In benzene sulphonic acid, the sulphonic group is attached to the benzene ring through which atom?

Benzene sulphonic acid (C₆H₅SO₃H) is an important aromatic compound formed when benzene undergoes sulphonation by reacting with concentrated sulfuric acid (H₂SO₄) or fuming sulfuric acid (oleum). During this reaction, a sulphonic group (-SO₃H) is introduced into the benzene ring, replacing one of the hydrogen atoms.

In the structure of benzene sulphonic acid, the sulphonic group (-SO₃H) is attached to the benzene ring through the sulphur atom. This means that the sulphur atom directly bonds with a carbon atom of the aromatic ring. The sulphonic group itself contains one sulphur atom doubly bonded to two oxygen atoms and singly bonded to a hydroxyl group (-OH). This gives it the structural formula –SO₂OH when attached to a ring.

The bond between sulphur and the aromatic carbon is a sigma bond, and due to the presence of electron-withdrawing oxygen atoms, the sulphonic group exerts a strong -I (inductive) and -M (mesomeric) effect, making the benzene ring less reactive toward further electrophilic substitution reactions. As a result, benzene sulphonic acid is deactivated and directs incoming substituents to the meta position.

Benzene sulphonic acid is highly soluble in water because of its ability to form hydrogen bonds through its –OH and =O groups. It serves as a key intermediate in the preparation of detergents, dyes, and sulfa drugs. The sulphonation process is also reversible under certain conditions, making it a useful step in aromatic substitution chemistry.

In conclusion, in benzene sulphonic acid, the sulphonic group (-SO₃H) is linked to the benzene ring through the sulphur atom, defining its chemical reactivity and functional behavior in organic chemistry.