A buckyball (top) can be ionized either by removing an electron (bottom left) or with the new superacid (bottom middle), producing a protonated buckyball (bottom right).

Acidity Without Ferocity

"Strong yet gentle" sounds like an ad for laundry detergent, but it perfectly describes carborane, a new acid that is one of the strongest ever synthesized. Unlike most acids, it's not corrosive, and hence may be safer to use and easier to store. The superacid has been used to create never-before-seen ions of carbon and may ultimately be useful for refining petroleum.

Chemists realized 30 years ago that a molecule called carborane would make a superb acid. All they had to do was add a proton to carborane's negatively charged ion (CB11H6X6-, with X either Cl or Br). But that is easier said than done; a team led by Christopher Reed, a chemist at the University of California, Riverside, tried for more than 10 years, but always failed. The problem was that the acid gave away its proton to (or, as chemists say, "protonated") any solvent they made it in. Once they hit upon the idea of creating it under ultradry conditions, the molecule kept its extra proton. As expected, the new acid was about a million times stronger than sulfuric acid, the traditional dividing line between acids and superacids.

The problem with most superacids is that they are very corrosive; they don't just give away a proton to other molecules, but proceed to rip them apart. Carborane, however, becomes extremely inert after giving away its proton. Reed and his group demonstrated this by using it to ionize "buckyballs," fragile, soccer-ball-shaped carbon molecules. Every previous attempt to protonate buckyballs had destroyed the molecules, but the carborane acid succeeded, turning the magenta buckyball powder into a red salt the color of Bordeaux, the team reports in the 7 July issue of Science.

Other chemists say the new acid is likely to be useful stuff. "The big picture is that [Reed] can now protonate anything," says Yves Rubin, a physical organic chemist at the University of California, Los Angeles. In the petroleum industry, carborane may be able to refine oil to boost its octane level, a process that usually requires strong and corrosive acids. For academics, it may be a way to stabilize and provide a "snapshot" of short-lived intermediate products of chemical reactions. "I think it will become the standard of nonreactive acids," Rubin says.

Related sites
The Reed group homepage
More about buckyballs