Relative thermodynamic stabilities of the isomeric dihydrofurans and isomeric dihydropyrans. An experimental and DFT study

Abstract

The relative thermodynamic stabilities of 2,5-dihydrofuran (1) and 2,3-dihydrofuran (2), and of 3,4-dihydro-6H-pyran (3) and 3,4-dihydro-2H-pyran (4), were determined at several temperatures by base-catalyzed equilibration in DMSO solution. For 1 → 2, \(\Delta G_m^ \circ\)= –15.4±0.1 kJ mol−1, \(\Delta H_m^ \circ\)=–12.6±0.5 kJ mol−1, and \(\Delta S_m^ \circ\)=9.5±1.3 J K−1 mol−1 at 298.15 K. The second-law reaction enthalpy agrees with literature data based on calorimetric enthalpies of hydrogenation of the isomeric forms in hexane. For 3 → 4, \(\Delta G_m^ \circ\)=–19.3±0.2 kJ mol−1, \(\Delta H_m^ \circ\)= –18.9±1.1 kJ mol−1 and \(\Delta S_m^ \circ\)=1.1±3.0 J K−1 mol−1 at 298.15 K: the experimental reaction enthalpy is in marked disagreement with literature data based on estimation. On the other hand, both of the experimental reaction enthalpies of the present study are in good agreement with DFT calculations using the B3LYP functional and 6-311+G(2d,p) basis set.