Proximate Charge Effects. 5. Enthalpies of Solvent Transfer of Reactants and Transition States in the Saponification of Betaine Ethyl Ester

Article abstract of DOI:10.1021/jo00172a022

The rates and activation enthalpies of the alkaline hydrolysis of betaine ethyl ester <(carbethoxymethyl)trimethylammonium chloride> were measured in water, in 60 mol percent ethanol-water, and in 60 mol percent dimethyl sulfoxide-water.Unlike the alkaline hydrolysis of ethyl acetate which exhibits a large increase in ΔH<*> on going from water as the solvent to 60 mol percent ethanol-water, no such solvent effect was observed in the present case.Calorimetric determination of the enthalpies of solvent transfer revealed that, whereas there was a large desolvation of the ethyl acetate saponification transition state on transfer into aqueous ethanol, no such desolvation of the betaine ethyl ester transition state took place, an observation which had been made earlier about the saponification of acetylcholine.In 60 mol percent dimethyl sulfoxide-water, a polar, poor hydrogen bond donor solvent, the ΔH<*> for betaine ethyl ester and ethyl acetate saponification were nearly the same as in water as the solvent.Calorimetric determination of the enthalpies of solvent transfer of the saponification transition states of betaine ethyl ester and ethyl acetate from aqueous dimethyl sulfoxide to hydrogen bond donor solvents revealed that the increase in solvation on going to the better solvent was about the same for both esters.