Abstract :
Podands and crown ethers are an important class of solvents and metal complexing agents. The vaporization enthalpies of a series of ethers that include 18-crown-6 (87.3±2.3), dibenzo-18-crown-6 (137±7.4), dicyclohexano-18-crown-6 (mixture of diastereomers 124.2±4.0), 1,2-bis-(2-methoxyethoxy)ethane (63.7±3.3), 2,5-dimethoxy-2,5-dihydrofuran (cis/trans mixture: 44.2±0.3) and 1,3,5-trimethoxybenzene (68.2±2.0) kJ mol−1 are evaluated by the technique of correlation gas chromatography. This technique utilizes gas chromatographic retention times measured as a function of temperature and the known vaporization enthalpies of a series of standards. Vaporization enthalpies are obtained for both liquids and solids. Sublimation enthalpies for benzo-12-crown-4 (104.3±2.6), 18-crown-6 (119.1±6.7), benzo-15-crown-5 (128.1±10.8), and dibenzo-18-crown-6 (178.8±6.9) kJ mol−1 are calculated by adding experimental fusion enthalpies to vaporization enthalpies and adjusting for temperature. Uncertainties represent two standard deviations. The results are compared to estimated values. Despite the fact that crown ethers are excellent complexing agents their thermodynamic properties associated with phase changes appear quite similar to their acyclic counterparts and similar to analogous hydrocarbons.
