Pyrophosphate formation for acetyl phosphate and orthophosphate anions in concentrated aqueous salt solutions does not provide evidence for a metaphosphate intermediate

Abstract

The formation of pyrophosphate (PPi) from the anions of acetyl phosphate (AcP) and orthophosphate (Pi) in concentrated aqueous sodium perchlorate is approximately first order in each reactant. The pH dependence of the reaction in the presence of 0.25 M Pi monoanion and 6.4 M sodium perchlorate at 54 OC shows different yields of PPi from AcP monoanion and dianion. This result is inconsistent with trapping of a free metaphosphate intermediate by Pi monoanion and water. The results are consistent with a concerted mechanism for PPI formation and yields of PPi from the different ionic species of 11.7% for AcP2- and Pi2-, 7.8% for AcPZ- and Pi-, and 4.2% for AcP- and Pi-. Measurement of rate constants and partitioning with AcP2-, 0.25 M Pi2-, and 6.4 M sodium perchlorate, in the presence and absence of pyridine, show that the phosphorylated pyridine monoanion yields 40% PPi, whereas AcP2- yields 10% PPi under the same conditions. The formation of PPi from AcP and Pi dianions is at least fourth order in sodium ion in concentrated sodium perchlorate solutions. The corresponding reaction of the monoanions is second order in sodium ion. The order of effectiveness of salts for facilitation of the formation of PPi from the dianions of AcP and Pi is NaCIO., > NaC1, KCI, CsC1, and NaCl > CsCI. However, increasing the concentration of sodium perchlorate from 0.1 to 6.6 M changes the rate constant for hydrolysis of AcP mono- and dianion by <30% at 39 and 54 OC. Values of AH* = 29 kcal mol-' and AS* = +I2 eu for PPi formation from the dianions of AcP and Pi and AH* = 19 kcal mol-' and AS* = -20 eu for PPi formation from the monoanions were obtained from the yields of PPi and the rates of hydrolysis at 39 and 54 OC. The high concentrations of sodium ion decrease electrostatic repulsion between the anions of AcP and Pi and may catalyze the formation of PPi by bridging the anions of AcP and Pi. These results provide no evidence for the existence of a metaphosphate intermediate in reactions in aqueous solution.