Abstract
The millionfold enhancement of the rate of hydrolysis of five-membered cyclic phosphate esters compared to their acyclic analogues is purely enthalpic and arises at least partially from destabilization of the four-coordinate cyclic reactant compared to the acyclic analogue, assuming that both react with hydroxide through unstrained five-coordinate transition states. Calorimetric analysis of the hydrolysis of cyclic and acyclic phosphates and phosphonates is reported in which the absolute value of the difference in enthalpy of hydrolysis of the cyclic and acyclic species is considerably less than the difference in activation enthalpies (about 2 kcal/mol discrepancy)
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