Structure of hydroxytriazenes and tautomerism
(i) 3-Hydroxy-l,3-diphenyltriazene (HDPT) as the parent compound of the hydroxytriazene class.
The compound prepared by the coupling of phenylhydroxylamine with benzene diazonium chloride is a light yellow solid with m.p. 119-120. It is very stable compound which can be preserved for quite a long time. Most of the complexes prepared with the compound are granular with a definite composition suitable for direct weighing as they are thermally stable. The spectral characteristics of the reagent using alcohol are Xmax 238 nm with log e= 4.328. The general structure of hydroxytriazenes or their analogues can be represented by the structure of 3-hydroxy-l,3-diphenyltri- azene which has been well-established by Purohit et al. [9,10]. The author has reported on the basis of UV spectral studies of HDPT by Calzolari and Furlani  that it exists either in a single form or in two forms which have identical spectra. Purohit et al.  have suggested the existence of a hydrogen-bonded form whether it is present in hydroxytriazene or in its tautomeric triazene oxide form. The same conclusion was published by Purohit and Sogani  on the basis of the low dissociation constants of
Figure 2.1 HDPT and other analogues exist in an intra-molecularly hydrogen bonded form (I) and its tautomers (II) and (III).
HDPT. It is thus suggested by the authors that HDPT and other analogues exist in an intra-molecularly hydrogen-bonded form (I) and its tautomeric form (II) and (III) (Figure 2.1).
The structural details described above have been supported by UV and IR spectral studies by the same authors. It is described in the paper that IR spectra in KBr showed bands at 3480 cm-1 and 3190 cm-1 for uOH and vNH, respectively. Further, the band at 3480 cm-1 did not exist in carbon tetrachloride, and Mitsuhasi  reported the position of a second band at 3250.6 cm-1 which has been established to be vNH through studies on 1SN (nitrogen remote from oxygen) in labeled HDPT. The existence of tautomeric forms is further strengthened due to the possibility of migration of the H atom via the formation of an intermediate chelate (IV).
The oxygen atom in the hydroxytriazene group is an electron donor, increasing the negative charge on the nitrogen atom, which increases the affinity of the H atom for nitrogen. Due to the greater electronegativity of oxygen compared to nitrogen a complete transfer of hydrogen to nitrogen does not occur. All these facts support a hydroxytriazene structure. The structure established can be represented as follows.
The strengthening of the N-N and N-O bonds is facilitated through the delocalization of electron density in the ring which equalizes them. Further, by this the л electron cloud of the molecule increases in symmetry. Thus a hydroxytriazene structure is predominant, as evidenced.
Tautomerism and hydrogen bonding in 3-hydroxy-1,3-diphenyltriazenes (HDPT)
Two leading publications on the tautomerism and association in hydroxy- triazenes are available. On the basis of UV absorption studies, Calozolari and Furlani  suggested two possibilities for the structure of FIPDT: (i) it exists in a single form, or (ii) two tautomeric forms have the same spectra. Purohit et al. suggested the existence of a hydrogen-bonded structure and its tautomeric forms based on spectral studies. The same conclusion was restated on the basis of the dissociation constant value of HDPT, pK = 11.41. The following resonating forms have been assigned.
Structures (I) to (VIII) show that charge on the anion is distributed over two benzene rings, making HDPT a stronger acid than phenol. However, the pK values indicate a contrary picture. A hydrogen-bonded structure and its tautomeric forms are suggestive of the suppression of acid character as shown in Figure 2.1 (II and III).
Further, on the basis of the spectrophotometric determination of HDPT , a hydroxytriazenic structure has been further evidenced.