Eaton’s creation of octanitrocubane called ‘a triumph of synthetic chemistry’

Octanitrocubane is easier said than done.

Some chemists once thought the compound, potentially the world’s most powerful non-nuclear explosive, would be impossible to create. Philip Eaton, Professor in Chemistry, has proven them wrong. He and his associate Mao-Xi Zhang, Research Associate in Chemistry, described their successful synthesis of the compound in the Jan. 17 international edition of the journal Angewandte Chemie.

“I never had any doubt that octanitrocubane could exist, although many people did,” Eaton said. “But we did know from the beginning that we were going to have to learn a lot about fundamental chemistry before we could do this.”

Eaton’s paper generated international media coverage that touted his synthesis of a powerful new explosive. Eaton and his colleagues, however, describe the work as an advance in fundamental chemistry.

“Octanitrocubane is a beautiful molecule in the symmetry of its structure and the energy content of its bonds,” said David Oxtoby, the William Rainey Harper Professor in Chemistry and Dean of Physical Sciences.

“It is also a triumph of synthetic chemistry. Phil Eaton and his research group have developed new methods of synthesis that will be useful for making other new molecules in the future.”

Eaton credits scientists at the U.S. Army arsenal in Picatinny, N.J., for coming to him decades ago with the idea that led to the synthesis. The standard military explosive today, HMX, is less powerful than octanitrocubane and is difficult to safely manufacture because of its shock sensitivity. Part of the military’s interest in cubane stemmed from the belief that its chemical offspring, octanitrocubane, would be insensitive to shock.

“In fact, you can hit octanitrocubane with a hammer and nothing happens,” Eaton said.

Nevertheless, the compound is probably too expensive to synthesize to make it a practical military explosive, Eaton said. And that is just fine with him. “I’m not militaristic and have absolutely no desire to make bombs or explosives,” he said. “I resisted working on this project initially because of my own prejudices about explosives, and more honestly, because I hadn’t the slightest idea how to do it.”

Then Eaton realized that in order to make octanitrocubane, he would first have to better understand the fundamental chemistry of its parent compound, cubane. At one time, chemists regarded cubane as impossible to synthesize because of its unusual geometry.

Cubane is cube-shaped, with carbon atoms forming its eight corners, and a hydrogen atom attached to each carbon. Carbon atoms naturally bond at 109-degree angles. Skeptics assumed that the 90-degree bonding angles of cubane would strain the molecule beyond its breaking point. Nevertheless, in 1964, early in Eaton’s career at Chicago, he managed to synthesize cubane, which proved to be remarkably stable.

Eaton went on to other projects for a number of years, but the opportunity to further investigate cubane’s surprising chemistry lured him back. While in the process of researching cubane’s fundamental chemistry, Eaton’s group made a series of seminal discoveries that had nothing to do with explosives, but with the limits of bonding within organic structures. The U.S. Army and Navy funded all of these studies.

“It turned out that we could use cubane as a starting material for looking at different kinds of limits in organic chemistry,” Eaton explained. “We found all kinds of things that nobody expected.”

In one of his experiments, Eaton came up with a result that differed from the theoretically predicted result by a factor of a quadrillion. “I think that is the largest discrepancy between theory and experiment that’s ever been found,” Eaton said with a laugh. “The theory was looked at and corrected appropriately, and now we have a better theory.”

Eaton’s latest achievement of synthesizing octanitrocubane began as a search to find ways to attach nitro groups––each consisting of a nitrogen atom connected to two oxygen atoms––to each of cubane’s carbons. These nitro groups are a component of almost every explosive, including TNT and nitroglycerin. But because of cubane’s special chemistry, attaching nitro groups to it required the development of entirely new techniques.

Some of the discoveries that flowed from cubane chemistry hold potential for making biologically active materials.

In a joint project with Northwestern University’s Rick Silverman, Eaton is studying interactions between cubane systems and some enzymes that play a key role in Parkinson’s disease. While the pharmaceutical implications of such research are uncertain at this stage, Eaton’s experience with octanitrocubane has shown how much knowledge can change.

“Twenty years from now, that may be very different,” he said.