Carborane acid

Some acids are very, very strong.

These are called carborane acids. They are much stronger than the acid in a battery. They can even change the air into something new.

These acids work in a gentle way. They do not break things too fast. They can change a tiny piece of carbon without destroying it. This makes them very special. Can you imagine a super strong acid?

Some acids are incredibly strong. We call these superacids.

Carborane acids are a special group of these superacids. They are much stronger than pure sulfuric acid. In fact, they can be a million times stronger!

These acids are unique because they are "gentle." Most superacids are very harsh and break things apart. But carborane acids can change a molecule without destroying it. For example, they can work on a carbon shape called a fullerene. They do this without breaking the carbon cage.

Why are they so strong? It is because of their shape. They have a cage made of boron and carbon atoms. This cage is very stable. It has many chlorine or fluorine atoms around it. These atoms help protect the cage. They also help the acid let out a tiny particle called a proton. This proton is what makes the acid work. One version of this acid can even change carbon dioxide. This makes it one of the strongest acids we know.

Carborane acids are a special group of superacids. A superacid is a substance that is much stronger than a regular acid. These acids are famous because they are incredibly powerful. Some are estimated to be one million times stronger than pure sulfuric acid.

They are known as Brønsted acids because they work by releasing a tiny particle called a proton. This proton is what makes the acid act so strongly on other things. Scientists study them to understand how the strongest substances in the world work.

The way these acids work comes down to their unique shape. They have a tiny cage made of eleven boron atoms and one carbon atom. This cage is called a carboranate anion.

To make the acid even stronger, scientists add atoms like chlorine or fluorine to the cage. These atoms act like shields and help pull energy away from the center. This makes it much easier for the acid to let go of its proton. Because the cage is so stable, the acid can perform its job without falling apart.

Scientists have been working with these shapes for a long time. A scientist named Walter Knoth first made the carboranate cage at DuPont in 1967. Later, a group of scientists in the Czech Republic improved how to add halogens to the cage. In 2004, Professor Christopher Reed and his team at the University of California, Riverside, discovered and made the carborane acid.

Making these acids is a very hard job. It can take 13 different steps to finish the process. Scientists must use special equipment called a glovebox to keep out even tiny bits of water.

There are many important facts about these powerful chemicals. The most studied version uses eleven chlorine atoms to surround the cage.

There is also a version made with fluorine that is even stronger. The fluorinated version can even change carbon dioxide, which is a very difficult task. This makes it possibly the strongest acid known to science. In the gas phase, some versions have a measured acidity of 241 kcal/mol. These numbers show just how much energy is involved in their reactions.

You can think of these acids as being both strong and gentle at the same time. Most superacids are very harsh and destroy the molecules they touch. However, carborane acids are unique because they can change a molecule without breaking it.

For example, they can work on a carbon shape called a fullerene without decomposing it. This is like a tool that can turn a screw perfectly without scratching the wood around it. Because they are so stable, they can even be stored in glass bottles. This makes them very useful for special jobs in chemistry labs.

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.