1,5-naphthalenedisulfonic acids

SPADNS is the acronym for the sodium salt of 2-(4-sulfophenylazo)-l,8-dihydroxy-3,6-naphthalenedisulfonic acid. 

In the manufacture of 2-naphthalenol, 2-naphthalenesulfonic acid must be converted to its sodium salt this can be done by adding sodium chloride to the acid, and by neutralizing with aqueous sodium hydroxide or neutralizing with the sodium sulfite by-product obtained in the caustic fusion of the sulfonate. The cmde sulfonation product, without isolation or purification of 2-naphthalenesulfonic acid, is used to make 1,6-, 2,6-, and 2,7-naphthalenedisulfonic acids and 1,3,6-naphthalenetrisulfonic acid by further sulfonation. By nitration, 5- and 8-nitro-2-naphthalenesulfonic acids, [89-69-1] and [117-41-9] respectively, are obtained, which are intermediates for Cleve s acid. All are dye intermediates. The cmde sulfonation product can be condensed with formaldehyde or alcohols or olefins to make valuable wetting, dispersing, and tanning agents. 

A naphthalene sulfonation product that is rich in the 2,6-isomer and low in sulfuric acid is formed by the reaction of naphthalene with excess sulfuric acid at 125°C and by passing the resultant solution through a continuous wiped-film evaporator at 245°C at 400 Pa (3 mm Hg) (26). The separation in high yield of 99% pure 2,6-naphthalenedisulfonate, as its anilinium salt from a cmde sulfonation product, has been claimed (27). A process has been patented for the separation of 2,6-naphthalenedisulfonic acid from its isomers by treatment with phenylenediarnine (28). 

Naphthalenedisulfonic acid (Ebert-Merz a-acid) is partially isomerized in sulfuric acid at 160°C to 2,6-naphthalenedisulfonic acid. The reaction takes place by a desulfonation—resulfonation mechanism. 

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