Question #45096

The textbook rule states that keto-forms are generally more stable than enols. A classic exception is the tautomerization of cyclohexa-2,4-dien-1-one to phenol, where the massive gain in Resonance Energy (RE) drives the equilibrium entirely toward the enol.

Consider the keto-enol equilibria ($K_{eq} = \frac{[\text{Enol}]}{[\text{Keto}]}$) for the following three systems in a non-polar solvent at 298 K: (I) cyclohexa-2,4-dien-1-one $\rightleftharpoons$ phenol (II) naphthalen-1(4H)-one $\rightleftharpoons$ 1-naphthol (III) 9-anthrone $\rightleftharpoons$ 9-anthrol

Given the approximate empirical resonance energies: Benzene ring $\approx 150 \text{ kJ mol}^{-1}$ Naphthalene system $\approx 255 \text{ kJ mol}^{-1}$ Anthracene system $\approx 360 \text{ kJ mol}^{-1}$

Assume the inherent bond enthalpy difference ($\Delta H_{\text{bonds}}$) favoring the keto form over the non-conjugated enol form is $+65 \text{ kJ mol}^{-1}$ for all systems, and entropy changes are negligible ($\Delta G^\circ \approx \Delta H^\circ$).

Determine the calculated standard enthalpy of tautomerization ($\Delta H^\circ_{\text{taut}}$) for System III, and identify the correct order of equilibrium constants ($K_I, K_{II}, K_{III}$).