Propellants, Explosives, Pyrotechnics 17, 17-19 (1992) 17

New Dependence of Activation Energies of Nitroesters Thermolysis and Possibility of its Application

Svatopluk Zeman

Department of Research and Development, CHEMKO, (3-072 22 Strazske (CSFR)

Eine neue Abhangigkeit der Aktivierungsenergie bei der Thermo- lyse von Nitroestern und Moglichkeiten ihrer Anwendung

Eine Beziehung wurde gefunden zwischen der Aktivierungsenergie E von Salpetersaureestern bei nichtautokatalytischer Niedrigtemperatur- Thermolyse und deren Sauerstoffbilanz. Durch diese Beziehung wird die molekular-strukturelle Abhangigkeit der E-Werte dokumentiert und gleichzeitig eine direkte Beziehung zwischen den E-Werten und der Schlagempfindlichkeit der Nitroester angezeigt.

Une nouvelle correlation entre I'energie d'activation dans la ther- molyse des esters nitres et leurs possibilites d'application

On a trouve une relation entre I'Cnergie d'activation E des esters daci- de nitrique et leur bilan en oxygkne en cas de thermolyse non autocata- lytique B basse tempkrature. Cette relation illustre la dependance qui exi- ste au niveau de la structure moleculaire entre les valeurs E et revkle en m&me temps la prCsence dune correlation directe entre les valeurs E et la sensibilite au chocs des esters nitres.

Summary

Relationship was found between activation energies E of low-tempe- rature non-autocatalyzed thermolysis of the nitric acid esters, on the one hand and their oxygen balances OB,,, on the other. By this relationship, molecular-structural dependence is evidenced of values E, at the same time existence is signalized of the direct relationship between the values E and the impact sensitivity of nitroesters.

1. Introduction

In our previous communication(') the relationship was

(1)

within the framework of examining the micromechanism of detonation initiation of secondary nitramines and nitrosami- nes, in which E is activation energy of low-temperature non- autocatalyzed thermolysis and OB,,, is the oxygen balan- ce(2,3). This equation is analogous to the relationship between the impact sensitivity and the structure of polynitroaliphatic substances as represented by Kamlet and his co-workers in the ~hape(~9~)

found

In E = a + b . OB,,,

log h,, % = 1.372 - 0.168 . OBI,, (2)

Where h,, % represents a 50 percent drop-height of the ham- mer having mass 2.5 kg. Considering the published relation- ships between the thermal decomposition parameters and detonation characteristics of organic explosives (see for example Refs. 4-6) as well as already quoted knowledge by Kamlet(2.3) and also by comparing the two equations, it was stated(') that the existence of the Eq. (1) is another piece of evidence in favour of identity of the primary fragmentation in the detonation conversion with the primary fragmentation occuring during low-temperature thermolysis of the nitro- and nitrosamines.

The validity of Eqs. (1) and (2) has not been verified so far for nitroesters. This category of compounds is known(7) to

start thermolysis by 0-NO2 bond homolysis. It was confir- med(4,6) that the activation energies of the low-temperature non-autocatalyzed nitroesters thermolysis do correlate with the characteristics of their detonation. A definite relationship between the kinetics of thermolysis of a number of alkylnitra- tes, on the one hand, and the reaction rate during their burn- ing and detonation, on the other hand, was taken notice of also by Kondrikov@), while studying the influence of the inert diluents on failure diameters of liquid 0- and C-nitrocom- pounds. It is only recently that has been found(9) that ignition sensitivity of the cast double base (nitrocellulose-nitroglyce- rin) propellants correlate well with the decomposition kine- tics and thermal stability and, to some extent, with the burn- ing rate of these propellants.

Therefore it follows from the ~ a p e r s ( l . ~ - ~ , ~ . ~ ) that the relati- onship (1) should also be valid for nitroesters. In order to confirm the said idea, results of the study of kinetics thermo- lysis are applied in the following part of the paper by means of the manometric method (mostly using the Soviet manome- tric method - SMM).

Like in Ref. 1, attention was paid marginally to the com- pensation effect of Arrhenius parameters. It is represented by the isokinetic relationship('0-12)

(3 ) E = e, + 2.303 . R . p . log A

where p is isokinetic temperature.

2. Results

The survey of the nitroesters being studied of the corre- sponding Arrhenius parameters E and log A of non-autocata- lyzed low-temperature thermolysis and calculated values of oxygen balance OB,,, according to Refs. 2,3 is contained in Table 1; the values E are given with the same number of decimal places as was done in the original papers.

