During the sulfonation of benzene, concentrated H₂SO₄ generates which electrophile?

Sulfonation of benzene is an example of an electrophilic aromatic substitution (EAS) reaction in which a sulfonic acid group (–SO₃H) is introduced into the benzene ring. The reaction typically occurs when benzene is treated with concentrated or fuming sulfuric acid (oleum). The key to this reaction lies in identifying the electrophile, which attacks the aromatic ring.

When concentrated sulfuric acid (H₂SO₄) is used, an equilibrium exists between sulfuric acid molecules. One molecule of H₂SO₄ can act as an acid and donate a proton (H⁺), while another molecule acts as a base and accepts that proton. This proton transfer produces sulfur trioxide (SO₃) and the hydronium ion (H₃O⁺) as shown below:

2H₂SO₄ ⇌ H₃O⁺ + HSO₄⁻ + SO₃

The SO₃ molecule formed here is the active electrophile in the sulfonation process. It is electron-deficient because the sulfur atom has a high oxidation state and can accept electrons from the benzene ring. Benzene, rich in π-electrons, reacts with SO₃ to form a sigma complex (arenium ion), followed by deprotonation to regenerate aromaticity. The final product is benzene sulfonic acid (C₆H₅SO₃H).

The reaction is reversible. In the presence of excess water and heat, desulfonation occurs, removing the –SO₃H group and regenerating benzene. This reversibility makes sulfonation a useful method for temporary protection or directing substitution in aromatic synthesis.

Thus, the electrophile responsible for attacking the benzene ring during sulfonation is sulfur trioxide (SO₃), generated by concentrated sulfuric acid. Understanding this mechanism helps in mastering the behavior of aromatic compounds in electrophilic substitution reactions, an essential concept in organic chemistry