Studies with dithizone—XII Formation of thiadiazolines by condensation with aldehydes and ketones

Abstract

The red colour that develops when mineral acids are added to solutions of dithizone (I; 3-mercapto-1,5-diphenylformazan) in certain samples of dioxan is mainly due to the formation of 2-methyl-3-phenyl-5-phenylazo-1,3,4-thiadiazoline (II) derived in part from adventitious traces of 2-methyldioxalane. A purple compound of molecular formula C(13)H(10)N(4)S is also formed from (I) by an independent and slower reaction. The thiadiazoline (II) is readily prepared from (I) and acetaldehyde, but analogous compounds from formaldehyde benzaldehyde, acetone and ethyl methyl ketone are obtained in better yield by starting from diphenylthiocarbazide. Di-p-tolyldithizone gives similar reactions. Reduction of the thiadiazoline (II) [which has spectra very closely resembling those of 1:1 complexes of (I) with arylmercury (II) cations] with ammonium hydrogen sulphide in ethanol yields diphenylthiocarbazide by opening of the hetero-ring and elimination of an alkyl residue from the intermediate 3-alkylmercapto-1,5-diphenylformazan. Other examples of nucleophilic displacements from the formazan group by SH(-) have been investigated.