Full Paper DOI: 10.1002/prep.202000197 Characterization of the Hexanitrate Esters of Sugar Alcohols Lindsay McLennan,[a] Audreyana Brown-Nash,[a] Taylor Busby,[a] Jeffrey Canaria,[a] Athina Kominia,[a] James L. Smith,[a] Jimmie C. Oxley,*[a] Faina Dubnikov,[b] Ronnie Kosloff,[b] and Yehuda Zeiri[c] Abstract: The hexanitrates of the six-carbon sugars man- ray diffraction (XRD) and Raman spectroscopy; however, no nitol and sorbitol were prepared and studied to gain in- differences were observed in the thermal behavior. Sorbitol sight in their relative stabilities. Synthesis and character- hexanitrate (SHN) has two distinct crystalline polymorphs ization of these materials resulted in the identification of that result in different melting behavior and differences in two new crystalline polymorphs, one for each hexanitrate. Raman and XRD. Thermal stability of these hexanitrates was Mannitol hexanitrate (MHN) exposed to elevated temper- also examined; despite being isomers, SHN and MHN differ atures exhibited a different structure when measured by x- in their long-term thermal stability. Keywords: Polymorphs · Nitrate ester · Sugar nitrates · thermal stability 1 Introduction Although its crystal structure was recently reported, xylitol pentanitrate (XPN) is notoriously difficult to isolate as a sol- Nitrate esters are formed from materials containing hydrox- id [14–16]. Mannitol hexanitrate (MHN) was first prepared in yl groups. Materials with multiple hydroxyl groups, polyols, 1847 by Italian scientist Ascanoio Sobrero, who had isolated have been nitrated to produce explosive compounds, NG a year earlier. Since it was solid at room temperature, which are used in both the commercial and military sectors. Sobrero was initially more interested in the potential of Some examples of common nitrate esters include pentaery- MHN rather than NG; however, following the accidental ex- thritol tetranitrate (PETN), ethylene glycol dinitrate (EGDN), plosion in his lab of 400 grams of MHN, his interest and nitroglycerin (NG) [1]. PETN is used in detonators, deto- waned.[17] MHN was used in detonators and even ex- nating cord, sheet explosives, and boosters and is even plosive rivet composition during World War II, before being found in medicinal devices. EGDN and NG found immediate replaced by more stable compositions [18]. Like NG and use in dynamite formulations and remain in many propel- PETN, MHN acts as a vasodilator and in the past has been lants, smokeless powders, and medicinal devices. Other pol- used in the treatment of angina pectoris [19]. Recently, a yols have also been subject to nitration, but due to certain co-crystal of MHN with 1-nitronaphthalene was reported; it undesirable properties, have not found military use. Polyols, had slightly decreased sensitivity but also poorer perform- such as erythritol, mannitol, and sorbitol, are referred to as ance [20]. Sorbitol hexanitrate (SHN) was reported in a 1930 sugar alcohols. They are available in the food industry as al- patent as oil that when purified, resulted in a solid ternative low-calorie sweeteners. Originally inaccessible be- (m.p. 54–55 °C) [21]. Like EGDN, SHN was considered for use cause their isolation from natural sources was expensive, bi- in non-freezing dynamites [17]. Recent nitrate ester studies oengineering found means to produce these sugars on a have examined the stereoisomers of ETN (L-ETN and D-ETN, large, efficient scale, making them accessible to the general comparing them to diastereomeric ETN) and the hexani- public [2]. Unfortunately, their widespread availability, com- trates SHN and MHN to determine if overall sensitivity (as bined with the ease of nitration makes these potential pre- judged by drop weight impact) varied among otherwise cursors for terrorist exploitation. identical molecules [22]. The discovery of nitrate esters shown in Figure 1 dates to the mid- to late-1800’s. Recently erythritol tetranitrate (ETN) has received considerable attention [3–11]. ETN has a [a] L. McLennan, A. Brown-Nash, T. Busby, J. Canaria, A. Kominia, similar explosive performance to PETN; albeit at the cost of J. L. Smith, J. C. Oxley increased sensitivity [12]. It melts significantly lower than Chemistry Department, University of Rhode Island PETN (ETN, m.p.=61 °C, PETN m.p.=141 °C, dec). This melt 140 Flagg Road, Kingston, RI 02881 is not accompanied by apparent decomposition, suggesting *e-mail: [email protected] [b] F. Dubnikov, R. Kosloff it might be melt castable. The wide commercial availability Hebrew University Jerusalem of erythritol and the low melting point of its nitrate ester [c] Y. Zeiri explains its use as a homemade explosive (HME) [13]. Ben Gurion University The nitrate esters derived from xylitol, mannitol, and Supporting information for this article is available on the WWW sorbitol have not received as much recent attention as ETN. under https://doi.org/10.1002/prep.202000197 Propellants Explos. Pyrotech. 