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?
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.
- 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).
- Oya Wahl, Thomas Sander, "Tautobase: An Open Tautomer Database", Chem. Inf. Model. 2020, 60, 1085–1089 (doi 10.1021/acs.jcim.0c00035).