oleum

Oleum, also known as oil of vitriol, is a solution of sulfur trioxide (SO3) in sulfuric acid (H2SO4). The term derives from Latin oleum meaning oil. Oleum is a more strongly acidic reagent than concentrated sulfuric acid and is used as a sulfonating and sulfating agent in organic synthesis. It exists as a fuming liquid with dissolved SO3, and its composition is commonly described by the percentage of SO3 by weight. Grades vary, typically ranging from tens of percent to higher concentrations, with higher SO3 content producing more vigorous fumes.

Preparation and nature: Oleum is produced by absorbing sulfur trioxide into sulfuric acid or by reacting sulfur trioxide with sulfuric acid under controlled conditions. It is a viscous, highly corrosive liquid that liberates heat upon reaction with water and releases sulfuric acid fumes. Because of its reactivity, handling requires corrosion-resistant equipment, a fume hood, and appropriate protective equipment. Storage must minimize exposure to moisture to prevent hydrolysis and loss of SO3.

Uses: In organic chemistry, oleum is employed to introduce sulfonic acid groups into substrates or to sulfate alcohols and phenols. It is a stronger sulfating agent than sulfuric acid alone and can enable sulfation under milder or more controlled conditions. Common applications include preparing sulfonated intermediates and certain detergent precursors; nitration and other reactions may use oleum within broader acid-sulfate contexts.

Safety and handling: Oleum is highly corrosive and toxic, with the potential to cause severe chemical burns and respiratory irritation. It requires careful handling, complete containment, and disposal protocols consistent with highly reactive inorganic acids.

See also: sulfuric acid, sulfur trioxide, fuming sulfuric acid, sulfonation, sulfation.