Complications Caused by Protonation at the Wrong Site

More commonly, protonation additional to that which gives the common cation has the potential to produce spurious fCp values and can be quite difficult to detect, especially when UV methodology with its relative lack of resolution is employed. Its effect is always to increase the observed pf value, and since as above it tends to act on the less basic, that is, more favored, tautomer, the result is generally to reduce the energy gap between the tautomers and so lower the estimated fCj value. 

72 The Basicity Method for Estimating Tautomer Ratio A Radical Re-appraisal 

Katritzky and coworkers set out to address in 4-phthalazinone 39 [28] those would have to be the subject of subsequent investigation. In this and in other such cases, the amide not the iminol would probably be the major tautomer. 

While oxopyrazines, oxopyridazines, and their benzofused derivatives are susceptible to protonation at aza-nitrogen, the corresponding pyrimidones are not, since in the 0x0 or thiono tautomers their nitrogen atoms give rise to an amidine structure whose conjugated system would be dismpted by further protonation. Other heterocyclic systems have yet to be explored. 

Tautomeric Problems to Which the Basicity Method Is Inapplicable 

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