Analysis of the mutual relationship of values E and OB,,, revealed that, in the sense of Eq. (I), the category of nitrates

0 VCH Verlagsgesellschaft mbH, D-6940 WeinheimJ992 0721 -3 1 15/92/0 102-001 7 $ 3.50+.25/0

18 S. Zeman Propellants, Explosives, Pyrotechnics 17, 17- 19 ( 1 992)

5-20

,r m a

5.15

5.10

I I

5*05 t Figure 1. Activation energies E of non-autocatalyzed low-temperature thermolysis of nitroesters as a function of their oxygen balance OB,,,:

0 the group A of nitroesters the group B ofnitroesters

A the group C of nitroesters 0 the data which do not correlate with anyone shape of the relation-

ship ( I ) the data from manometric method('5.'6) which is different from SMM.

being examined does split into four groups (see also Fig. 1): - the group of A are nitrates with neighbouring nitroxy

groups for which a = 5.127 f 0.003, b= -0.014 k 0.001 and the correlation coefficient r = -0.9802;

- the group of B are nitrates consisting predominantly of 1,3-dinitroxypropane derivatives for which a = 5.104 k 0.002,

b = -0.003 k 0.001 and the correlation coefficient r =

- the group of C are nitrates which consists of two nitrates only (2-nitroxyethyl-X-derivatives, where X = heteroatom);

- group of nitrates the data of which do not correlate with anyone shape of the relationship (1).

Mathematical treatment of the data resulting from the SMM application to the study of thermolysis kinetics produ- ced a given shape of the relationship (3) with e, = 95.33 f 19.14 kJ/mol, p = 240.1 f 64.3 K with correlation coefficient r = 0.6822.

-0.7789;

3. Discussion

In the paper of Ref. 1 secondary nitramines and nitrosamines were studied having grouping HC - NY - CH within the molecule (here Y = NO, or NO). To this corresponded one shape of Eq. (1) for nitramines and one shape for nitrosami- nes. It is therefore logical that more than one shape of Eq. (1) should correspond to a more varied series of molecular struc- tures of nitroesters in Table 1.

In nitroesters with the vicinal nitroxy groups (group A of nitrates), the mutual negative-inductive effect of these nitroxyles may probably be the reason for relatively tight cor- relations of their data in the sense of Eq. (1). With this set of data, the data of methylnitrate are well correlated. Methyl- nitrate can therefore be considered to be the building unit of the molecules of compounds in group A. From this group, the compound 13 deviates to some extent; its kinetic data corre- spond to thermolysis in solid state (the data of the other mem- bers of group A of nitrates come from the thermolysis in the liquid state).

It is a generally acknowledged fact that induction effects in bonds which are more distant from the beta-carbon are of little significance. That is why the dependence of E values of thermolysis of 1,3-dinitroxypropane derivatives upon their

Table 1. Survey of the OB,,, Values and the Arrhenius Parameters of the Studied Esters of Nitric Acid

Com- Nitroester Oxygen Method of State of Temperature E Log A pound balance thermolysis thermal range [KJ/mol] [s-'1 No. OB,,, following decompn. [K]

1 2.1 2.2 3 4.1 4.2 5.1 5.2 6 7 8 9 10.1 10.2 11 12 13 14 15 16 17

Methyl nitrate Ethylene glycol dinitrate

Diethylene glycol dinitrate 1.2-Propylene glycol dinitrate

1.3-Propylene glycol dinitrate

1.4-Butylene glycol dinitrate 2.3-Butylene glycol dinitrate 2-Hydroxy- I .3-propylene glycol dinitrate Glycerol trinitrate Pentaerythritol tetranitrate

Manitol hexanitrate Dipentaerythritol hexanitrate Nitrocellulose (13.35 % N) N.N-Diethanolnitramine dinitrate Trimeth ylolnitromethane trinitrate Ethylnitrate 1.1. I-Trimethylolpropane trinitrate

1.298 2.221

- 1.020 0.000

0.000

-2.220 -2.220 1.189 3.082 1.898

3.538 0.381 0.612 0.000 2.796

-3.295 -1.857

Refer- ence

SMM SMM manometric SMM SMM manometric SMM manometric SMM SMM SMM SMM SMM SMM SMM extrapol SMM SMM manometric SMM

liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid liquid solid liquid liquid liauid

485-512 343-413 3.58-378 3.53-413 345-4 13 353-373 345-4 13 363-383 373-413 3.53-413 353-4 I3 343-413 353-413 353-413 353-413

408-433 373-433 348-368 343-383

165.27 163.28 163.18 175.72 168.72 156.48 163.70 159.41 163.18 174.05 177.40 163.30 165.29 163.17 159.10 164.59 164.12 173.75 152.30 167.36

