CHAPTER-4 GENERAL DISCUSSION CHAPTER-4 GENERAL DISCUSSION High-energy dense materials (HEDMs) capable to deliver high VOD compared to today's workhorse explosives RDX and HMX san undesirable vulnerability to unplanned stimuli are the need of tomorrow. CL-20 qualifies for such demands ^ It is also emerging as HEM candidate for clean - burning chlorine - free propellants and offers better alternative to AP compared to ADN and HNF in terms of handling and sensitivity respectively. Consequently, R&D work on CL-20 and formulations based on it is pursued at a high pace all over the globe^' ^ CL-20 can be categorized as caged nitramine class of compounds. The synthesis approach of caged nitramines may involve synthesis of a caged amine followed by nitration or cyclization of nitramine (acyclic) units. A combination of these methods may also be applicable. Owing to the limitations of the second approach for cyclic nitramines with complex structure due to amenable possibilities of side reactions, the first approach is the preferred one"*. However, a viable methodology to introduce nitro groups on all the endocyclic nitrogens of the cage to obtain molecules like CL-20 remains a severe challenge. The most serious one is the vulnerability of aminal nitrogen structures within the cage to ring-opening reactions under nitration reaction conditions as the acidic and oxidizing medium of nitration agent could easily offer the conducive environment for collapse of cage structure. The condensation reactions of an^ines with glyoxal to yield hexaazaisowurtzitane derivatives appear to be limited to benzyl amine. The basic cage structure as prelude to CL-20 is realized by this approach. Nearly 170 stoichiometric quantities of benzyl amine with 40% aqueous glyoxal in aqueous acetonitrile solvent at 25"C in presence of an acid catalyst yield HBIW in high yield. The pH of 9.5 was found optimum for efficijsnt formation of HBIW as reported by other researchers. However, the best results were obtained with formic acid [(formic acid 0.1 molar % of the amine)] during this work (Fig.l)^. PQ o 13 40 1- ± 1 0 5 10 ±IS 20 MOLAfl RATIO OP BENZYiAMINE TO FORMIC ACiO Fig.l: % yield of HBIW vs Molar ratio of Benzyl amine to Formit acid^ The mechanism^ of formation of HBIW is proposed to invMve trimerization of diamine (Scheme-1). 171 QHjCHiNHz + OCHCHO •*• C6H5CH2NHCHOHCHOHNHCH2C6H5 5- S+ CfiHsCHaN = CHCH= NCH2QH5 + 2 H2O II QHsCHzN = CHCH= NCH2C6H5 5+ 5- C6H5CH2N CHCH=NCH2C6H5 C6H5CH2 N-CHCH=NCH2C6H5 QH5CH2N =CHCH=NCH2C6H5 H IV III II+IV CH2QH5 CHiCftHs I I C6H5CH2 CH2C6H5 |-2H* C5H5CH2 ^ 'CH2C6H5 N N' QHsCHj—N ;N-CH2C«H5 \^ .N V CH2C6H5 C(iH5CH2 HBIW Scheme-1: Mechanism of Formation of HBIW It is inferred that benzyl group exerts its characteristic stabilizing as well as activating influence on ionic intermediates. A substituted 172 benzylamine yields diamines tiiat fail to trimerize due to steric effects like N- alkyl benzylamine of tert-butyl class. Several attempts of synthesizing CL-20 without collapse of cage structure of precursor led to the approach of the reductive debenzylation of HBIW under a wide variety of reaction conditions. Although, palladium on charcoal is a preferred choice over palladium metal alone, it gives yields up to 40-50% level. The best results (-60% yield) are obtained with the catalyst generated by reduction of palladium hydroxide on carbon (Pearlman's catalyst). In the present work, Pearlman's catalyst (20% Pd (OH)2 on C) was used along with HBr introduced as bromobenzene to convert HBIW to TADBIW. The maximum yield of 59 % was obtained with one-fourth the weight of catalyst with respect to HBIW and reaction time of 18 hours. Hg ,Pd/C C^OfN HBIW CL-20 R = H2b) =CH92c) HBr performs the task of a co-catalyst in the hydrogenation cum acetylation of HBIW to TADBIW. The HBr released under reaction conditions reacts with the acetic anhydride to form acetyl bromide, and thus 173 provides active carbonyl carbon for nucleophilic attack. The concentration of HBr is critical and maximum yields are obtained at its one-eighth molar concentration with respect to the number of moles of HBIW. The solid product, TADBIW remained unaffected after prolonged reaction time, and the reaction was allowed to proceed overnight. The TADBIW was converted to CL-20 by reacting it with NOBF4/ NO2BF4 in sulfolane medium during this work on the lines of reported methods^. It has been found that special nitrating agents like nitrosonium / nitroniun tetrafluoroborate (NOBF4/ NO2BF4) are essential for the conversion of tetraacetyl derivative having lone pair of electrons in conjugatin with carbonyl group to CL-20 as they provide adequate concentration of NOV N02'^ to enable nitration of ring nitrogen. Nitration of amines by nitronium salts was first reported by Olah and Kahn^, and was subsequently investigated by Olsen et al . It is reported to progress as 2 R2NH + N02^BF4_ • R2N-NO2 +R2NH.HBF4 Sulfolane is a relatively good solvent for nitronium salts (7% solubility). Moreover, it does not react with the nitronium salt and provides homogeneous solution. Acetonitrile, another promising solvent for nitronium salts was also assessed. However, the nitrile group strongly interacts with N02^ and causes acetonitrile to slowly oligomerise even at room temperature. Although NOBF4 / NO2BF4 combination is effective for TADBIW ~^ CL-20 conversion, the work up of the product is tedious. Moreover, reagents are cost intensive and are not produced indigenously. Therefore, alternate precursors akin to nitration using normal nitrating agents were investigated. 