter after amplification. Based on the volume of the system, weight of H N E used, the calculated final pressure is 0.849 atm., based on eq. 2 and ideal behavior of the gases.
Table 11: Decomposition of H N E a t 100°o t , min.
z/a
35 56 95 160 265 300
a
- 1-e-ktb
k X 108,m h - 1
0.165 ,212 ,291 ,459 ,690 ,674
5.17 4.24 3.62 3.85 4.43 3.74 4.18 -I: 0.44
Analysis of H N E by infrared.
Fraction decomposed.
~~
Table 111: Decomposition of H N E a t
30.0 59.9 90.0 120.0 150.0 174.0 268.0 300.0
0.130 ,229 ,307 ,405 ,464 ,506 657 ,712
looo" 4.66 4.34 4.08 4.32 4.16 4.05 4.00 4.15 4 . 2 2 f 0.16
~
Titrimetric analysis of NO
+ NO*.
Fraction decomposed.
of the infrared spectra of the precipitate to that of an authentic sample of adipic acid. No enhancement of rate was observed for an increased sample size (10 to 100 mg.), increased surface (ground glass), or the presence of strong acids (trifluoroacetic acid). Activation Parameters. The logarithms of the specific first-order rate constants were plotted against the reciprocal of the absolute temperature, giving straight lines as shown in Figure 1. From the slopes of the curves, which are equal to -E*/2.303R, the energies of activation were determined. In Table V, the specific first-order rate constants are given in terms of the Arrhenius equation. Included in this table are the entropies of activation at 85' and the relative rates of HNE decomposition a t 85'.
IV. Discussion From the kinetic parameters given in Table V; the thermal decomposition of HKE appears to fall into two classes, one being the decomposition in CCl, and that of the pure solid and the other being the decoinposition of H N E in hydrocarbon solvents. Volume 65, Number I
January 1565
H. P. MARSHALL, F. G. BORGARDT, AND P. NOBLE,JR.
28
10-8
a C-C bond which is weaker than a normal C-C, much like in the system of hexaarylethanes. Also, intuitively, one would predict that the trinitromethyl radical should be fairly stable because of resonance stabilization of the radical species. However, the experimental facts are difficult to explain on the basis of a process as outlined in eq. 3. If the trinitromethyl radical is formed during the course of reaction, it should combine with some of the NOz to form tetranitromethane (TNM). It is realized that either CO of C-N bond formation can occur from the reaction of NO2 with the trinitromethyl radical, but it would be expected that sufficient T N l I would be formed to be identified. The rate of decomposition of T N W is about one-hundredth as fast as that of the HIVE and, therefore, would survive the course of reaction. No evidence for T N M has been observed although a great amount of effort was expended looking for it during the work on the identification of the reaction products. A more plausible mechanism for the decomposition of H N E is given in eq. 4 or 4a
-
a
cn
lo-'
- 0 SOLID HNE
--- A0 -
CARBON TETRACHIBRIDE HEPTANE A CYCLOHEXANE 0 CHLOROFORM H PERFLUORO KEROSENE 1
1
\
(NO2)3C-C (NO2)3 + 1
+ 2N02 -+
(N02)2C-C(N02)2
Table V : Thermodynamic Functions for the Thermal Decomposition of H N E
NO2
Solvent
None"
cc1,
Heptane Cyclohexane
k, sec.-l
1013.6exp( -38,900/RT)b 1013J exp( -37,80O/RT)* 1 O l 8 exp(-28,000/RT) 109.1 exp( -21 ,OM)/RT)
NO2
\
/
C-C-N02 Relative rates
AS* at 358O
at 8 5 O
24.2 23.7 -3.1 -19
1.0 3.0 11.5 23.0
J. M. Rosen gives k = lo1'.' exp( -39,00O/RT) Bec.-l, private communication. * Calculated by least-squares method. Average deviation in E* is about 10%.
A priori, the decomposition of H N E as the solid and in CCl, solutions might be considered as proceeding through the formation of trinitromethyl radicals as
. . . -+ products
..
\
+ 2NOZ-.
. . --+ products
NOz where the species in brackets is the activated complex. T N M would not be a product for a reaction proceeding as outlined in either (4) or (4a). The diradical written as an intermediate for the reaction (eq. 4) should give rise to tetranitroethylene. The existence of this ethylene drivative is doubtful since no reference to it is made in the literature, and our own efforts to prepare this compound have been unsuccessful. However, the behavior of dichlorotetratiitroethane6 seems to follow the same course of reaction (eq. 5) as outlined for HNE.
