A Bigger Bang

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Robert F. Service
2000-01-19 19:00
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Alfred Nobel made his fortune by stabilizing nitroglycerine, creating an explosive paste that he patented as dynamite. The industrialist and founder of the prize that bears his name would no doubt be pleased by a feat of organic chemistry reported in this week's Angewandte Chemie International Edition: the synthesis of what may be the most powerful nonnuclear explosives ever made. The compounds could one day form a key part of the U.S. Army's next-generation munitions.

The new explosives--heptanitrocubane and octanitrocubane--have been on the drawing board for more than a decade. Their inspiration was a compound with a molecular core consisting of a cube of eight carbon atoms studded with hydrogens, first synthesized in 1964 by Philip Eaton, an organic chemist at the University of Chicago, and his colleagues. Eaton and others later realized that if they could replace the hydrogens with explosive nitro groups--each containing a nitrogen and two oxygens--they'd have an ultradense, and therefore ultrapowerful, explosive.

Swapping nitros for the hydrogens proved a herculean task. Eaton's team struggled with the synthesis for some 15 years until at last, in 1998, they found a reaction that tacked on all but the eighth nitro. Now, Eaton--along with chemist Mao-Xi Zhang and crystallographer Richard Gilardi of the Naval Research Laboratory in Washington, D.C.--has discovered a more efficient way to construct the seven-nitro heptanitrocubane, as well as the magic mix of ingredients and conditions that tacks on the eighth to form octanitrocubane. They haven't produced enough of either compound to test their blasting power, but calculations suggest they should deliver up to 30% more bang than HMX, the most explosive conventional military ammunition in regular use. For now, the synthesis of octanitrocubane remains too impractical to ramp up for military-scale production. But Eaton says his team is already pursuing ideas to streamline the synthesis.

"I think it's fantastic," says Leo Paquette, an organic chemist at Ohio State University, Columbus. "To get all the way to eight nitro groups is clearly a feat. ... It's asking a lot of the molecule to squeeze all those nitro groups into a limited space." Eaton notes that the eight-nitro compound should be able to adopt a more compact crystalline structure than the one they've isolated. If they manage to find that, they should be able to wring out even more explosive power.

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