Thermodynamic study on aqueous dilute solutions of organic compounds. Part 2.—Cyclic ethers

Free energy, enthalpy and entropy variations associated with solution and hydration processes have been determined for the following compounds: tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydropyran, 1,3-dioxolan and 1,4-dioxan. Negative ΔSh and ΔHh values result from the transfer of the above-mentioned ethers from the gaseous state to dilute aqueous solutions. Their absolute values, however, are markedly smaller for the diethers than for monoethers, this being particularly so for ΔSh values. Comparisons are made between the thermodynamic behaviour in aqueous solutions of cyclic ethers and amines and the analogue, open-chain derivatives. On the basis of views currently held on water structure, hypotheses are suggested to explain effects associated with (i) changing from open to cyclic systems, (ii) lengthening of the chain and methyl substitution, and (iii) the presence, in cyclic systems, of several hydrophilic centres.