Monosulfonation

Monosulfonation is the introduction of a single sulfonic acid group (−SO3H) into an organic molecule, most commonly an aromatic ring, by electrophilic aromatic substitution. The reaction typically uses sulfuric acid, fuming sulfuric acid (oleum), or sulfur trioxide as the sulfonating agent. The process yields arylsulfonic acids such as benzenesulfonic acid (C6H5SO3H). The term distinguishes monosubstitution from disulfonation or polysulfonation, where multiple sulfonic groups are introduced.

Mechanistically, a sulfonyl electrophile (SO3 or HSO3+) adds to the aromatic ring to form a sigma complex, which is subsequently deprotonated to restore aromaticity. The result is an aryl sulfonic acid. The SO3H substituent is a strongly deactivating group and largely suppresses further substitution at the same ring under standard conditions; if additional sulfonation occurs, it typically requires different conditions or yields mixtures.

Regioselectivity is governed by existing substituents on the ring. Electron-donating groups (such as alkyl or alkoxy) direct electrophilic attack to the ortho and para positions, while electron-withdrawing groups tend to direct to the meta position. In simple benzene systems, the outcome can depend on temperature, acid strength, and the presence of directing groups on the substrate. Desulfonation, the reverse process, can occur by heating the sulfonic acid in dilute acid or water, sometimes allowing recovery of the parent hydrocarbon.

Industrial relevance includes the production of aryl sulfonic acids used in detergents, dyes, and as intermediates in organic synthesis. Monosulfonation remains a foundational transformation for preparing sulfonic acid derivatives, though it is balanced against reversibility and the influence of substituents on regioselectivity.