Kinetics of Proton Transfer Reactions of Carbon Acids. V. Variation of Primary Deuterium Isotope Effect with Solvent Composition for the Reaction of Di-(4-nitrophenyl)methane with t-Butoxide in t- Butyl Alcohol–Toluene Mixtures
The second-order rate constants, activation parameters, and primary deuterium isotope effects are reported for the proton transfer reaction from di-(4-nitrophenyl)methane to t-butoxide ion in a series of solvents containing varying amounts of toluene in t-butyl alcohol. Increasing toluene content in the solvent decreases the rate constant and increases the enthalpy of activation, while the entropy of activation becomes less negative. The isotope rate ratio kH/kD increases from 7.3 in 10% v/v toluene to 9.4 in 50% v/v toluene at 25 °C, corresponding to a change in (ΔHD≠ − ΔHH≠) from 0.46 to 1.0 kcal mol−1 and a change in (ΔSD≠ − ΔSH≠) from −2.4 to −1.1 cal mol−1 deg−1. It is suggested that the negative values for (ΔSD≠ − ΔSH≠) are due to a time lag for solvent reorganization relative to the proton transfer.