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Abstract

5-Phenyl-1,3,4-thiadiazol-2-diazonium ion (I) is transformed into 2-amino-5-phenyl-1,3,4-thiadiazol (II) in diluted mineral acids. The reaction rate measured in solutions of diluted sulphuric acid reaches its maximum at concentrations of 2 to 2.5M-H₂SO₄. The reaction intermediate is 5-phenyl-1,3,4-thiadiazol-2-diazo hydroxide (III). The rate-limiting step in formation of III consists in base-catalyzed reaction of the diazonium ion I with water; it is 4.3 times slower in 0.1M-D₂SO₄ than in 0.1m-H₂SO₄. Ratio of the rate constants of the transformation of the diazo hydroxide III into the diazonium ion I and into the amine II increases rapidly with increasing sulphuric acid concentration.