Heats of formation of enols. The double-bond stabilizing effect of the hydroxyl group

Aqueous solution free energies of ketonization have been combined with enthalpies of reduction of the keto forms and other thermochemical data to yield estimates of the heats of formation of the corresponding enols. A double‐bond stabilization term, DOH = 5.4 ± 0.5 kcal mol−1 (1 kcal = 4.184 kJ), is obtained for the OH group. This quantity is close to Hine's double‐bond stabilization parameter for the methoxyl group (4.9 ± 0.2 kcal mol−1). Comparison with available experimental gas‐phase enthalpies of formation suggest that, relative to the keto form, enols are more stable in the gas phase than in water. The differences are unlikely to be due entirely to entropic effects. A wholly enthalpic, gas‐phase double‐bond stabilization term, DOH = 8.1 ± 0.6 kcal mol−1, may be calculated from the available experimental data. This result indicates that the hydroxyl group more effectively stabilizes the CC bond in the gas phase than in aqueous solution. An important limitation is that the results reported bere apply only to simple enols, that is, monofunctional enols not stabilized by extra resonance or hydrogen‐bonding effects.