Formation and stability of organic zwitterions - The carbon acid pK _as of the trimethylsulfonium and tetramethylphosphonium cations in water

We report second-order rate constants of k_DO = 7.5 × 10^-4 and 9.9 × 10^-5 (mol/L)^-1 s ^-1 for exchange for deuterium of the first methyl proton of the trimethylsulfonium and tetramethylphosphonium cations, respectively, in D ₂O at 25°C and 1 = 1.0 (KCl). The data were analyzed to give the following carbon acidities for these cationic carbon acids in water: (CH ₃)₃S⁺, pK_a = 28.5; (CH ₃)₄P⁺, pK_a = 29.4. These acidities are close to those of the neutral carbon acids acetonitrile and dimethylacetamide. This provides evidence that a portion of the stabilization of the cyanomethyl carbanion is due to resonance delocalization of negative charge from carbon to cyano nitrogen.