SO3

Sulfur trioxide, SO3, is a highly reactive oxide of sulfur and a key intermediate in the industrial chemistry of sulfuric acid. In practice it exists as a reactive monomer in the gas phase and as polymeric forms in the condensed phase; the molecule in its monomeric form is trigonal planar with three S=O interactions, giving a formal oxidation state of +6 for sulfur.

SO3 is produced primarily by oxidation of sulfur dioxide (SO2) with oxygen in the presence of a catalyst, commonly used in the contact process for sulfuric acid manufacture. The gas is highly exothermic to dissolve in water, and in industry the gas is often absorbed in sulfuric acid to form oleum (H2SO4·SO3), which is then hydrated to give sulfuric acid. In the laboratory, SO3 can be generated and handled under strictly dry conditions due to its extreme affinity for water.

Reactivity and applications: SO3 reacts vigorously with water to form sulfuric acid (SO3 + H2O → H2SO4). It also readily sulfonates organic compounds and reacts with alcohols to form sulfate esters. Because of its strong electrophilicity, it is used to introduce sulfate groups in the synthesis of esters, cellulose derivatives, and various polymers, and it serves as a versatile sulfonating agent in organic and industrial chemistry.

Safety and handling: SO3 is highly corrosive and fuming; it damages skin and eye tissue and liberates heat upon contact with moisture. It reacts with moisture in air to form sulfuric acid mist, posing inhalation hazards. It is stored and used only in dry, well-ventilated equipment designed to exclude moisture, typically under inert gas or dry air.

Occurrence: SO3 is not stable as a pure, long-lived solid or liquid under ordinary ambient conditions and is mainly encountered as a reactive intermediate in industrial processes or in concentrated sulfuric acid systems.