Sumnzary. The therinal rearrangement of [18]annulene in solution gives benzene and 1,2-bcnzo-1,3,7-cyclooctatriene. This reaction has bcen investigated using a differential scanning calorimeter and the associated cnthalpy change has been nicasured. The enthalpy of the reaction (in the gas phase a t 298 "I<) is AH&, 298")From this value and the cnthalpies of lorination of the reaction products, the enthalpy of formation of [18]annulene is obtained :The stabilization encrgy of [18]annulenc, defined as the difference between the enthalpy ol formation of the liypothctical Kekuli The very large stabilization energy of [18]annulene (100 4: 6 kcal i~i o l -~) previously reported in the literature, based on measurements of the hcat of combustion, is not correct since the aniiulene 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-l; the activation enthalpy observed for this process ( A H " = 16.1 kcal mol-1) indicates that the stabilization is not completely destroyed in the transition state. The mechanism of thc thcrmolysis of [18]annulene, invcstigated by kinetic mcasurements and by analysis of the shape of the thermograms, is discussed.
Die Thermolyse des Annulens (II) in deuteriertem Tetrahydrofuran ergibt das Produkt (I) und Benzol, die aus einer intermediären, mit dem Annulen isomeren, tetracyclischen Verbindung hervorgehen, deren Struktur nicht eindeutig geklärt werden konnte.
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