Considering the following example while comparing the enolic content in the product, am I supposed to compare its stability with reference to its initial state or am I supposed to compare it between the final products obtained?

Supposed question


Given are keto form of keto-enol tautomerism of reference compounds. Answers are given according to the percentages of enol forms. Your duty is to find which enol form is dominant.

In cyclohexa-2,5-dienone, the enol form (phenol) dominate 100% in solid or in solutions, regardless of the solvent because of the aromaticity (100% B).

In the case of pyridones, however, the dominance form of the compound depends on which phase it is in. For instance, Ref.1 states that:

In the gas phase, there is a preference of the hydroxy form in both 2- and 4-hydroxypyridine, while in a nonpolar solvents such as cyclohexane and chloroform, the two tautomers exist in comparable amounts. However, the two equilibria are shifted entirely to the right in favor of the oxo-form, 2- and 4-pyridone, respectively in polar solvents as well as in the crystalline state.

In general, pyridones' (e.g., 4-pyridone) predominance strongly over their tautomeric hydroxy forms (e.g., 4-hydroxypyridine) can be rationalized by considering the mesomerism of the alternative forms. The charge-separated form of 4-pyridone (far left; negative charge on more negative oxygen), being aromatic, is greatly preferred over the nonaromatic, charge-separated form of 4-hydroxypyridine (far right; negative charge on relatively more positive nitrogen).


Thus, it is safe to say %B > %A and hence you have the correct answer.

Note: There are compilations about this type of keto-enol tautomerism. The freely available Tautobase for example is provided as searchable database in DataWarrior. Also read answers in this question.


  1. Jiali Gao, Lei Shao, "Polarization Effects on the Tautomeric Equilibria of 2- and 4-Hydroxypyridine in Aqueous and Organic Solution," J. Phys. Chem. 1994, 98(51), 13772–13779 (https://doi.org/10.1021/j100102a051).
  2. Oya Wahl, Thomas Sander, "Tautobase: An Open Tautomer Database", Chem. Inf. Model. 2020, 60, 1085–1089 (doi 10.1021/acs.jcim.0c00035).

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