The vaporization enthalpies of some crown and polyethers by correlation gas chromatography

Podands and crown ethers are an important class of solvents and metal compl exing agents. The vaporization enthalpies of a series of ethers that includ e 18-crown-6 (87.3 +/- 2.3), dibenzo-18-crown-6 (137 +/- 7.4), dicyclohexan o-18-crown-6 (mixture of diastereomers 124.2 +/- 4.0), 1,2-bis-(2-methoxyet hoxy)ethane (63.7 +/- 3.3), 2,5-dimethoxy-2,5-dihydrofuran (cis/trans mixtu re: 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 techniq ue utilizes gas chromatographic retention times measured as a function of t emperature and the known vaporization enthalpies of a series of standards. Vaporization enthalpies an obtained for both liquids and solids. Sublimatio n 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 va porization enthalpies and adjusting for temperature. Uncertainties represen t two standard deviations. The results are compared to estimated values. De spite the fact that crown ethers are excellent complexing agents their ther modynamic properties associated with phase changes appear quite similar to their acyclic counterparts and similar to analogous hydrocarbons. (C) 2000 Elsevier Science B.V. All rights reserved.