| Abstract: | To determine the factors governing the enhanced reactivity of 5-membered ring phosphates, the lowest activation free-energy profiles for the alkaline hydrolyses of methyl ethylene phosphate (5-MEP), its acyclic analog, trimethyl phosphate (a-TMP), and its 6-membered ring analog, methyl propylene phosphate (6-MPP), have been computed using ab initio and continuum dielectric methods. The calculations yield product distributions and activation free energy differences in accord with experiment. They show that solvent stabilization of the 5-membered ring transition state plays a key role in the million-fold enhanced rates of alkaline hydrolysis of 5-MEP relative to its acyclic or 6-membered ring analogs. Furthermore, strain energy calculations show that ring strain contributes partly to the observed rate enhancement of 5-MEP relative to 6-MPP but not to that of 5-MEP relative to a-TMP. |
