MOLAR ABSORPTIVITY AND PK(R-DIMETHOXYTRITYL CARBENIUM ION IN METHANOLIC WATER, AND ITS EQUILIBRIUM WITH CHLORIDE-ION() OF THE 4,4')

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'-dimeth oxytritylamine. The pK(R+) (-0.95; K(R+) = 8.98) and molar absorptivit y (epsilon(R+) = 297 000) of the cation have been measured directly un der equilibrium conditions in this highly aqueous medium at ionic stre ngth = 1.0 mol dm-3 by a convenient spectrophotometric method at 25-de grees-C. In addition, the pK(R+) has been measured at other temperatur es, which allows the determination of the standard enthalpy (DELTAH(fo rmal) = -11.4 kJ mol-1) and entropy (DELTAS(formal) = -20 J K-1 mol-1) of the reaction of the cation with the solvent, and at other ionic st rengths. The equilibrium constant for the formation of 4,4'-dimethoxyt rityl chloride from the cation and chloride has also been measured in the same medium (K(RCl) = 8.20 at 25.0-degrees-C, ionic strength = 1.0 mol dm-3) by generating the cation from the alcohol and perchloric ac id in the presence of 0.10 mol dm-3 sodium chloride.