Thermochemistry of amines: strain in six-membered rings from experimental standard molar enthalpies of formation of morpholines and piperazines

Abstract

The standard (p°=0.1 MPa) molar enthalpies of formation ΔfHm°(l) at the temperatureT=298.15 K were measured by means of combustion calorimetry for morpholine (A), N-methyl-morpholine (B), N-ethyl-morpholine (C), and N,N-dimethyl-piperazine (D). The standard molar enthalpies of vaporization ΔglHm° of these compounds and, in addition, the enthalpy of sublimation of piperazine (E) ΔgcrHm°=(72.1±1.3) kJ·mol−1, were obtained from the temperature dependence of the vapour pressure measured in a flow system. Strain enthalpies (ring-correction terms) of cyclic amines were derived from the measured enthalpies of formation of the gaseous compounds. These experimental results, together with data available from the literature, provided a quantitative understanding of the inter-relations of structure and energetics of amines and a further improvement of the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.