摘要 : The standard enthalpies of formation ΔH_f°(g) of mono- and gem-di(alkoxycarbonyl)-substituted cyclopropanes 1, cyclobutanes 2 and cyclopentanes 3 have been calculated from the standard enthalpies of combustion ΔH_c°, which were me... 展开 The standard enthalpies of formation ΔH_f°(g) of mono- and gem-di(alkoxycarbonyl)-substituted cyclopropanes 1, cyclobutanes 2 and cyclopentanes 3 have been calculated from the standard enthalpies of combustion ΔH_c°, which were measured calorimetrically, in combination with the standard enthalpies of vaporization ΔH_(vap)°. The latter were obtained for 1a-c, 2b-c and 3b-c from the temperature dependence of the vapor pressures, which were measured in a flow system. Contrary to suggestions in the literature, only weak stabilization (≤ 8 kJ · mol~(-1)) of small rings by gem-alkoxycarbonyl substituents was observed. In this context, we give a revised value for the enthalpy of combustion of methyl cyclobutanecarboxylate. It is concluded that the known high rates of ring closure to gem-dialkoxycarbonyl cyclopropanes are not attributable to a 'stabilizing effect' resulting from conjugation between the alkoxycarbgonyl substituents and the cyclopropane ring, as has been suggested in the literature. The operation of a Thorpe-Ingold- or gem-dimethyl-type effect would seem to offer a more satisfactory interpretation. 收起