Aromatic sulfonation

Stoichiometry and mechanism

Typical conditions involve heating the aromatic compound with sulfuric acid:

C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O

Sulfur trioxide or its protonated derivative is the actual electrophile in this electrophilic aromatic substitution.

To drive the equilibrium, dehydrating agents such as thionyl chloride can be added.

C₆H₆ + H₂SO₄ + SOCl₂ → C₆H₅SO₃H + SO₂ + 2 HCl

Chlorosulfuric acid is also an effective agent:

C₆H₆ + HSO₃Cl → C₆H₅SO₃H + HCl

In contrast to aromatic nitration and most other electrophilic aromatic substitutions this reaction is reversible. Sulfonation takes place in concentrated acidic conditions and desulfonation is the mode of action in a dilute hot aqueous acid. The reversibility is very useful in protecting the aromatic system because of this reversibility.

Specialized sulfonation methods

Many method have been developed for introducing sulfonate groups aside from direction sulfonation.

Piria reaction

A classic named reaction is the Piria reaction (R. Piria, 1851) in which nitrobenzene is reacted with a metal bisulfite forming an aminosulfonic acid as a result of combined nitro group reduction and sulfonation.

Tyre sulfonation process

In the Tyrer sulfonation process (1917), at some time of technological importance, benzene vapor is led through a vessel containing 90% sulfuric acid the temperature of which is increased from 100 to 180°C. Water and benzene are continuously removed in a condenser and the benzene layer fed back to the vessel. In this way an 80% yield is obtained.

Applications

Aromatic sulfonic acids are intermediates in the preparation of dyes and many pharmaceuticals. Sulfonation of anilines lead to a large group of sulfa drugs.

Sulfonation of polystyrene is used to make sodium polystyrene sulfonate, a common ion exchange resin for water softening.

Reactions of aryl sulfonic acids

As a functional group, an aryl sulfonic acid exhibits two distinctive reactions:

Desulfonation when heated in water near 200 ºC. The reaction, a hydrolysis, is the reverse of the sulfonation. The temperature of desulfonation correlates with the ease of the sulfonation:

RC₆H₄SO₃H + H₂O → RC₆H₅ + H₂SO₄H

This reactivity is exploited in the regiospecific conversion of 2-chlorotoluene by chlorination of p-toluenesulfonic acid, followed by hydrolysis of the intermediate.

When treated with strong base, benzenesulfonic acid derivatives converts to phenols (via the phenoxides).

C₆H₅SO₃H + 2 NaOH → C₆H₅ONa + NaHSO₄H