Solvent Effects on other Tautomeric Equilibria

Solvent effects similar to those described for the keto/enol equilibria can also be found for other tautomerisms, e.g. lactim/lactam, azo/hydrazone, ring/chain equilibria, etc. [62-64], The pecularities arising here can only be illustrated by means of a few representative examples. 

One of the classic studies of lactimilactam tautomerism is the determination of the 2-hydroxypyridine (Ha) 2-pyridone (Hb) equilibrium [63-65, 141-145, 251-255], 

IR and UV/Vis [65a], mass spectrometric [65b], photoelectron [65c], microwave [65d], as well as low-temperature matrix-isolation IR spectroscopic measurements [65e] reveal that 2- and 4-hydroxypyridine (as well as 2- and 4-mercaptopyridine [65f]) exist in the gas phase and in inert matrices (N2, Ar) under equilibrium conditions mainly in the lactim (hydroxy or mercapto) form, in contrast to the situation in solution. While in nonpolar solvents such as cyclohexane and chloroform both tautomers exist in comparable amounts, the tautomeric equilibrium is shifted entirely in favour of the lactam (0x0 or thioxo) form in polar solvents such as water, as well as in the crystalUne state [66, 67, 141-145, 251-255], Supercritical-fluid 1,1-difluoroethane can be used to adjust the tautomeric constant Ki = [(llb)]l[(lla)] iso thermally over a continuum from gas-phase values to those measured in polar solvents, simply by increasing the pressure [254]. The gas-phase and solution equilibrium constants of 2- and 4-hydroxypyridine are given in Table 4-4. 

The gas-phase equilibrium constants differ from those in aqueous solution by as much as 10 The large differences between the stabilities of the tautomeric forms in the gas phase and in solution once more reveal the dominant influence of solvation on relative molecular stabilities. 

The precise determination of the protomeric equihbrium constants Ki for 2(4)-hydroxypyridine 2(4)-pyridone is rather difficult because of self-association even in highly diluted solutions of the tautomers in nonpolar solvents such as cyclohexane. Self-associated tautomers may have Ki values which are substantially different from those of the unassociated isomers [142, 255], 

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