Sulfonic acid derivatives of naphthalene

Sulfonation followed by nitration produces further naphthalene derivatives which are used predominantly as dyestuff intermediates. 

Among the naphthalenesulfonic acid derivatives, particular note should be taken of the so-called letter acids. Table 9.6 shows some important letter acids, which are used as coupling components in the production of azo dyes. 

1 Sulfonation reactor 2 Nitration reactor 3 NOx-separation 4 Neutralizing vessel 5 Filtration vessel 6 Reduction reactor 7 Precipitation vessel 8 Tri-Na-salt formation 9 Concentration vessel 10 Melt reactor 11 Precipitation vessel 

In the first reaction stage, naphthalene is sulfonated with sulfuric acid. Subsequent nitration and neutralization of the naphthalene-trisulfonic acid with lime yields the calcium salt of l-nitronaphthalene-3,6,8-trisulfonic add, which is traditionally reduced to T-add (Koch-add) with iron filings and hydrochloric acid modem processes use continuous catalytic hydrogenation with a nickel catalyst. The H-acid is obtained by subsequent fusion of the T-acid with sodium hydroxide and neutralization with sulfuric acid. It serves as a coupling component in the production of azo dyes such as Direct Blue 15 or Acid Black 1. 

An example of an azo dyestuff in which y-acid is the coupling component is Acid Red 337, which is obtained through the reaction of diazotized o-trifluoro-methylaniline and y-add. 

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