We know, generally, that the keto tautomer is more stable than the corresponding enol tautomer. I would appreciate an explanation that goes beyond the thermodynamic reasoning, too.

In the nitroso-oxime tautomerism, the nitroso form is analogous to keto isomer and the oxime is analogous to the enol form. Then why is the oxime more stable than the nitroso isomer?

  • $\begingroup$ I think the more suitable reason is that nitro compounds are stable when there is no alpha hydrogen. $\endgroup$ – aqsa munawar Nov 11 '19 at 8:35

In keto-enol tautomerism generally the keto tautomer is more stable than its enol counterpart because in the former a significant ionic character exists which stabilises it. However, there are certain special cases. For instance if the enol form is aromatic(phenol), or very bulky mesityl groups are attached, etc.

In oxime-nitroso tautomerism however, the electronegativity difference comes into play. enter image description here

In oxime, nitrogen is attached to carbon by a double bond;enter image description here

and in nitroso, nitrogen is attached to oxygen by a double bond.

Now it is known that greater the difference in the electronegativity values, stronger is the bond. This property makes the nitrogen carbon double bond stronger hence making oximes more stable than nitroso.

(electronegativity values: N=3.04, O=3.44, C=2.55)

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  • $\begingroup$ sorry but I can not at all agree with the above answer! The greater the difference in electronegativity between 2 atoms, the weaker is the bond and not the reverse, when we compare atoms of the same period (C, N, O, F). For example H-F is the weakest (most acidic) linkage i.e., F-H <O-H <N-H <C-H. The C-H bond is the strongest and therefore the most difficult to break. $\endgroup$ – lemos Nov 29 '16 at 14:38
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    $\begingroup$ @lemos The farther away the electronegativity values of 2 atoms, the stronger the bond generally is. Cesium has the lowest, and Fluorine has the highest and the make the strongest ionic bond. Be nice to people. $\endgroup$ – Piyush Mishra Dec 18 '16 at 12:00
  • $\begingroup$ For your acidic strength thingy: chemistry.stackexchange.com/questions/39711/… $\endgroup$ – Piyush Mishra Dec 18 '16 at 12:05
  • $\begingroup$ "it is known that greater the difference in the electronegativity values..." -- a reference would be good here, even a titorial type reference that has primary sources documented. $\endgroup$ – Oscar Lanzi Nov 11 '19 at 11:01
  • $\begingroup$ @lemos The H-F bond is the strongest of the hydrogen-halogen bonds. Look in any bond enthalpy table. $\endgroup$ – FusRoDah Nov 11 '19 at 11:43

I wonder if the $\alpha$ effect is in play here. Two hetero atoms with lone pairs bonded to each other have a weaker bond because of lone pair repulsion. This probably destabilizes the corresponding $\pi$ bond as well. That would favor the oxime.

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