The Stabilization Energy of [18] Annulene. A thermochemical determination

The thermal rearrangement of 18]annulene in solution gives benzene and 1,2-benzo-1,3,7-cyclooctatriene. This reaction has been investigated using a differential scanning calorimeter and the associated enthalpy change has been measured. The enthalpy of the reaction (in the gase phase at 298°K) is From this value and the enthalpies of formation of the reaction products, the enthalpy of formation of 18]annulene is obtained: The stabilization energy of 18]annulene, defined as the difference between the enthalpy of formation of the hypothetical Kékulé 18]annulene (with single and double bonds of normal lengths) and the heat of formation of the real molecule is then a value close to the corresponding quantity found for benzene. The very large stabilization energy of 18]annulen (100 ± 6 kcal mol^?1) previously reported in the literature, based on measurements of the heat of combustion, is not correct since the annulene is certainly oxidized in the bomb prior to combustion. The isodynamic conformational mobility of 18]annulene is not incompatible with a stabilization energy of 37 kcal mol^?1; the activation enthalpy observed for this process (ΔH^≠ = 16.1 kcal mol^?1) indicates that the stabilization is not completely destroyed in the transition state. The mechanism of the thermolysis of 18]annulen, investigated by kinetic measurements and by analysis of the shape of the thermograms, is discussed.