Abstract: The hydrolysis of five-membered cyclic phosphate and phosphonate esters is about 106-fold more rapid than the hydrolysis of related acyclic esters. The origin of the enhanced reactivity of the cyclic esters has been ascribed to enthalpic factors, associated with ground-state strain, and to entropic factors, associated with optimal orbital orientations. The temperature dependence of the rates of alkaline hydrolysis of ethyl and methyl esters ...