Carborane acid

Carborane acids are a unique class of superacids that represent some of the most powerful Brønsted acids ever discovered. A Brønsted acid is a substance that works by donating a proton, which is a tiny hydrogen ion. While many acids are strong, carborane acids are in a category of their own. Some versions are estimated to be at least one million times stronger than 100% pure sulfuric acid. This strength is measured using the Hammett acidity function, denoted as H0. For these acids, the H0 value is estimated to be at or below −18.

The extreme strength of these acids comes from their specific molecular structure. Each acid features a cage-like structure called a carboranate anion. This anion consists of eleven boron atoms and one carbon atom arranged in an icosahedral shape. To increase the acidity, scientists attach electronegative groups to the boron atoms. These groups can include chlorine, fluorine, or CF3. These attachments pull electron density away from the center, which helps the acid release its proton more easily.

This process of electron movement is called delocalization. In a carboranate anion, the electrons are spread out across the twelve atoms in the cage. This creates a state called σ-aromaticity, which makes the cage extremely stable. Because the resulting anion is so stable and lacks oxidizing properties, the acid is described as "gentle." Most superacids are so harsh that they destroy the molecules they react with. However, carborane acids can protonate sensitive molecules like C60 fullerene without decomposing them.

The history of these substances involves several decades of chemical discovery. The parent molecule, the icosahedral carboranate anion, was first synthesized by Walter Knoth at DuPont in 1967. For many years, research into these molecules slowed down. In the mid-1980s, a group of Czech scientists named Plešek, Štíbr, and Heřmánek improved the process for halogenation. This improvement allowed scientists to add halogens to the carborane molecules more effectively. Finally, in 2004, Professor Christopher Reed and his colleagues at the University of California, Riverside, discovered and synthesized the carborane acid itself.

There are different types of carborane acids based on their substituents. The most widely studied version is the highly chlorinated derivative. This version has eleven chlorine atoms attached to the boron atoms. There is also a fluorinated version that is even more powerful than the chlorinated one. The fluorinated carborane acid is capable of protonating butane to form a tert-butyl cation at room temperature. It is also the only known acid that can protonate carbon dioxide to create a bridged cation. This makes it a candidate for the strongest known acid.

Synthesizing these acids is an incredibly difficult and technical process. The most well-studied carborane acid requires a 13-step synthesis. The process starts with decaborane, which is a highly toxic substance. Scientists must perform the final steps inside a glovebox to keep the environment extremely pure. The environment must have less than 1 part per million of water. Even very weak bases, such as benzene or dichloromethane, can react with the acid and ruin the experiment.

Beyond their raw strength, carborane acids have many potential uses in science. They are proposed as catalysts for hydrocarbon cracking and isomerization. This means they could help turn n-alkanes into branched isoalkanes, such as isooctane. They are also useful in mechanistic organic chemistry for studying reactive cationic intermediates. Because the conjugate base is so stable and inert, these acids allow chemists to isolate exotic chemical species. Their ability to be stored in glass bottles makes them much easier to handle than many other superacids.