Sulfonation of Benzene

Aug 26, 2022, 16:45 IST

Sulfonation of Benzene

Benzene reacts with fuming sulfuric acid at room temperature to produce benzenesulfonic acid. Fuming sulfuric acid is sulfuric acid that contains added sulfur trioxide (SO3). Sulfonation also takes place in concentrated sulfuric acid alone, but more slowly . In fuming sulfuric acid, step 1 is unimportant because the dissolved sulfur trioxide reacts directly.

Because all of the steps are reversible, the position of equilibrium can be influenced by the conditions we employ. If we want to sulfonate benzene we use concentrated sulfuric acid (or better fuming sulfuric acid). Under these conditions the position of equilibrium lies appreciably to the right and we obtain benzenesulfonic acid in good yield.

On the other hand, we may want to remove a sulfonic acid group from a benzene ring. To do this we employ dilute sulfuric acid and usually pass steam through the mixture. Under these conditions with a high concentration of water the equilibrium lies appreciably to the left and desulfonation occurs. The equilibrium is shifted even further to the left with volatile aromatic compounds because the aromatic compounds distill with the steam.

We shall see later that sulfonation and desulfonation reactions are often used in synthetic work. We may, for example, introduce a sulfonic acid group into a benzene ring to influence the course of some further reaction. Later, we may remove the sulfonic acid group by desulfonation. Like iodination, sulfonation exhibits a kinetic isotopic effect.

Mechanism of Sulfonation of Benzene