Illustrated Glossary of Organic Chemistry

Sulfur trioxide: SO3. A gas at room temperature, and a Lewis acid that reacts with water to produce sulfuric acid. Rain dissolves atmospheric sulfur trioxide, forming acid rain.

Molecular Structure of Sulfur Trioxide

Lewis structure
Ball and spoke
Space filling model

Sulfur trioxide can be used for aromatic sulfonation via the electrophilic aromatic substitution mechanism. In this example, reaction of toluene with sulfur trioxide in sulfuric acid (fuming sulfuric acid or oleum) affords p-toluenesulfonic acid as the major product.