Date of Award
2022
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Chemistry
Department
Chemistry
First Advisor
Jimmie C. Oxley
Abstract
Homemade explosive (HME) is a general term used to encompass energetics easily prepared from materials that consumers can purchase, but because of sensitivity, stability, and/or performance, often have no legitimate commercial or military use. HME preparation has been described in many books and message board threads for years with varying levels of detail and correctness. The class of HMEs is broad and includes fuel/oxidizer mixtures (e.g. ammonium nitrate/fuel oil or ANFO), nitrate esters (e.g. erythritol tetranitrate, ETN), and peroxides (e.g. triacetone triperoxide, TATP). Though the energetic properties for many of these compounds have been known for over a century, they are still studied today. Often focus is on detection schemes that can be applied to field techniques to be used at checkpoints or for ensuring that canine training aids are safe and reflective of the target molecule. With improvements to analytical instrumentation, new detection schemes are being explored today. Sometimes studying the synthesis of these materials could drive legislation towards control of some precursors if the threat is deemed high enough.
One broad class of HMEs that is prevalent are nitrate esters. Nitrate esters are molecules that generally have carbon backbones with multiple -ONO2 groups present on the molecule. Nitroglycerin (NG) and pentaerythritol tetranitrate (PETN) are two examples of nitrate esters that have legitimate uses and are quite common. Both are prepared from the nitration of their corresponding sugar alcohols. The availability of the sugar alcohols. in large amounts has been a driving force for the popularity of some of the compounds. As consumers move towards healthier sugar-free alternatives, the availability of sugar alcohols, such as erythritol, xylitol, mannitol, and sorbitol has become widespread. Through simple nitration reactions, these sugar alcohols can be readily converted into explosives with good performance. One of the most common of these nitrate ester HMEs is erythritol tetranitrate (ETN). While it currently has no military applications, it melting point of 60°C makes it a melt-castable material that, if decomposition and sensitivity issues can be handled, continues to remain relevant.
Since erythritol is readily nitrated, ETN is common in hobbyist chemist circles and message boards. With a forensics group in the Netherlands, a large study was undertaken to explore whether or not product ETN could be traced back to synthesis methods. Crude and recrystallized ETN were prepared via two synthesis routes, the nitrate salt method (MNO3 and sulfuric acid) and the mixed acid route (HNO3 and H2SO4). Precursors were obtained from the United States and the European Union. Spectroscopic methods were unable to differentiate between ETN prepared by the different methods. More in-depth analysis by liquid-chromatography-mass spectrometry (LC-MS) showed the ETN prepared by the nitrate route method was less pure, marked by higher levels of the impurity erythritol trinitrate (ETriN). Recrystallization was able to remove some of the ETriN, but it often did not remove all of the impurity. Finally, stable isotope analysis by isotope ratio mass spectrometry (IRMS) was able to show that there was not a large variation in the 13C/12C ratio of erythritol, meaning that a link would not be able to be established between precursor and ETN. However, there was a difference observed between nitrate sources, whether from a nitrate salt or nitric acid was maintained through synthesis, meaning that a link could be established nitrogen isotope ratios. Sugar alcohols isomers mannitol and sorbitol can also be nitrated to form mannitol and sorbitol hexanitrates (MHN and SHN). The stereochemistry about the carbon backbone results in two hexanitrates with different properties. MHN melts at 112°C and was once considered for military use, but now is mostly classified as an HME. SHN is a low melting solid that is difficult to recover as a pure solid and sees less use. The literature was scant on the nitrate ester and a further study into their structure was undertaken. For both MHN and SHN, a polymorph structure was identified for each molecule from XRD, FT-IR, and Raman analysis. MHN was found to go through a phase transition at 105°C, just prior to melting. Due to the inherent difficulty with crystallizing SHN, the factors that led to an SHN polymorph were difficult to isolate. The thermal properties of MHN and SHN were explored through isothermal analysis and compared to data found in the literature.
Just as popular as the nitrate esters, and perhaps more famous, are the peroxide explosives. The most common is TATP, which has been used extensively in attacks in the EU and across the globe. A related peroxide is hexamethylene triperoxide diamine (HMTD), which is more sensitive than TATP, but has HME applications in homemade detonators. Most studies have focused on the decomposition of HMTD in order to develop training aids for canine security purposes, but few have focused on the formation of HMTD. A study was undertaken, using Raman spectroscopy to follow the formation of HMTD under acid catalyzed and uncatalyzed conditions. An oxidized intermediate, hexamine N-oxide, was identified through Raman, DFT calculations, and mass spectrometry techniques.
Through the study, a polymorph of HMTD, only described through literature to date, was isolated from the reaction of ammonium hydroxide, formaldehyde, and hydrogen peroxide. The polymorph was identified as the kinetic product formed under alkaline conditions. Though single crystal XRD could not be completed, the identification was made from Raman, FT-IR, and XRD. The product was found to not be stable and converted to the normal form after a few weeks.
Recommended Citation
McLennan, Lindsay R., "SYNTHESIS AND CHARACTERIZATION OF HOMEMADE EXPLOSIVES" (2022). Open Access Dissertations. Paper 1367.
https://digitalcommons.uri.edu/oa_diss/1367
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