Molar absorptivity and pKR+ of the 4,4′-dimethoxytrityl carbenium ion in methanolic water, and its equilibrium with chloride ion

Abstract

The 4,4′-dimethoxytrityl carbenium ion has been generated in 80 : 20 (v/v) water–methanol containing perchloric acid in a rapid reaction from 4,4′-dimethoxytrityl alcohol and in a slower reaction from 4,4′-dimethoxytritylamine. The pK_R⁺(–0.95; K_R⁺= 8.98) and molar absorptivity (ε_R⁺= 297 000) of the cation have been measured directly under equilibrium conditions in this highly aqueous medium at ionic strength = 1.0 mol dm^–3 by a convenient spectrophotometric method at 25 °C. In addition, the pK_R⁺ has been measured at other temperatures, which allows the determination of the standard enthalpy (ΔH^⊖=–11.4 kJ mol^–1) and entropy (ΔS^⊖=–20 J K^–1 mol^–1) of the reaction of the cation with the solvent, and at other ionic strengths. The equilibrium constant for the formation of 4,4′-dimethoxytrityl chloride from the cation and chloride has also been measured in the same medium (K_RCI= 8.20 at 25.0 °C, ionic strength = 1.0 mol dm^–3) by generating the cation from the alcohol and perchloric acid in the presence of 0.10 mol dm^–1 sodium chloride.