Tautomers, basic properties

Purine has a pK value of 2.39. Thus, the molecule is more basic than pyrimidine (pK = 1.31), but less basic than imidazole (6.99). Electron-withdrawing groups decrease, electron-donating groups increase basicity. Hydroxy and sulfanyl groups which form oxo or thioxo tautomers weaken the acidic properties. The pK values shown in Tables 3-5 were determined spectropho-tometrically, potentiometrically, by NMR, etc (cf. also Table 15, p 320 ff). 

For the first, third, and fourth catalysts in (84) the two tautomers are chemically identical, and the same is true for ions such as HCOJ, HPO4", H2PO2, and H2ASO4, which have been reported to have an abnormally high catalytic activity in some reactions.It is clear that the effectiveness of this kind of catalyst is related to its particular electronic structure rather than to its acid-base properties, and the process is more appropriately described as tautomeric catalysis than as bifunctional or concerted acid-base catalysis. It is of interest that a theoretical treatment of some molecules in which acidic and basic groups form part of the same TT-electron system shows some parallelism between catalytic activity and the coupling constants of the molecular orbital theory moreover, a very general treatment of concerted proton transfers indicates that simple bifunctional acid-base catalysis is likely to be of importance only under very restricted conditions. ... 

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