1-Aminonaphthalene-2-sulfonic acid [81- 06-1]. ortho-naphthionic acid, C10H9NO3S, Mr 223.24, mp 262 – 265℃ (decomp.), is sparingly soluble in cold water but dissolves in hot water to give a 3 % solution. The sodium salt is more readily soluble in water (1.7 % at 20℃ and 10 % at 100℃). The calcium salt is similarly soluble, but the barium salt is almost insoluble.
1-Naphthylamine can be sulfonated by heating with sulfuric acid at 180 – 185℃, but the transient intermediate naphthalene-1-sulfamic acid is best prepared from 1-aminonaphthalene-4-sulfonic acid (naphthionic acid). When sodium naphthionate is heated in a solvent such as o-dichlorobenzene or xylene at 225℃, thermal rearrangement occurs to give the 1,2-isomer in high yield.
