L-Phenylazo-2-naphthol

Dyes based on 4-phenylazo-l-naphthol (6) have been used extensively to study azo/hydrazone tautomerism since they exist as an equilibrium mixture of both the azo and hydrazone tautomers.8 However, they are of little use commercially and of no use whatsoever for metal complex azo dyes since the hydroxy group is not ortho to the azo group so these cannot act as chelating ligands. 

The phenylazonaphthols present a particularly interesting series. Despite the partial loss of aromaticity in the hydrazone structures that can be drawn for l-phenylazo-2-naphthol, 2-phenylazo-l-naphthol and 4-phenylazo-l-naphthol, these structures are not much higher in energy than the azo forms [50], so that there should be no significant preference for one form over the other. When the visible absorption curves of 4-phenylazo-l-naphthol are measured in a variety of solvents, they all pass through a common point (isosbestic point) as in Figure 4.1 [51,52]. l-Phenylazo-2-naphthol shows similar behaviour. This finding is clear evidence that in solutions of these compounds both tautomers are present. 

The solvent effect on the azo-hydrazone equilibrium of 4-phenylazo-l-naphthol has been modelled using ab initio quantum-chemical calculations. The hydrazone form is more stable in water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form, The calculated results were in good agreement with the experimental data in these solvents. Similar studies of l-phenylazo-2-naphthol and 2-phenylazo-l-naphthol provided confirmation. Substituent effects in the phenyl ring were rationalised in terms of the HOMO-LUMO orbital diagrams of both tautomeric forms [53]. 

Figure 4.1 Effect of solvent on 4-phenylazo-l-naphthol spectrum...

HadZi used IR spectroscopy to study some azophenols and azonaphthols (112). The phenylazophenols were found to be phenolic in the solid state. On the other hand the molecule of 4-phenylazo-l-naphthol is present in the crystal as the hydrazone tautomer. Had2i suggested that in l-phenylazo-2-naphthol and 2-phenylazo-1 -naphthol both tautomers are present. Similar suggestions have been made by others (113,114). 

Both forms are present in 4-phenylazo-l-naphthols the hydrazone form being bathochromic relative to the azo form (2.12), and the proportion of each dependent on substituents and solvent. 

Azo oil dye The synthesis and purification of 4-phenylazo-1-naphthylamine (4-NH2), 4-phenylazo-l-naphthol (4-OH) were described in our previous paper (14). Water used in this experiment was twice distilled and was deionized by an ion-exchange instrument (NANO pure D-1791 of Barnstead Co., Ltd.) its resistivity was about 18.0 megohm cm and its pH was 6.7. 

Because azo dyes are of commercial importance as colouring materials, the azo/ hydrazone tautomerism of hydroxy-substituted azo compounds has been intensively studied [71, 228]. In the case of 4-phenylazo-l-naphthol (16a), an increase in the solvent polarity displaces the tautomeric equilibrium towards the more dipolar quinone hydra-zone form (16b) [72-74, 74a, 74b, 150-154]. In addition, the NH and OH groups of both tautomers are capable of forming hydrogen bonds with suitable solvents. Due to... 

The azo/hydrazone tautomeric equilibrium 4-phenylazo-l-naphthol 1,4-naphthoquinone-1-phenylhydrazone [cf. (16a) (16b) in Section 4.3.2] has been studied in various liquids and in liquid and supercritical carbon dioxide [910]. The less dipolar azo tautomer 10 ° Cm) is dominant in the dilute gas phase, in com-... 

O Shea K, Kirmse K, Fox MA, Johnston KP. Polar and hydrogen-bonding interactions in supercritical fluids effects on the tautomeric equilibrium of 4-(phenylazo-l-naphthol). J Phys Chem 1991 95 7863. 

Dolinsky and Jones [58] make the interesting observation that ort/zo-hydroxy or aminoazo-compounds were abnormal and different from the p-substituted compounds, which led them to conclude that zwitterion structures involving the —N=NH— structure were involved. Hadzi [59] had investigated this further using deuteration techniques. He found that complex mixtures of tautomers involving the hydrazones are formed in this way, whilst compounds such as 4-phenylazo-l-naphthol and the o-methyl derivative of 1- and 4-phenylazophenol exist as true —N=N— compounds. He was, however, unable to identify any characteristic —N=N— frequency in any of these compounds. 

Unimolecular reactions that have been used to investigate the solvation properties of supercritical fluids include tautomeric reactions (67-71), rotational isomerization reactions (72-78), and radical reactions (79-83). O Shea et al. used the tautomeric equilibrium of 4-(phenylazo)-l-naphthol to examine the solvent strength in supercritical ethane, CO2, and fluoroform and to determine its dependence on density (67). The equilibrium is strongly shifted to the azo tautomer in supercritical ethane and the hydrazone tautomer in supercritical chloroform and... 

In the case of solvatochromism a shift in the equilibrium between two types of molecules can occur (for example, keto enol equilibrium, associated nonassociated form, or the stabilization of polarized or the unpolarized form of the dye can take place under the influence of the polarity of the solvent. For example, the behavior of 4-phenylazo-l-naphthol (I) (12, 14) represents a clear example of the shift in the tautomeric equilibrium under the influence of the solvent. When dissolved in pyridine this compound gives a yellow solution and contains exclusively azonaphthol form (la). 

For the first time, such an approach was used by Metzler and collaborators [20] who studied the two-component tautomeric equilibrium between the neutral form (37b, Chapter 1) and the dipolar ion (37a, Chapter 1) of 3-hydroxypyridine in water and water/methanol binary mixtures at various temperatures. Very similar is the procedure used by Reeves et al. [21] for the analysis of the keto-enol tautomerism of 4-phenylazo-l-naphthol-2,4 -disulfonate VSzquez Segura and collaborators [22] in the case of 5 -deoxypyridoxal and Vilanova et al. [23] for the analysis of tautomeric members of vitamin Bg family. In all these cases, the individual bands belonging to tautomers are mathematically resolved using a suitable band-shape function (in this case log-normal [18]), and the corresponding spectral areas I are calculated. The tautomeric constant Kj. is then given as... 

Figure 13.1 Experimental (thick lines) and simulated [thin lines PCM (EtOH)-TDDFT CAM-B3LYP/5-311+G(d,p)//B3LYP(5-31+G(d,p)] UV-vis spectra of hydrazo (full lines) and azo (dotted lines) tautomers of 4-phenylazo-l-naphthol.

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