Thermodynamic and kinetic acidities in dimethyl sulphoxide. Part 3. Alcohols and phenols

Abstract

Proton transfer from and to the oxygen atom of a series of four alcohols (ROH) in anhydrous dimethyl sulphoxide was followed by dynamic n.m.r. spectroscopy under conditions of both acid (trifluoromethanesulphonic) and base (n-butylamine) catalysed exchange. A series of four phenols was also investigated in acidic conditions. Two kinetic processes were observed, one catalysed by the hydrogen ion DMSO ⋯ H⁺ and the other by the conjugate alkoxide base. The results at 25 °C for alcohols are summarized by the equation, rate of hydroxylic proton exchange =k_A[DMSO ⋯ H⁺]+k₂[RO^–], with k_A 3.13 × 10⁶ and k₂ 1.36 × 10¹⁰ l mol^–1 s^–1 at 25 °C for EtOH. These rate constants, and the high pK values (21.95 for EtOH) account for the slow exchange on the n.m.r. time scale over a wide range of pH (5.5–11.5), and thus for the successful use of DMSO as a solvent to observe O–H fission. This property arises not from the low rate constants in DMSO with respect to those in water or alcohols, but from the relatively smaller amounts of alkyloxonium and alkoxide ions. The rate constants k_A and k₂ follow the order of decreasing acidities, and obey Hammett or Ingold–Taft relationships, ρ*=–0.227 and –0.289 for alcohols: ρ–0.852 for phenols. The k_A values of phenols are consistent only with the concerted mechanism (i) which is also DMSO ⋯*H⁺+ [graphic omitted]–H ⋯ DMSO → DMSO ⋯*H–[graphic omitted] + H⁺⋯ DMSO (i) suggested for alcohols.