Kinetic hydrogen isotope effects for proton transfer between carboxylic acids and the monoprotonated crysptand (2,1,1)H+ in methanol

Abstract

The rates of proton transfer from substituted acetic and benzoic acids, HA, to the monoprotonated cryptand (2,1,1)H⁺ have been measured in methanol. The reaction involves a slow direct proton transfer from HA into the ligand cavity to give the di-endo-protonated cryptand (2,1,1)H₂²⁺. The measured rates are several orders of magnitude lower than those expected for diffusion-controlled proton transfer between oxygen and nitrogen bases. Good Brönsted plots with α values close to 0.5 are observed for both series of acids. The reactions show substantial kinetic hydrogen isotope effects which pass through a maximum with increassing strength of HA.