Superacids: A New Generation

"Acids are the most important catalysts in the chemical industry," says Christopher A. Reed, Ph.D., professor of chemistry in USC's College of Letters, Arts and Sciences, who announced the discovery in a paper presented today at the national meeting of the American Chemical Society in Dallas. "This new generation of superacids has potential applications in fuel cell technology and the chemical and petroleum industries."

Acids do not usually react with hydrocarbons such as petroleum oil, but superacids do. Most superacids are thick, viscous and highly corrosive fluids that are difficult to use. Dr. Reed explains that superacids break hydrocarbons into positively charged hydrocarbon cations, which usually exist for only an instant before the continuing chemical reactions destroy them.

"The fleeting existence of these cations, called carbocations, has long fascinated scientists because they help explain how one hydrocarbon can be transformed into a more useful one," says Reed. Decades ago, scientists discovered that superacids could stabilize carbocations so that they could be studied.

An acid dissociates into a positively charged cation and a negatively charged anion. Because the free ions are so reactive, acid breaks down numerous other compounds; hence its utility in the chemical industry, says Reed. An acid is defined as a superacid if it is stronger than 100% sulfuric acid, the strongest classical acid, which is also considered to be a superacid. The diluted acid used to kill bacteria in swimming pools spreads just a few ions through a lot of water and the solution is only weakly acidic.

The acid's cation, the positively charged hydrogen ion, gets most of the credit for triggering reactions. However, Reed says the role of the negatively charged anion can be crucial and has been underappreciated.

"For every positive charge, there must be a corresponding negative charge, and that anion is always there," says Reed. "Anions have limited the usefulness of currently known superacids, because they react with the carbocations produced."

Reed and Nathanael [cq] Fackler, Ph.D., a USC postdoctoral fellow, have discovered an exceptionally inert anion in the carborane family. This anion makes possible a new generation of superacids that will produce hydrocarbon cations that are not broken down.

"Carboranes may be the most inert anions in chemistry," says Reed.

In a second paper, to be delivered on April 1, Robert Bolskar, Ph.D., a USC graduate student working with Reed, demonstrates how the new anion stabilizes a novel carbocation. Bolskar has isolated two new cations from buckminsterfullerene, a hydrocarbon chemical curiosity first identified in 1985. This compound has 60 carbon atoms arranged in a symmetrical polyhedral structure that resembles a soccer ball. Usually called a "buckyball," it was named for American architect Richard Buckminster Fuller, the father of the geodesic dome.

"Usually, the buckyball compound is rapidly chewed up by a superacid," Reed says. "We've made two different cations that people did not think could be put in a bottle."

Carborane anions are boron compounds originally synthesized by chemists at E.I. du Pont de Nemours & Co. in the 1960s. Reed says they are now too expensive to be widely used in the commercial chemical industry today. Nevertheless, a Massachusetts company, Strem Chemicals Inc., is currently marketing them.

"I think we have shown the way for a clever industrial chemist to exploit superacids," Reed says. "Just as important is the opportunity these new superacids will give chemists in academia to understand how acid-catalysed reactions work."

Reed's research was funded by a grant from the National Science Foundation and the Exxon Corp.