FSO3H

FSO3H is the chemical shorthand for fluorosulfuric acid, also known as fluorosulphonic acid. Its formula is FSO3H, reflecting a sulfur center bonded to three oxygens and a bonded fluorine atom. It is one of the strongest Brønsted mineral acids and is often described as a superacid. In addition to the term fluorosulfuric acid, it is sometimes referred to by its descriptive name or as a component of superacid systems such as magic acid when paired with a Lewis acid.

Preparation of FSO3H typically involves the direct combination of hydrogen fluoride (HF) with sulfur trioxide (SO3) under strictly anhydrous conditions. The reaction yields fluorosulfuric acid, which is most commonly handled as a concentrated, moisture-free liquid due to its extreme reactivity with water.

Physically, fluorosulfuric acid is a colorless to pale yellow liquid that is highly hygroscopic and corrosive. It is miscible with water but reacts vigorously with moisture, releasing hazardous gases such as hydrogen fluoride. It is also highly reactive with many materials, including glass, and is usually stored in fluorinated polymer-lined vessels or other compatible containment. In solution, it behaves as an extremely strong acid and a powerful dehydrating agent.

Applications center on its role as a superacid catalyst in organic synthesis. As a Brønsted acid, it can protonate hydrocarbons and stabilize carbocation intermediates, enabling reactions that are difficult or impossible with weaker acids. When combined with antimony pentafluoride (SbF5) to form magic acid, its acidity is enhanced further, enabling highly reactive catalytic environments for specialized fluorination and rearrangement processes.

Safety and handling require strict moisture control, use of appropriate personal protective equipment, and operation in a well-ventilated fume hood. Contact with skin or eyes can cause severe burns, and exposure to hydrofluoric compounds necessitates careful emergency procedures.