Abstract
It is known that monosulfonation of 1-naphthol affords 1-naphthol-4-sulfonic acid along with 1-naphthol-2-sulfonic acid, but this reaction has not been applied to industrial preparations yet.
1-Naphthol was dissolved in an inert solvent such as ο-nitrotoluene, and less than 2 equivalents of sulfuric acid was added. The mixture was stirred for ca. 3 hours, and then poured into water.
The solvent was separated and KCl was added to the aqueous solution. The precipitated potassium 1-naphthol-2-sulfonate was filtered. Addition of NaCl to the filtrate yielded sodium 1-naphthol-4-sulfonate. The yield of potassium 1-naphthol-2-sulfonate and sodium 1-naphthol-4-sulfonate was 85-90%.
When the reaction was performed at higher temperatures in the range of-570°C, the yield of the 2-sulfonate decreased and that of the 4-sulfonate increased. Each crude product was purified by the salting-out method (more than 90% recovery and less than 0.5% contamination of the isomer).
