Why are benzenesulfonamides derived from primary amines acidic?

Question

Why are benzenesulfonamides derived from primary amines acidic?

Answer 1

First of all, amides derived from primary amines actually have a hydrogen attached to the nitrogen atom, whereas those derived from secondary amines don't. Why is this hydrogen acidic? It is because of the strong electron-withdrawing influence of the -OSO2 group, which contains four electronegative atoms: three oxygens and sulfur (and in this case sulfur's electronegativity is elevated by the high oxidation state). Compare this to the -COO group which also has a strong electron-withdrawing influence, but not as strong as -OSO2 (although _two_ carboxyl groups are comparable in effect to _one_ sulfonyl group). Drawing electron density away from an electronegative atom, such as nitrogen, helps the proton attached to that atom be more acidic (for the negative charge in the anion is more effectively stabilized, being more delocalized over the structure.) Hence, sulfonamides, while not acidic enough to liberate carbon dioxide from sodium bicarbonate, are sufficiently acidic to be extracted into aqueous sodium hydroxide.

This in fact forms the basis of the test for primary, secondary, and tertiary amines: primary amines react with benzenesulfonic acid to give a product (the sulfonamide) which is soluble in "caustic soda"; secondary amines react to give a sulfonamide which has no acidic hydrogens and is consequently insoluble in caustic soda; tertiary amines fail to react.