14.4 14.5 15.9 16.5 15.8 15.2 14.9 15.2 15.1 16.7 16.8 15.4 15.8 15.6 15.9 15.0 15.0 16.5 15.3 14.7

.- __ extrapol liquid 165.29 14.9

13 14 15.16 17 18 15.16 18 15.16 18 18 18 19 14 20 14 6

21 22 15.16 23 6

Propellants, Explosives, Pyrotechnics 17, 17-19 (1992) Activation Energies of Nitroesters Thermolyse 19

molecular structure is less pronounced [more loose correlati- ons in the sense of Eq. (l)]. With this category are well corre- lated the data of ethylnitrate - the nitrates of group B can be generated by an imaginary substitution upon beta-carbon of compound 16. There is also a good correlation of compound 9 data, even better than with the shape of Eq. (1) for the group A of nitrates. However, data of compound 8 do not fit in the set of data for group B which may be caused by inter- action going on between nitroxyles and hydroxyl by another mechanism than inductive one.

Compounds 3 and 14 should form probably a new shape of Eq. (1) for 2-nitroxyethyl derivatives of general formula 0,NO-CH,CH,-X-CH,CH,-ONO,, where X is a heteroatom.

There is no correlation between the found shapes of Eq. (1) and compound 6 because its structural difference.

The kinetic data from the thermolysis of compounds 4.2, 5.2 and 15 were obtained using a manometric method diffe- rent of the SMM. That is also why they do not correlate with any other shape of the Eq. (1). From application of this equa- tion shape for the B group of nitrates on the case of com- pound 15, value E = 166.07 kJ/mol results for it.

The E values of the nitrates being studied are more or less linked up with the intensity of the inductive intramolecular effect wheras the molecular structure is not reflected in the values of log A in a more pronounced manner. The neigh- bourhood of the reaction centre therefore exercises very little influence on the activation entropy of the thermolysis (this is in agreement with the homolysis of the 0-NO, bond as the primary process). Single application of the Eq. (3) for ther- molysis of these compounds is therefore dubitable as a result of that was previously stated.

4. Conclusion

The found relationship between the activation energies E of low-temperature non-autocatalyzed thermolysis of nitro- esters, on the one hand, and their oxygen balances, on the other, signals the existence of a direct relationship existing between these E values and the impact sensitivity of the above-mentioned nitrocompounds. Possibility is thereby given of its application as indirect evidence material in the realm of the study of the micromechanism in initiation of nitroesters detonation (i. e. an analogy in the knowledge con- tained in the paper of Ref. 1). The found relationship, among other things, also demonstrates the molecular-structural dependence of the E values of thermolysis of nitric acid esters.

5. References

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(2) M. J. Kamlet; Proc. 6th Symposium (International) on Detonation, San Diego, Calif. August 24-26, 1976.

(3) M. J. Kamlet, in A. A. Borisova (Ed.): “Detonatsyia z vzryvchatye veschestva (Detonation and Explosives)”, Izdat. Mir, Moscow, 1981, p. 142.

(4) S. Zeman, J. Fedik, and M. Dimun; Zbornik Radova (Collect. Pap., Techn. Fuc.,Bor) 18, 119 (1982).

(5) S. Zeman; Thermochirn. Acta 49,219 (1981). (6) S. Zeman, M. Dimun, and S. Truchlik; Thermochim. Acta 78, 181

(1984). (7) B. A. Lur’e, B. S. Svetlov, and V. P. Schelaputina; in G. B. Mane-

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(9) S. Y. Ho and C. W. Fong; Combust. Flame 75, 139 (1989). (10) E. Koch; “Non-Isothermal Reaction Analysis”, Academic Press,

London 1977, pp. 57, 170. (1 1) 0. Exner; ”Korelachni vztahy v organitske khemii (Correlation

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(18) A. G. Afanas’ev, B. A. Lur’e, and B. S. Svetlov; Tr. Mosk. Khim.- Tekhnol. Inst. Mendeleeva 53,63 (1967).

(19) B. S. Svetlov; Kinet. Katal. 2,38 (1961). (20) G. B. Manelis; ”Problemy kinetiki elementarnykh khimicheskikh

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(21) K. K. Andreew and B. S. Samsonov; Tr. Mosk. Khim.-Tekhnol. Inst. Mendeleeva 53,7 (1967).

(22) B. S. Svetlov and B. A. Lu’re; Zh. Fiz. Khim. 37, 1979 (1963). (23) B. A. Lur’e and B. S. Svetlov; Tr. Mosk. Khim.-Tekhnol. Inst.

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(Received December 4, 1990; Ms 56/90)