On the origins of enhanced reactivity of five-membered cyclic phosphate esters. The relative contributions of enthalpic and entropic factors

The hydrolysis of five-membered cyclic phosphate and phosphonate esters is about 10 6 -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 of ethylene phosphate and of ethyl and methyl esters of propylphosphonate have been determined. The enthalpies of activation for the cyclic esters are much less than those for the corresponding acyclic esters while differences in entropies of activation are dependent on the nature of the alkyl substituent, varying from less than 1 cal mol −1 deg −1 (eu) for the methyl esters to about 8 eu for the ethyl esters