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

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


The sulfonyl group is kind of bulky due to the size of the sulfur, so steric hindrance is probably making it energetically disfavored for the group to be placed ortho. It's unlikely you can see this easily from a model or a drawing. If you have access to computational resources, you might be able to see the issue.


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