Thermodynamic and kinetic acidities in dimethyl sulphoxide. Part 2. Acetylenic compounds

Abstract

The pK values and proton transfer rates in anhydrous dimethyl sulphoxide are reported for three monosubstituted acetylenes (1)–(3) RCCH (R = Ph, MeOCH₂, or Me₂NCH₂). The pK values were measured using a set of overlapping indicators: pK= 22.6, 23.5, and 24.2 for (1)–(3) respectively. Proton transfers were carried out by conjugate acetylide ions, arising from small additions of methylsulphinylmethanide (dimsyl) anion. Rate constants k₁ followed the order of decreasing acidities: k₁ 2.92 × 10⁶, 3.93 × 10⁴, 2.92 × 10³ l mol^–1 s^–1 at 25 °C for (1)–(3) respectively. The contribution from the dimsyl anion, though negligible, was also taken into account.