Basicity constants quinoline

The piC values of polymethine dyes depend on terminal group basicity (64) thus the protonation abHity diminishes if the basic properties of the residues decrease, passing from benzimidazole, quinoline, benzothiazole, to indolenine. On the other hand, the piC of higher homologues increases with chain lengthening. The rate constant of protonation is sensitive to other features, for example, substituents and rings in the chain and steric hindrance for short-chain dyes. 

Attempts have been made to deduce the structure of the predominant form of a potentially tautomeric compound from the shifts which occur in the ultraviolet spectrum of the compound in question on passing from neutral to basic or acidic solutions. The fact that no bathochromic shifts were observed for 2- and 4-hydroxy quinoline and 1-hydroxyisoquinoline under these conditions was taken as evidence that they existed in the oxo form [similar work on substituted quinol-4-ones led to no definite conclusions ]. A knowledge of the dissociation constants is essential to studies of this type, and the conclusions can, in any case, be only very tentative. A further dif-... 

The reactivity of 1,8-naphthyridine (27) is greater than expected.61,79 Here, unlike the diazines, the two heteroatoms with their unshared electron pairs are in separate rings. After correction for the two kinetically equivalent reactive sites, the rate constant for 27 is nearly the same as that for pyridine and four times larger than that for quinoline. These results are surprising, especially when it is remembered that the diaza substrate is substantially less basic than the comparison compounds. Correcting for the diminished nucleophilicity expected to be associated with the lower basicity of 27 serves to make the reactivity comparisons even more striking.61... 

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