174 TAIW emerging as potential precursor for alternate nitration route was selected. Being a free amine, TAIW can be smoothly nitrated in strongly acidic nitrating media. The compound was synthesized by acetylation of TADBIW with acetic acid /Pd (0H)2 catalyst combination during this work and was nitrated with HNO3/H2SO4 system at 78 ±2°C to obtain CL-20 in -85% yield^'^. The mechanism of complex multiple concurrent reaction pathways are not understood well because the intermediates have not been isolated. Generally, an amine is expected to be immediately protonated on dissolving it in the acid solution, which is normally expected to prevent subsequent clean nitration. However, nitration of the acetyl substituted positions is likely to release acetyl groups into solution and it is envisaged that during the later stages of nitration, the reaction medium resembles the one in which nitration by acetyl nitrate occurs (a common medium for nitration of secondary amine). In addition, the basicity of the amines gets remarkably reduced as the cage is nitrated allowing nitration in the same way as for non-basic nitrogens such as urethanes. A further possibility is that the basicity and reactivity of the amine nitrogen on the piperazine ring of the substituted hexaazaisowurtzitne cage is unusual because of the conformation into which they are locked by the fused ring system. Despite the clean nitration of most of the amines, there appears to be a degree of decomposition of the cage during nitration, and thereby the yield of CL-20 obtained on nitration of TAIW is not quantitative. The decomposition of the cage and deacetylation of TAIW do not appear to be accompanied with NOx evolution or obvious gassing, and thus the acid stable by-products are envisaged to be derived from hydrolysis of the protonated amine rather than oxidation. HPLC conducted on the lines of the reported method" established >98 % purity of the CL-20 synthesized by both the routes during this work. 175 Although nitric-sulfuric acid combination continues to be the most widely used cost effective practical nitrating reagent, serious environmental problems are posed during spent-acid disposal'^. N2O5 has acquired significance in this scenario as versatile nitrating agent for synthesis. Simple isolation of the products, if N2O5 is used as nitrating agent in solvent medium or reduction in the quantum of required acid even if used in acidic medium are adveintageous. It was attempted as nitrating agent for ADN synthesis as mentioned by Golfier^. In order to assess feasibility of N2O5 nitration of precursors to CL-20, N2O5 was synthesized during this work by clean gas phase reaction of O3 and N2O4. The dichotomy of N2O5 pertaining to non­ selective nitration in acidic medium to nitrate deactivated precursors and selective nitration in an organic solvent (especially chlorinated hydrocarbons) medium was investigated for CL-20 synthesis. The nitration reactions of HBIW, TADBIW and TAIW with N2O5 undertaken during this research work suggest the feasibility of nitration of TAIW by N2O5 dissolved in nitric acid. Particle size of energetic materials plays an important role in the processing of explosives and propellant formulations. Crystallization methods need to be optimized to obtain desired particle size during synthesis/manufacturing stage. The method established during this work offered CL-20 particles of 30±10 |xm size, which is optimum for processing of compositions. 176 (a) (b) CL-20 before crystallization (a) and after crystallization'^ (b) o (a) (b) CL-20 synthesized in the lab (a) before crystallization and (b) after crystallization CL-20 exhibits polymorphism like HMX. Its c polymorph is the preferred choice in view of better density as well as relatively low vulnerability and stability characteristics. One of the widely used methods to obtain 8 CL-20 is the non-solvent crystallization'"*. The relative dipole moment of non-solvent has bearing on the yield of 8 HNIW. Crystallization of CL-20 dissolved in ethyl acetate (dipole moment: 1.88) by addition of n- 177 heptane (dipole moment: 0) as non-solvent led to the formation of e CL-20 free from other polymorphs during this work. FTIR spectral feature of the synthesized product obtained during this work matched with that reported'^ for 8 CL-20.