4- Hydroxypyrazole, hydroxy tautomer

The problem of tautomerism is simpler in the case of 1-substituted pyrazolin-3-ones since only two forms, the OH (140a) and the NH (140b), are possible. The OH form is the more stable and is the only one present in the crystal (Section 4.04.1.3.1). In protic solvents, like water or methanol, the equilibrium position is much more evenly balanced between the OH and NH forms. Finally, 4-hydroxypyrazoles (141) exist as such. A CNDO/2 calculation justifies the result that 4-hydroxy tautomers are relatively more stable than... 

Tautomerism of 3-hydroxypyrazoles unsubstituted on nitrogen is more complex. A detailed investigation of 3-hydroxy-5-methylpyrazole disclosed that the major tautomers in aqueous solution (polar medium) are 264 and 265, whereas in cyclohexane solution (nonpolar medium) the major tautomers are 264 and 266 <1976AHC(S1)346>. 

Although UV data are available for the 3-hydroxy derivatives of this ring system, no IR details have been published. Accordingly it is unclear which of the possible tautomeric structures (14-16) best represents these compounds. Possibly, as with 5-hydroxypyrazoles, the hydroxy form 14 is less stable than oxo tautomers such as 16. Ultraviolet and infrared data are, however, available for the 6-hydroxy derivatives (17). In the solid state there is surprisingly no evidence for a carbonyl group in the infrare(. The presence of a broad band from 2300 to 3100 cm (v C=NH) is taken as indicating the importance of the dipolar structure 18 at the expense of the neutral forms 17 and 19. 

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