Activation parameters and kinetic isotope effect in the system tropaeolin O‐OH −

The kinetics of the deprotonation of tropaeolin O by OH − ions was investigated between 9° and 30°C, and by OD − ions at 24.7°C. The pH range was 10.7–12.5, and the ionic strength 0.1 M throughout. All results were obtained by the temperature jump method. On the basis of a mechanism suggested earlier, rate constants k 31 for the reaction between OL − and the internally bonded weak acid and k 32 for the opening of the internal hydrogen bond were evaluated. The activation energies in ordinary water were found to be Δ H ≠ 31 = 3.6 kcal/mol, Δ S ≠ 31 = –19 eu, and Δ H ≠ 32 = 27 kcal/mol, Δ S ≠ 32 = 46 eu. The kinetic isotope effect was k 31   H   2 O / k 31   D   2 O∼ 1.5 and k 32   H   2 O / k 32   D   2 O∼ 0.9. The unusual results for reaction pathare discussed in terms of solvent participation.