Quinazoline hydration equilibrium ratios

Quinazoline 3-oxides, hydration equilibrium ratios (table), 65 hydration of, 10, 11, 13, 22 oxidation of, 13 ultraviolet spectra of, 10,11... 

Very few values are known for anhydrous organic bases which can undergo covalent hydration, so that, in general, and for such systems cannot be calculated using Eqs. (12) and (13). However, in cases where the pA of the hydrated species can be measured, Eq. (14) can be used to obtain an approximate estimate of K, the equilibrium ratio of hydrated to anhydrous neutral molecules. This treatment has been applied to quinazoline, the nitroquinazolines, and some triazanaphthalenes. 

In principle, if an estimate could be made of K, the equilibrium concentration ratio of hydrated to anhydrous cations, relation (15) would enable the approximate pA of the anhydrous species to be calculated. Although such an estimate may be derivable from absorption spectral data, no such calculation appears to have been reported. Conversely, if an upper estimate of pA is made from the (pAa)eqm value for the corresponding, appropriately methyl-substituted base, Eq. (15) can be used to furnish a lower limit to the extent of hydration in the cation. Taking quinazoline as an example ... 

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