Evidence for a Solvent-Induced Change in the Rate-Determining Step in the Water-Catalyzed Hydrolysis of the Dimethylimmonium Ion of Benzophenone

Article abstract of DOI:10.1021/jo00265a011

The first order rate constants (k₁) and the kinetic solvent isotope effects (KSIE) for the hydrolysis of the dimethylimmonium ion of benzophenone (2) have been determined in mixed water-acetonitrile solvents varying between pure water and 90percent acetonitrile-10percent water by volume.Between 0 and 50percent acetonitrile the k₁ increases from (1409 +/- 6) x 1E-6 s^-1 to (3487 +/- 42) x 1E-6 s^-1 and the KSIE decreases from 2.19 to 1.97.Between 50 and 90percent added acetonotrile the k₁ decreases from (3487 +/- 42) x 1E-6 s^-1 to (1662 +/- 8) x 1E-6 s^-1 and KSIE increases from 1.97 to 2.32.The ΔG(activ) remains almost constant while ΔH(activ) and ΔS(activ) decrease regularly with the increase of acetonitrile in the solvent mixture.The proton inventory studies generate downwardly bowed k_n vs n plots.However, the degree of bowing decreases with the increase of acetonitrile in the solvent mixture.The results are interpreted by invoking a mechanism that involves a rate-limiting conversion of the polarized substrate (A') into a protonated carbinolamine (B') via a cyclic transition state (4) at lower volume percent, 0-50percent, of acetonitrile and two rate-limiting steps, the conversion of A' into a carbinolamine (B) via an acyclic transition state and the conversion of B into zwitterion (C) via a cyclic transition state at higher volume percent, 50-90percent, of acetonitrile.These latter processes occur at comparable rates.