THERMOCHEMISTRY OF AMINES - STRAIN IN 6-MEMBERED RINGS FROM EXPERIMENTAL STANDARD MOLAR ENTHALPIES OF FORMATION OF MORPHOLINES AND PIPERAZINES

The standard (p degrees = 0.1MPa) molar enthalpies of formation Delta( f)H(m)degrees(I) at the temperature T= 298.15K 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 Delta(l)(g)H(m)degrees of these comp ounds and, in addition, the enthalpy of sublimation of piperazine (E) Delta(cr)(g)H(m)degrees = (72.1 +/- 1.3)kJ.mol(-1) were obtained from the temperature dependence of the vapour pressure measured in a flow s ystem. Strain enthalpies (ring-correction terms) of cyclic amines were derived from the measured enthalpies of formation of the gaseous comp ounds. These experimental results, together with data available from t he literature, provided a quantitative understanding of the inter-rela tions of structure and energetics of amines and a further improvement of the group-contribution methodology for estimation of the thermodyna mic properties of organic compounds. [GRAPHICS] (C) 1998 Academic Pres s.