Double oxadiazole could replace TNT

Trinitrotoluene (TNT) has been a standard explosive used in munitions for more than 100 years, but the military is looking to phase out its use due to its toxicity. A new 24-atom molecule has ignited the interest of chemists as a possible TNT replacement. The compound, known as bis(1,2,4-oxadiazole)bis(methylene) dinitrate, has a melting point of 84.5 °C and a decomposition temperature of 183.4 °C, making it an excellent melt-castable material.

Melt-castable explosives, such as TNT and bis(1,2,4-oxadiazole)bis(methylene) dinitrate, are solid at room temperature but melt within a certain temperature range so they can be easily loaded into munitions. Although the textbook definition of a melt-castable explosive requires a melting point between 70 and 120 °C, the practical melting range is much tighter, says Jesse J. Sabatini, a chemist with the U.S. Army Research Laboratory (ARL) at Aberdeen Proving Ground.

“You need to make sure the melting point is high enough that when you store this stuff in a bunker in the desert it won’t melt,” he says. But the melting point should be below 100 °C so steam can be used to do the melting, Sabatini adds. Meanwhile, the decomposition temperature should be significantly higher than the melting point for safety reasons—chemists don’t want the material to explode during processing. TNT, for example, has a melting point of 80.4 °C and decomposes at 295 °C.

Sabatini and ARL colleagues, working with David E. Chavez of Los Alamos National Laboratory via the Joint Munitions Program, created bis(1,2,4-oxadiazole)bis(methylene) dinitrate based on a similar molecule they’d previously made that had one fewer nitrogen atom in each of the compound’s rings. That earlier compound had a good temperature profile, but its explosive performance was inferior to that of TNT. The chemists reasoned that the logical next step was to swap out some carbon atoms for nitrogen. “We know that’s going to increase performance,” Sabatini says.

The biggest challenge proved to be working out a good synthesis of the compound. The original procedure had a meager 4% overall yield. “The molecule, on paper, looked like it was worth the effort,” to rework the synthesis, Chavez says. After a few iterations the chemists boosted the overall yield to 44% and were able to make 25 g of the material (Org. Proc. Res. Dev. 2018, DOI: 10.1021/acs.oprd.8b00076).

The compound is better than its precursor in terms of melting and decomposition temperatures. Computational studies indicate bis(1,2,4-oxadiazole)bis(methylene) dinitrate should perform better than TNT, but the chemists need to make the material on a kilogram scale to test its explosive properties. Toxicity studies are also planned.

Thomas M. Klapötke, an expert on explosive compounds at the Ludwig-Maximilians University, calls the new compound “a superb material” with high thermal stability, low sensitivity, and great properties for melt-casting.