Plotting these pKa data against σx values yields the graph shown in Fig. 6. It is obvious that all the substituents fall nearly on one correlation except the substituents with resonance effect such as p-CH3CO and p-CN and p-NO2, which are capable of direct interaction with the negatively charged sites in H− and A− (Scheme 4). However, when pKa data
were plotted versus σ− constants, a better correlation was obtainedx (Fig. 7). This can be explained by the fact that the electron density on the aromatic rings can be increased by the electron releasing groups
such as OEt and CH3 due to their electron donating ability which destabilizes anionic forms of the dyes (Scheme 4).
Besides the aforementioned findings [21,24–26] and as a result of the linearity observed between pKa values and σ− constants, it can bex concluded that the azo-quinolone dyes under study exist predominantly as the hydrazone forms T1 and T2 in solution.