Proximate Charge Effects. 3.¹ Enthalpies of Solvent Transfer of Reactants and Transition States in the Saponification of Acetylcholine

The rates and activation enthalpies of the alkaline hydrolysis of acetylcholine were measured in water, in 60 mol % ethanol-water, and in 85 mol % ethanol-water. Unlike the alkaline hydrolysis of ethyl acetate, which exhibited a large increase in δH* on going from water solvent to 60 mol % ethanol-water solvent, no such solvent effect was observed in the case of acetylcholine. Calorimetric determination of the relative enthalpies of the reactants in water and in aqueous ethanol revealed that whereas there was a large desolvation of the ethyl acetate saponification transition state in aqueous ethanol relative to water, no such desolvation of the acetylcholine saponification transition state took place.