After more than a century of searching, chemists have finally nabbed a legendary acid.
The acid called cyanoform or tricyanomethane appears widely in textbooks as one of the strongest carbon-based acids known. Yet despite attempts to make the acid dating back to 1896, cyanoform has evaded chemists until now. Researchers report September 18 in Angewandte Chemie International Edition that they isolated the acid by figuring out crucial experimental conditions.
The main problem was temperature, says coauthor Andreas Kornath, an inorganic chemist at Ludwig Maximilian University of Munich. Researchers previously assumed that cyanoform is stable at room temperature. “It is just not,” Kornath says. Using trial and error, he and his team found that cyanoform is stable only below –40°Celsius.
The acid has a central carbon atom attached to a hydrogen atom as well as to three cyano groups, each consisting of a carbon triple-bonded to a nitrogen. The molecule loses its hydrogen atom very easily, making it a strong acid and demonstrating a fundamental rule of carbon acids. The rule describes how electron-loving groups (in this case, the cyano groups) attached to a central hydrogen-toting carbon pull on that carbon’s electrons. The molecule’s electrons settle into a cozy position close to the cyano groups, leaving the link to the hydrogen extremely weak.
But at room temperature, cyanoform simply decomposes, quickly forming junk molecules, Kornath says.
This is probably what happened in 1896 when chemist Hermann Schmidtmann tried to make cyanoform. Schmidtmann mixed sulfuric acid with a stable relative of cyanoform called sodium tricyanomethanide. That molecule, considered a salt of cyanoform, has the same structure as the acid except it has lost the positive hydrogen ion, resulting in a negative molecule, which is paired with a positive sodium ion.
Schmidtmann expected that sulfuric acid would stick a hydrogen atom onto the negative tricyanomethanide, forming cyanoform. Instead, he ended up with a greenish concoction, probably containing only remnants of the unstable acid.
Several research groups have tried to isolate cyanoform since, repeating Schmidtmann’s experiment or trying new strategies. All failed until now, says Jack Dunitz, a chemical crystallographer retired from ETH Zurich in Switzerland. Cyanoform has been “listed in all the books,” he says. But “it’s doubtful whether it’s ever been made.”
At frigid temperatures, though, Kornath and colleagues finally made the acid, which is a colorless liquid. Similar to Schmidtmann’s method, the researchers reacted a weak acid, in this case hydrogen fluoride, with a salt of cyanoform. Using multiple chemical analyses, they found that the resulting molecule perfectly matched the structure of cyanoform.
“It’s very noteworthy,” says physical chemist Daniel Kuroda at Louisiana State University in Baton Rouge. Theoretical chemistry is simply not good enough to predict temperatures at which substances decompose, he says. But experimental information like this gives chemists new ideas, including thoughts about how to make other acids and salts for fuel cells, for instance. “It opens a lot of doors for chemistry,” he says, “and that’s the most important thing.”
Editor’s Note: This story was updated November 13, 2015, to correct the description of hydrogen fluoride as a strong acid. While it can cause serious problems to people exposed to it, hydrogen fluoride is technically classified as a weak acid.
