The u.v. spectra of 12 halogen-substituted benzoic acid hydrazides are reported. A very intense band in the region 225–260 mμ and a second one of low intensity at 270–290 mμ are assigned to π → π* and n → π* transitions, respectively.The pK1 and pK2 of the mentioned hydrazides, corresponding to the Brönsted acids [Formula: see text] and XC6H4C(OH)=NNH2 respectively, were obtained spectrophotometrically (X = F, Cl, Br, or I in o-, m-, and p-position). The influence of the nature and the position of the halogen atom on the pK values is discussed.
A series of substituted 3-amyldithiocarbazates has been synthesized and studied. The corresponding acid dissociation constants have been determined potentiometrically. Semiempirical PM3 molecular orbital calculations suggest the existence of several tautomeric forms of the compounds. Geometrical parameters, proton affinities, and static reactivity indices have been examined. Structural properties and protonation sites are well described by calculations. The strong correlations between the pKa values and the Hammett a constants as well as the N(3) calculated proton affinities indicate that the N(3) atom is the most probable protonation site. The thermodynamics of the protonation process are mainly controlled by HOMO-LUMO rather than electrostatic interactions. According to PM3 results, 3-aroyldithiocarbazic acid should be quite stable in the gas phase, while a mechanism for its decomposition in solution is proposed.
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