[(r\'o2)3c. . . . . . c(x02)3] + (~0~)3c-c(p\To2)3 2(K0z)3C.4. . . . +products (3)
C1(NOz)zC-C (NOz)2C1 +
where the species in the brackets is the activated complex. This path for the decomposition would be proposed on the basis of the bulkiness of the groups and the electron-withdrawing effect of the nitro groups giving
Although our analysis of the reaction products is incomplete for this reaction there is no doubt that the main course of the reaction proceeds as outlined above. For this reaction, a C-C bond cleavage would give
The Journal of Physical Chemistry
Cl(NOz)C=C(NOz)Cl
+ 2N02
(5)
THERMAL DECOMPOSITION OF HEXANITROETHANE
29
rise to the dinitrochloromethyl radical as an intermediate with a potentially high degree of resonance stabilization, but this is not the case, as indicated by the products of reaction. On the basis of the present data the reaction path giving rise to a carbene as indicated in eq. 4a cannot be excluded. Further work is under way in these laboratories to attempt to resolve this problem. Looking a t the data in Table VI, the entropy factors for activation of H N E decomposition in CCI, and for the pure solid are quite large, on the order of 25 e.u., and require further comment. If we consider reaction 4 or 4a, the entropy for the unfreezing of two S O 2 groups is about 10-12 e.u.l2 This unfreezing process would be according to the reaction
H N E in CCl,. Solvolytic reactions of charge-transfer complexes’5 show rate enhancement and are associated with a decrease in the activation entropy, when compared to the solvolytic reaction of the uncomplexed material. Both these facts are in line with the finding for the deconiposition of H N E in cyclohexane. Thus, possibly, the rate of product formation from H N E solutions in hydrocarbons (HC) with time may be given by
where the entropy of activation has been measured as 10 e.u. and ascribed to the vibrational and rotational entropy contributions of the free NOz groups, as conipared to the NOz groups “frozen” in the NZO4niolecules. The additional 15 e.u. of activation observed for HIVE decomposition must come about because of either additional unfreezing of the remainder of the carbonnitro system or the leaving NOn groups being sufficiently removed from carbon in the transition state to have attained entropy of translation, in the transition state. The thermal dt?coniposition of H N E in the hydrocarbon solvents is probably proceeding by a path conipletely different from its decomposition in CCl, or as the solid. Recent findings indicate the formation of a complex from T K M and cyclohe~ane’~ or heptane. l 4 Because of similarity of structure it would be anticipated that a complex also would be formed from H N E and cyclohexane. The thermal decomposition of the HNEcyclohexane coniplex could very well proceed more rapidly and by a different mechanism than that for
d(product) = kl(HNE) dt
+ kz(HNE)(HC) + ka(HNE.HC)
(7)
where ICl is the first-order rate constant for HIVE decomposition, k2 is the biniolecular rate constant, and k3 is the rate constant for the decomposition of the complex. Work is in progress in these laboratories to determine the nature of the decoiiipositions of H X E in hydrocarbon systems.
V. Special Remarks The deconiposition of solid H N E appears to represent a unique case of thermal deconipositionI6 of an organic solid. The decomposition of Hn’E proceeds without the formation of a liquid phase or the exhibition of an induction period. The reaction is one of a solid decomposing into all gaseous products. Also, the mode of formation of NzO in the decomposition of H X E is not clear a t present. We hope that future work on similar compounds will lead to a reasonable explanation for the formation of S 2 0 froiii the decomposition of HNE and other polynitro compounds. (12) S. W. Benson, “The Foundations of Chemical Kinetics,” McGrew-Hill Book Co., Inc., New York, N. Y., 1960, p. 260. (13) D. F. Evans, J . Chem. SOC.,4229 (1957). (14) J. N. Chaudhuri and S. Basu, ibid., 4232 (1957). (15) A. K. Colter and S. S. Wsng, J . A m . Chem. SOC., 85, 114 (1963). (16) For a resume of thermal decomposition of organic solids, see W. E. Garner, Ed., “The Chemistry of the Solid State,” Butterworth and Co. Ltd.. London, 1955. See, particularly, Chapter 10.
Volume 69, Number 1
January 1966
