Kinetics and mechanism of thermal decomposition of 2-substituted 5,5-dinitro-1,3-dioxanes

Abstract

Thermal decomposition of 2-substituted 5,5-dinitro-1,3-dioxanes in melt and in solution follows a homolytic mechanism with initial dissociation of the C3N bond. The reaction in solution is accompanied by oxidation of the initial compound with nitrogen dioxide formed as a result of decomposition. The rate of decomposition weakly depends on the substituent in position 2 of the heteroring. On the whole, the C(NO 2 ) 2 moiety in 1,3-dioxanes is less stable than in alkanes due to conformational features of the heteroring.We examined thermal decomposition of 2-substituted 5,5-dinitro-1,3-dioxanes I3VII in the liquid phase with a view to elucidate the effect of the 2-substituent on the rate and mechanism of the process. : : ; =; ; 9 ; < < 9 O O d e h O 2 N O 2 N R I!VII I, R = H; II, R = CH 3 ; III, R = C 6 H 5 ; IV, R = p-O 2 NC 6 H 4 ; V, R = 4-furoxan-5-yl; VI, R = CH 3 N(NO 2 )CH 2 CH 2 ; VII, R = CH 3 C(NO 2 ) 2 CH 2 CH 2 .Interest in this class of compounds originates from their possible practical application as components of high-energy compositions for various purposes [1].Thermal decomposition of compounds I3IV and VII in melt is accompanied by autoacceleration and is characterized by S-like kinetic curves, whereas geminal dinitroalkanes are known to decompose in the gas phase according to first-order kinetics [2]. The initial rate of decomposition of I3IV does not change with variation of m/V (the ratio of the sample weight to the reaction volume) and S/V (the ratio of the reactor surface to its volume). Gaseous decomposition products accelerate substrate decomposition. Qualitative mass-spectrometric analysis of the reaction mixtures showed that the major gaseous products of decomposition of compounds I, II, and VII (conversion 13325%) are NO 2 , NO, N 2 O, CO, CO 2 , and H 2 O. Among these, the most reactive is nitrogen dioxide; therefore, we examined its effect on the character and rate of decomposition of compound I (Fig. 1).It is seen that the rate of decomposition increases as the concentration of nitrogen dioxide increases; however, the shape of the kinetic curve does not change. We can conclude that the acceleration of the decomposition of 2-substituted 5,5-dinitro-1,3-dioxanes results from reaction of the substrate with nitrogen dioxide liberated during the process. This conclusion was confirmed by experiments performed with dinitrodioxanes I3VII in dilute (235 wt %) solutions in such relatively inert solvents as dibutyl phthalate and 1,3-dinitrobenzene. Under these conditions, molecules of the substrate are isolated from each other and from decomposition products, while the solubility of nitrogen dioxide at 1903210oC is low. Here, the decomposition process followed first-order kinetcs. The rate of decomposition did not depend on the substrate concentration (238 wt %) and dielectric constant of the solvent (e = 6.1 and 20.6, respectively). These data suggest that the decomposition of dinitrodioxanes I3VII involves a nonpolar activated complex typical of radical and molecular mechanisms.

Effect of additive polyfunctional substituents on the thermal decomposition rate of R1,R2,R3-nitromethanes

“…At first glance, reaction initiated at carbon atom of gem-dinitromethyl group in 1,3-dioxane ring could be expected to compete with the major pathway of compounds I-VI thermolysis. However, we previously demonstrated that gem-dinitromethyl group in 1,3-dioxane ring was more stable under the experiment conditions [7]. Rate constant of its decomposition (solution in dibutyl phthalate, 160ºC) was of 4.9 × 10 -7 s -1 , about 5 times lower than that in the case of compound VI.…”

Substituent effect on the rate of thermal decomposition of 5-(R-phenyl)-2-methyl-1,2,3,4-tetrazoles

“…A relation has been found between the rate of thermal decomposition and nature of the substituent in the benzene ring. Effects of solvent and temperature on thermal decomposition of substituted tetrazoles have been revealed.We have earlier shown that decomposition mechanism and thermal stability of energetic compounds based on 5-R-2-methyltetrazoles strongly depends on the nature of the R substituent [1][2][3]. Extending these studies, herein we report on thermal decomposition of compounds I-VIII aiming to reveal a quantitative relationship between the tetrazoles reactivity and nature of the substituent in the 5-phenyl ring.…”