ISSN:
1434-193X
Schlagwort(e):
Enthalpy of formation
;
Enthalpy of vaporization
;
Enthalpy of combustion
;
Cycloalkanes
;
Chemistry
;
General Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
The standard enthalpies of formation ΔHf°(g) of mono- and gem-di(alkoxycarbonyl)-substituted cyclopropanes 1, cyclobutanes 2 and cyclopentanes 3 have been calculated from the standard enthalpies of combustion ΔHc°, which were measured calorimetrically, in combination with the standard enthalpies of vaporization ΔHvap°. 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 alkoxycarbonyl 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.
Materialart:
Digitale Medien
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