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.