The Herz reaction, named after the chemist Richard Herz, is the chemical conversion of an aniline to the benzodithiazolium salt by its reaction with disulfur dichloride. The salt is called a Herz salt. Hydrolysis of this Herz salt give the corresponding sodium thiolate, which can be further converted to the 2-aminothiophenol.^[1]

The 2-aminothiophenols are suitable for diazotization, giving benzothiadiazoles.^[2] Instead the sodium 2-aminothiophenolate can be converted to a 1,3-benzothiazole.

Dyes

Aniline 5 is converted to compound 6, in three steps;

1. conversion to an ortho-aminothiol through the Herz-reaction (aniline 5 and disulfur dichloride), followed by
2. conversion to an ortho-aminoarylthioglycolacid and
3. conversion of the aromatic amine function to a nitrile via the Sandmeyer reaction.
4. In a last step the nitrile is hydrolysed resulting in 6. This compound is converted to 7 via a ring-closing reaction and decarboxylation.

The compound, (thioindoxyl, 7) is an important intermediate in the organic synthesis of some dyes. Condensation with acenaphthoquinone gives 8, a dye of the so-called Ciba-Scarlet type, while condensation of 7 with isatin results in the thio-Indigo dye 9.