Chemistry · Amines · NEET
Aniline first reacts with concentrated sulphuric acid to form anilinium hydrogensulphate (a salt). On heating this salt with sulphuric acid at 453-473 K, it produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product. So the final answer NCERT expects is sulphanilic acid (the para isomer).
The -NH2 group is ortho/para directing. But sulphonation happens through the anilinium salt at high temperature (453-473 K). Sulphonation is reversible, and at high temperature the more stable para product is favoured because it avoids steric crowding next to the amino group. So para is the major product, giving p-aminobenzene sulphonic acid.
Sulphanilic acid has a basic -NH2 group and a strongly acidic -SO3H group on the same benzene ring. The -SO3H proton transfers internally to the -NH2 group. This makes the molecule carry a positive -NH3+ end and a negative -SO3- end at the same time, with no overall charge. Such a dipolar ion is called a zwitterion (or inner salt).
Because it exists as a zwitterion, it behaves like an ionic salt rather than a neutral molecule. The +NH3 and -SO3 charges attract each other strongly between molecules (strong electrostatic forces). This gives sulphanilic acid a high melting point, low volatility, and poor solubility in organic solvents, unlike ordinary aromatic acids.
Benzene is sulphonated by heating with fuming sulphuric acid (oleum) to give benzenesulphonic acid directly. Aniline first forms anilinium hydrogensulphate with conc. H2SO4, and only on heating at 453-473 K does it give the para product, sulphanilic acid. The extra salt step and the zwitterion nature of the product make aniline's case a favourite NEET point.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
p-Aminobenzene sulphonic acid, commonly called sulphanilic acid, formed by heating anilinium hydrogensulphate at 453-473 K.
A zwitterion is a dipolar ion that carries both a positive charge (-NH3+) and a negative charge (-SO3-) on the same molecule, with no net charge overall. Sulphanilic acid is a classic example.
The anilinium hydrogensulphate salt is heated with sulphuric acid at 453-473 K to give sulphanilic acid as the major product.
It is sparingly soluble in cold water and better in hot water, but it is poorly soluble in organic solvents because its zwitterion structure makes it behave like an ionic salt.
Sulphanilic acid is used to make azo dyes and is the parent of sulpha drugs (sulphonamides), so its zwitterion structure and para geometry are commonly tested in NEET.
