In the present work the tautomerics equilibria in 3-hydroxy-4-methyl-4-pentenonitrile and 4-methyl-3-oxo-4-pentenonitrile have been studied. The first compound presents two possible theoretical tautomers, nitrile and ketenimine. The second compound presents four possible theoretical tautomers ketonitrile, nitrile-enol (E and Z) and keto-ketenimine. The study of the equilibrium in gas phase was performed by gas chromatography-mass spectrometry (GC-MS), and in solution by proton nuclear magnetic resonance spectrometry ((1)H NMR). In gas phase, the ketonitrile tautomer was favoured, a result which was supported by theoretical calculations by the use of AM1 semi-empiric calculation. The experimental tautomerization heat values were in good agreement with the theoretical ones. The (1)H NMR spectra gave the additional evidence for the coexistence of the tautomers ketonitrile and enolnitrile for 4-methyl-3-oxo-4-pentenonitrile. The nitrile-ketenimine equilibrium for both compounds could not be observed by (1)H NMR spectra because of the low sensibility of this method. The ketonitrile-enolnitrile tautomerization heat of 4-methyl-3-oxo-4-pentenonitrile has been calculated and compared with the corresponding one in gas phase to evaluate the solvent effect.
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