Enthalpies of hydrogenation and formation of enones. Resonance energies of 2-cyclopentenone and 2-cyclohexenone

The enthalpies of hydrogenation of 2-cyclopentenone, 2-cyclohexenone, 2-methyl-3-isobutoxycyclohex-2-enone, and 5-methylhex-5-en-2-one have been measured and found to be -(97.1 ± 2.7) kJ·mol^-1, -(109.2 ± 2.9) kJ·mol^-1, -(106.1 ± 3.0) kJ·mol^-1, and -(113.2 ± 2.7) kJ·mol^-1, respectively. In combination with the enthalpies of formation of the corresponding alkanones, we obtained the enthalpies of formation Δ_fH^o _m (cyclopentenone) = -(100.3 ± 3.0) kJ·mol^-1 and Δ_fH^o _m (cyclohexenone) = -(121.9 ± 3.0) kJ·mol^-1. By using isodesmic reactions, the resonance energy of cyclic enones is estimated to be (13 ± 4) kJ·mol^-1.