Aromatic sulfonation

Stoichiometry and mechanism

Typical conditions involve heating the aromatic compound with sulfuric acid: : 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: : Historically, mercurous sulfate has been used to catalyze the reaction.

Chlorosulfuric acid is also an effective agent: :

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 reaction is very useful in protecting the aromatic system because of this reversibility. Due to their electron withdrawing effects, sulfonate protecting groups can be used to prevent electrophilic aromatic substitution. They can also be installed as directing groups to affect the position where a substitution may take place.

Reactions of aryl sulfonates

Desulfonation reaction

The sulfonic acid group can be removed by desulfonation acid hydrolysis: : (Ar = aryl)

Alkaline fusion

One of the main reasons for sulfonation is the subsequent hydrolysis to obtain the phenol illustrated by the following steps: : (Ar = aryl) :

The first step is achieved by alkaline fusion, which entails the dissolution of the sulfonic acid in molten alkali metal base, such as a mixture of sodium hydroxide-potassium hydroxide at 250 °C. This approach is employed to produce ethylphenol from ethylbenzene.

Specialized sulfonation methods

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

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

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 and the benzene 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.

References

References

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