Tautomerism in Schiff Bases and Related Azo Compounds

In sharp contrast to the P-diacyl compounds of Table 11.5, aU those of Table 11.7 possess positive tt and a coefficients. The first demonstrates that the aminoenone moiety of the b tautomers is better conjugated, so more polar, than the aromatic but otherwise weakly conjugated a forms, while the second reflects the fact that carbonyl, unlike aza-nitrogen, has a second lone pair available for bonding to the solvent. The absence of a /3 term, for example, for acetylacetone 13 indicates that no solvent employed can open its chelated tautomer its absence in the Schiffbases 19 and 20 extends that conclusion to both. However, all azo compounds possess a f term, not only 23 which cannot form a chelate, and its negative sign for 21 and 22 is evidence that the OH N chelate of the a tautomer is wholly or mainly affected. 

This is unexpected because OH N is generally considered much stronger than NH O hydrogen bonding [35] and OH here will certainly be a strong proton 

Two apparent problems remain. The first is the much poorer statistics for the azo compounds than the Schifi bases, and this is certainly not because the experimental data are inferior. Any hnear free-energy relationship depends for its success on precise characterization of the system being examined. If that is compromised, for example, by the presence of molecular species not part of the analysis, that analysis may be at risk. This is most obviously trae for 21 and 22 since both contain unchelated material [33] which, for one component or both, may vary with solvent in a way that does not reflect the behavior of the dominant subspecies. Also, this unchelated material, being spectrally almost identical with the corresponding chelate but without any clear isosbestic points, caimot be recognized as distinct by the chemometric approaches used for the estimation of Ky. To a lesser extent this may also be true for 23, where conformational complications are possible. By contrast, the tight chelate bonding observed for both tautomers of 19 and 20 leads to clearly defined systems whose statistics show a marked improvement. A similar contrast is shown between dimedone 11 and the bicyclic fS-ketoester 16 

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