Benzenethiol reactions with nitriles

Substituted thioindigoid dyes are usually obtained via the appropriate benzenethiol in a Heumann-type synthesis. The final cyclisation of the phenylthioglycolic acid derivative can often be achieved in concentrated sulphuric acid or by using chlorosulphonic acid. Several routes make use of the Herz reaction (Scheme 6.22), in which a substituted aniline is converted into the corresponding o-aminothiophenol by reaction with sulphur monochloride followed by hydrolysis of the intermediate dithiazolium salt [47]. After reaction between the thiol and chloroacetic acid, the amino group is converted into a nitrile group by a Sandmeyer reaction. Hydrolysis of the nitrile leads to the formation of the required thioindoxyl derivative. 

The sulfonation reaction is reversible and benzenesulfonic acid may be desulfonated by treatment with dilute acid at 150 °C. The group can be displaced by fusion of its salt with sodamide to give the corresponding amine, with sodium hydroxide to give the phenol, sodium cyanide to give the nitrile, and potassium hydrogen sulfide to give the benzenethiol (Scheme 5.10). 

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