Hydrogen isotope exchange between fluoroform (CF3H) and water. 2. Catalysis by hydroxide ion in the presence of added dimethyl sulfoxide

Article abstract of DOI:10.1021/ja00401a035

Hydroxide ion catalyzed isotope exchange between CF₃D or CF₃T and Me₂SO/water has been studied as a function of solvent composition and temperature in a stirred reactor. The solubility of CF₃H gas in the catalyst solutions has been measured over a range of temperatures to enable conversion of the experimental exchange rate constants to in-solution values The resultant k_obsd (CF₃D) for 0.011 mol·L^{-1 -}OH at 49.1°C increases by a factor of 10⁶ on changing from water to 70 mol % Me₂SO. The heat of solution of CF₃H at 50°C passes through a minimum in the region of 30-35 mol % Me₂SO; the "salting-out" effect on CF₃H solubility is also strongest in this region of greatest Me₂SO/H₂O association. Hydrolysis of CF₃H is a factor of ca. 600 slower than isotope exchange at 50°C in 30 mol % Me₂SO. The slopes of linear plots of log k_obsd(CF₃D) vs H_- and vs. Cox and Stewart's excess basicity function are 0.80 and 0.99, respectively; their significance is discussed. The Arrhenius activation parameters (CF₃D) determined as a function of solvent composition have been broken down to enthalpy, entropy, and free energy of transfer functions for the reactants and transition state. Comparisons are made with similar data available for chloroform and molecular hydrogen. The primary kinetic isotope effect k_D/k_T, calculated from rate constant data for CF₃D and CF₃T exchange, increases smoothly from an inverse value in 20 mol % Me₂SO (ca. 0.4) to ca 1.5 in 70 mol % Me₂SO. The above results are consistent with a product-like transition state for CF₃D isotope exchange.