Sulfonation

Sulfonation is a chemical process that introduces a sulfonic acid group, typically written as -SO3H, into an organic molecule. The reaction often forms sulfonic acid derivatives such as aryl sulfonic acids or alkyl sulfonic acids and can also yield related sulfonate esters under different reagents and conditions. In many contexts, sulfonation denotes electrophilic sulfonation, whereas other routes may lead to sulfonyl derivatives.

In aromatic compounds, sulfonation proceeds by electrophilic substitution of the aromatic ring. The most common sulfonating agents are sulfuric acid, fuming sulfuric acid (oleum), or sulfur trioxide sources. The sulfonic group is introduced onto the ring with the reaction typically governed by directing effects and the substituents already present on the ring. The sulfonic acid group is strongly deactivating and meta-directing for subsequent substitutions. The reaction is generally reversible at elevated temperatures, and desulfonation can occur under hot aqueous conditions to regenerate the parent hydrocarbon in some cases.

In aliphatic substrates, direct sulfonation can be achieved using reagents such as sulfur trioxide or the sulfuric acid/sulfur trioxide system, leading to alkyl or aryl sulfonic acids. This route is employed in the petrochemical and fine-chemical industries to modify solubility, acidity, or reactivity.

Applications of sulfonation include the production of detergents and dyes, pharmaceutical intermediates, and the preparation of sulfonated polymers such as ion-exchange resins. These polymers, including sulfonated polystyrene derivatives, are valued for their acidic groups and hydration properties. Safety considerations are important, as sulfonation reagents are highly corrosive and the reactions are often exothermic.