# Reactions of Aromatic Compounds - Examples of Ar-SE Reaction

## Sulfonation

When benzene is treated with hot concentrated sulfuric acid for a certain period, benzenesulfonic acid is formed.

Fig.1
Reaction of benzene with concentrated sulfuric acid.

The reaction runs considerably more rapidly, when oleum is applied instead of concentrated sulfuric acid. Oleum is concentrated sulfuric acid enriched with sulfur trioxide ($SO3$). The reaction rate rises with an increase of $SO3$ content. Sulfonation may also be carried out the application of $SO3$ in an inert solvent.

These observations indicate that $SO3$ is the actual electrophile in the sulfonation of aromatic compounds even if concentrated sulfuric acid is used. In concentrated sulfuric acid, $SO3$ is generated in small amounts through autoprotolysis.

Fig.2
Autoprotolysis of sulfuric acid.

If the electrophile is an electrically neutral species, such as $SO3$, a zwitterionic σ complex is formed.

Fig.3
Formation of a zwitterionic σ complex.

Dissolved $SO3$ can then also act as a which abstracts the proton from the zwitterionic σ complex yielding an aromatic sulfonate. Subsequent protonation of the aromatic sulfonate yields an aromatic sulfonic acid, such as benzenesulfonic acid, and restores $SO3$.

Fig.4
Intermolecular proton abstractions.

The deprotonation of the σ complex and the protonation of the aromatic sulfonate may alternatively occur in one step by an intramolecular proton shift.

Fig.5
Intramolecular proton shift.
Fig.6

The animation illustrates the sulfonation mechanism of the para sulfonation of toluene with an intramolecular proton shift.

In contrast to nitration, sulfonation of aromatic compounds is reversible. As a result, the sulfo group ($SO3H$-) can be exchanged for hydrogen by steaming the aromatic sulfonic acid. Thus, sulfonation may be used to control the regioselectivity of electrophilic aromatic substitution even if the desired product does not contain any sulfo group. To this end, a sulfo group is introduced into an aromatic compound first and then controls the regioselectivity of the introduction of additional substituents through electrophilic aromatic substitution. Finally, the sulfo group is removed by steaming.

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