Sulfonation of Benzene

As with halogenation, there needs to be something that makes nitric acid more electrophilic—in other words, to create the nitronium ion. HNO3 is a nitrogen atom with three oxygens around it, one of which is attached to a hydrogen ion. When mixed with sulfuric acid, the NO2 made by pulling the OH group off of the nitric acid as is seen in figure 59:

Figure 59.

This electrophile (NO2) is positively charged and is apt to bind to benzene. The sulfuric acid ion is strong enough now to pull off the extra hydrogen ion on benzene, leaving behind sulfuric acid (completely reconstituted) and nitrobenzene.

SULFONATION OF BENZENE

The end result of the sulfonation of benzene is benzenesulfonic acid. In such cases, a catalyst is necessary (as is the case with all electrophilic substitution reactions). Sulfur trioxide (SO3) is used up in the equation, while hot sulfuric acid (H2SO4) is necessary to make the reaction occur. In the reaction, sulfur trioxide is mixed with sulfuric acid to make a mixture called “oleum”. The sulfur in SO3 is considered electrophilic because it is partially charged by the electronegative oxygen atoms. When it combines with benzene, the SO3- ion on the benzene molecule will take the hydrogen off the benzene molecule. What this looks like is seen in figure 60: