As I was looking for the answer of the question I found that it is explained using the Electronegativity difference concept.

That's correct but I am unable to prove the same through the bond energy data.

Can someone prove the same through it.


1 Answer 1


Counter example: Realistically, alkyl oximes are more stable than their nitroso counterparts. For example, computational studies has been shown that formaldehyde oxime is $\pu{15.8 kcal/mol}$ more stable than nitrosomethane when the aqueous solvation correction of $\pu{3.8 kcal/mol}$ is applied to the G2 energies. Unsolvated formaldehyde oxime is estimated to be $\pu{11.1 kcal/mol}$ more stable than nitrone (Ref.1). In solution chemistry, one of the difficulties association with synthesis of C-nitrosoalkanes is their likeliness to convert to the more stable corresponding oxime tautomer (Ref.2), apart from their high reactivity (Rings the bell?). You can find some $\Delta H_f(s)$, $\Delta H_f(l)$, and $\Delta H_f(g)$ of C-nitrosoalkanes in Ref.3. It is also noteworthy that $\ce{C-N}$ and $\ce{N-O}$ bond lengths are quite typical of oximes (overall values are $\pu{128.1 pm}$ and $\pu{141.6 pm}$, respectively), yet $\ce{N-O}$ in oximes are somewhat shorter than that of corresponding hydroxylamines. This indicates that some charge delocalization in oximes. However, the NMR evidence of syn-anti isomerization in oximes (e.g., syn-isome being the one with $\ce{H}$ and $\ce{OH}$ in the same side in acetoxime $\ce{CH3-CH=N-OH}$) confirms the predominance of oxime tautomer.

In closely relevant cases of ortho-nitrosonaphthols and acenaphthenequinonemonooxime, computational studies have been shown that the predominance of oxime form (Ref.4 and 5, respectively). For example, energy diagrams of 2-nitroso-1-naphthol and 1-nitroso-2-naphthol are illustrated in following diagrams:




  1. Judy A. Long, Nathan J. Harris, Koop Lammertsma, "Formaldehyde Oxime $\ce{<=>}$ Nitrosomethane Tautomerism," J. Org. Chem. 2001, 66(20), 6762-6767 (https://doi.org/10.1021/jo010671v).
  2. Brian G. Gowenlock, George B. Richter-Addo, "Preparations of C-Nitroso Compounds," Chem. Rev. 2004, 104(7), 3315-3340 (https://doi.org/10.1021/cr030450k).
  3. Suzanne W Slayden, Joel F. Liebman, "Chapter 3: The organic thermochemistry of hydroxylamines, oximes, hydroxamic Acids and their derivatives," In The Chemistry of Hydroxylamines, Oximes and Hydroxamic Acids, Part 1; Zvi Rappoport,Joel F. Liebman, Editors (Patai Series: The Chemistry of Functional Groups; Series Editor: Zvi Rappoport); John Wiley & Sons, Ltd.: Chichester, England, 2009, pp.53-83.
  4. Galya Ivanova, Venelin Enchev, "Does tautomeric equilibrium exist in ortho-nitrosonaphthols?," Chemical Physics 2001, 264(3), 235-244 (https://doi.org/10.1016/S0301-0104(01)00245-2).
  5. V. Enchev, G. Ivanova, A. Ugrinov, G. D. Neykov, "Tautomeric and conformational equilibrium of acenaphthenequinonemonooxime," Journal of Molecular Structure 1999, 508(1-3), 149-161 (https://doi.org/10.1016/S0022-2860(99)00008-3).

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