RELATIVE THERMODYNAMIC STABILITIES OF ISOMERIC ALKYL ALLYL AND ALKYL (Z)-PROPENYL ETHERS

The relative thermodynamic stabilities of ten allyl ethers (ROCH(2)CH=CH2) and the corresponding isomeric Q-propenyl ethers (where R is an alkyl group, or a methoxysubstituted alkyl group) have been determined by chemical equilibration in DMSO solution with t-BuOK as catalyst. From the variation of the equilibrium constant with temperature, the values of the thermodynamic parameters Delta G(theta), Delta H-theta and Delta S-theta of isomerization at 298.15 K were evaluated. The propenyl ethers are highly favored at equilibrium, the values of both Delta G(theta) and Delta H-theta for the allyl --> propenyl reaction being ca. -18 to -25 kJ mol(-1). The favor of the propenyl ethers is increased by bulky alkyl substituents, and decreased by methoxysubstituted alkyl groups. In most cases the entropy contribution is negligible; however, for R =(MeO)(2)CH and R =(MeO)(3)C the values of Delta S-theta are ca. -5 J K-1 mol(-1).