Three-Way Tautomerism in the Pyrazolone

It can be argued that the work of Freyer et al. [39] on the pyrazolone 25 is the single most important application yet of LSER to tautomerism. It covers 

LSER is a methodology that requires a lavish experimental input - work in what can be a dozen carefully chosen solvents being routinely required - and so is never likely to become widespread in its use. Nevertheless, it can solve problems of tautomer ratio inaccessible to other techniques, illustrated here in particular by the pyrazolone 25, only one of whose three equilibria can be investigated by the basicity method of Chapter 12. Indeed, these two techniques are in many ways complementary, as in that case and perhaps generally when linked equilibria, not all involving clearly identifiable common cations, are present. 

Two other disadvantages ofLSER need pointing out. One is its inability to handle any situation in which one tautomer is so dominant that minor tautomers are close to invisible in any solvent. This rules out the great majority of simple ketones 

To the small number of existing studies, some of which are described in detail here, it would be pleasant and profitable, therefore, to add some new ones. We can only hope that, at a time when the experimental science of physical organic chemistry seems almost to be in abeyance, it may ultimately be possible to reverse this trend. 

(30) becoming a mixed function of n and a. Later, a number of twin probes were tried, perhaps the most important being those from which the DN ( solvent donicity ) and AN ( acceptor number ) scales of Gutmann [40] were generated. These involve electron donation by solvent to probe, for example, to SbClj, and electron donation by probe, for example, EtjPO to the protonic solvent, respectively. Quite good correlations exist between and DN [5], and between a and AN [1], 

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