HSO3Cl

HSO3Cl, commonly called chlorosulfonic acid, is a strong sulfonating and chlorinating agent used in organic synthesis. Its formula is HSO3Cl, and it features a sulfur(VI) center bonded to three oxygens and a chlorine, often depicted as HO–S(=O)2–Cl. The compound is a colorless to pale yellow liquid that fumes in air and is highly reactive with water and many organic materials. It is highly hygroscopic and must be handled under strictly dry and inert conditions.

Preparation of chlorosulfonic acid is typically achieved by reacting sulfur trioxide with hydrogen chloride, often at controlled temperatures. In practice it is commonly supplied as a solution in sulfuric acid or handled as a neat liquid under appropriate protective measures.

Reactions and applications of HSO3Cl center on its dual functionality as a chlorinating and sulfonating agent. It hydrolyzes readily in the presence of water to form sulfuric acid and hydrogen chloride, releasing heat. In organic synthesis, it is used to introduce sulfonyl chloride functionalities into substrates and to activate alcohols, converting them into alkyl chlorosulfonates or related intermediates. It can also sulfonate aromatic rings under suitable conditions, enabling the preparation of aryl sulfonyl chlorides and related products. Due to its extreme reactivity, reactants and solvents must be chosen with care to avoid unwanted side reactions.

Safety and handling of chlorosulfonic acid are paramount. It is highly corrosive to skin, eyes, and mucous membranes and reacts violently with water. It releases corrosive HCl fumes upon decomposition or hydrolysis. Handling requires appropriate personal protective equipment, proper ventilation, dry glassware, and storage away from moisture, bases, and incompatible organic materials.