Equilibria nitroso/oxime tautomeric

Nitroso-Oxime Tautomerism. This equilibrium lies far to the right, and as a rule nitroso compounds are stable only when there is not a hydrogen. 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

Primary and secondary nitroso compounds usually stabilize themselves, even under the conditions of their preparation, by passing irreversibly into their isomers, the oximes. In tertiary aliphatic nitroso compounds this transformation is accompanied by fission of the neighboring carbon-carbon bond by hydrolysis or decarboxylation. Aromatic nitroso compounds are more stable, and when oxime formation does occur this is usually reversible within the framework of a true tautomeric equilibrium for instance, the tautomeric pair -nitrosophenol/p-benzoquinone monooxime is obtained both by nitrosation of phenol and by oximation of />-benzoquinone. 

Several other types of tautomerization are shown in Figure 3.7. In each case, the equilibrium lies well to the right. For keto-phenol tautomers, the favored phenol form is aromatic whereas the keto form is not. Enamines that have at least one hydrogen on the nitrogen are generally not stable and revert to the imine form. Aliphatic nitroso compoimds with a hydrogen attached to the adjacent carbon are also usually not stable and exist primarily as oximes. 

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