According to a textbook by K. L. Chugh:

> Sulfonation [of aniline] is a reversible reaction. The *p*-isomer, being more stable, does not get desulfonated easily. Hence, *p*-sulfanilic acid is the major product.

I am curious as to why the *ortho* isomer (2-aminobenzenesulfonic acid) is not more thermodynamically stable. There is a possibility of intramolecular hydrogen bonding (in a six-membered ring):

[![Intramolecular hydrogen bond in 2-aminobenzenesulfonic acid][1]][1]

Why is the *para* isomer (4-aminobenzenesulfonic acid) favoured?

  [1]: https://i.sstatic.net/lK2gy.png