2021, 46, 1–15 © 2021 Wiley-VCH GmbH 1 These are not the final page numbers! �� Full Paper L. McLennan et al. Figure 1. Nitrate ester explosives and their melting points. Nitrate esters as a class of improvised explosives is not chilled to 0 °C. White fuming nitric acid (10.6 mL, 0.254 mol) new, but rather its popularity is driven by the availability of was slowly added so that the temperature did not rise the precursor sugar alcohols. In the United States, the wide above 10 °C. The mixture was stirred for 30 minutes before availability of erythritol combined with the favorable prop- the polyol (2 g, 0.011 mol) was added. The addition of poly- erties of ETN has directed nitrate ester research. Xylitol, ol caused a slight temperature increase before the temper- mannitol, and sorbitol are less common but can be easily ature re-equilibrated to 0 °C. The reaction mixture was stir- purchased online in powder form. While there has been re- red at 0 °C for 2 hours and then at room temperature for searching into the mass spectrometry and thermal analysis another 2 hours. The reaction mixture was then poured into of these compounds, there is still a lack of characterization 400 mL of rapidly stirring ice water to recover the product. of these compounds. This work aims to bridge that gap In the case of MHN, and sometimes with SHN, a solid pre- with material characterization on both an experimental and cipitate formed directly from the reaction mixture. This sol- theoretical scale. id was recovered via vacuum filtration and rinsed with wa- ter, saturated aqueous sodium bicarbonate, and again with water to remove any excess acid. The solid was dried under 2 Experimental Methods vacuum for 30 minutes and then at room temperature overnight. 2.1 Synthesis of MHN Via Mixed Acid Method MHN was prepared via mixed acid nitration according to 2.3 Purification and Crystallization of SHN published literature procedures [23,24]. Fuming nitric acid was chilled in an ice bath and mannitol was slowly added, SHN did not always crystallize out when the crude reaction maintaining a temperature less than 10 °C. The resultant was poured into water. When SHN formed oily masses yellowish slurry was warmed to between 10–15 °C and sul- which hardened rather than forming a suspended solid in furic acid was slowly added. The mixture became a nearly the water, it was recovered via a modified literature proce- solid slurry and manual stirring was used to mix the re- dure [25]. The SHN was dissolved in methylene chloride and action. After an hour, the reaction was poured onto ice wa- transferred to a separatory funnel where it was washed ter, and the product recovered by vacuum filtration. The three times with water, followed by three times with satu- solid was rinsed with water, saturated sodium bicarbonate, rated sodium bicarbonate solution. The product solution and additional water to remove leftover acid impurities. The was dried over anhydrous sodium sulfate and the meth- solid dried on a vacuum for a minimum of 30 minutes and ylene chloride was evaporated under reduced pressure. The then at room temperature overnight. SHN was collected as a clear oil that was re-dissolved in minimal methylene chloride and chilled in an ice bath. Hex- ane was added slowly which resulted in a white precipitate. 2.2 Synthesis of Hexanitrate Esters Via Acetyl Nitrate The SHN was collected by vacuum filtration, rinsed with a Method small amount of cold hexane, and dried on a vacuum for 20 minutes. The final product collected was a fluffy white solid Acetic acid (21 mL, 0.367 mol) and acid anhydride (21 mL, (m.p. 53–55 °C). 0.222 mol) were placed in a 100 mL round bottom flask and 2 www.pep.wiley-vch.de © 2021 Wiley-VCH GmbH Propellants Explos. Pyrotech. 2021, 46, 1–15 �� These are not the final page numbers! Characterization of the Hexanitrate Esters of Sugar Alcohols 2.4 Synthesis of Mannitol Pentanitrate (MPN) for a total run time of 7 min. Electrospray (ESI) in negative ionization mode was used for all MHN and SHN adducts de- MPN was synthesized via a slightly modified literature pro- tections; all relevant Exactive Orbitrap conditions are pro- cedure [26]. Mannitol hexanitrate was dissolved in acetone vided in Table 1. and 0.5 mL of aqueous ammonium carbonate (0.2 g/mL) Of the expected ions ([MÀ H]À , [M+Cl]À , [M+CHOO]À À solution was added. Almost immediately the solution and [M+CH3COO] ) the chloride and formate adducts were turned orange. The solution was allowed to stand at room the predominate ones found in the analysis of MHN/SHN temperature overnight before the solvent was removed at and their lesser nitrated counterparts (i.e. mannitol pentani- reduced pressure. The resulting dark orange oil was recon- trate and sorbitol pentanitrate, MPN and SPN, and the cor- stituted in ethanol.