Benzoquinone Oxime and 4-Nitrosophenol

Conceptually, the simplest pair to discuss is that of p-benzoquinone oxime and 4-nitrosophenol and their substituted derivatives, here all denoted by p-OCgH4=NOH/4-HOC6H4NO . In practice, this comparison is complicated by the fact that this species cannot be sublimed without decomposition and so we are prevented from analyzing the energetics of gas phase isodesmic reactions such as equations 38 and 39. 

The enthalpy of reaction 38 is ca 13 kJ mol for the case of cyclohexanone wherein R R = —(CH2)5—. This difference is compatible with intermolecular hydrogen bonding as would be found for the benzoquinone oxime tautomer. In order to understand the energetics of reaction 39, we derived the solid phase enthalpy of formation of nitrosobenzene 

This resulted in an enthalpy of reaction 39 of ca 70 klmol . While this is much larger than what would be predicted for the correction due to nitroso-phenol conjugation and the energies of most hydrogen bonds, the value for the related reaction for p-nitrophenol (equation 40) is the rather comparable 80 klmol .  

As such, thermochemical analogies don t help us decide which tautomer is more stable. From the literature we conclude that the quinone oxime tautomer is somewhat the more stable, the precise enthalpy and free energy difference depending on the solvent, substituents and reaction conditions (see the recent paper. Reference 60, and references cited therein). 

We note that the difference between the enthalpy of formation of the unsubstituted benzoquinone/nitrosophenol and that of its isopropyl methyl cymene-like disubstituted derivative is 146 kJmol, comparable to that of the corresponding hydrocarbons, benzene and its substituted counterpart as liquids, 127.0 1.2 kJmol, and as the corresponding phenols as solids, 144.4 9.6 kJmol .  

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