The carbenium ⇄ oxonium ion equilibrium. Part I. Reversible reaction of non-paired triphenylmethyl cation with ethers and acetals

Abstract

Equilibria between the non-paired triphenylmethyl cation and the corresponding oxonium ions, involving various linear and cyclic ethers and actals, have been studied in CH₂CI₂ over a wide temperature range. The standard enthalpy (ΔH_e°) and entropy (ΔS_e°) of equilibrium have been determined for twelve nucleophiles and it was found that ΔH_e° is a linear function of the basicity expressed as pK_a or pK_b(measured at 25°) of the corresponding nucleophile. The correlation factor is rather poor (0·7) and reflects the uncertainty of the basicity data. ΔH_e° and ΔS_e° change from –5·8 ± 0·2 kcal mol^–1 and –24 ± 1 cal mol^–1 K^–1 for 2-trichloromethyl-1,3-dioxolan to –17 ± 1 kcal mol^–1 and –60 ± 5 cal mol^–1 K^–1 for tetrahydropyran respectively. From a linear plot of ΔH_e° against ΔS_e°(correlation factor 0·97) the isoequilibrium temperature has been determined as 311 K.