Reaction of phosphonated acetals. Part 3. Acid-catalysed hydrolysis of diethyl 3,3-diethoxypropylphosphonate

Abstract

The kinetics of the acid-catalysed hydrolysis of diethyl 3,3-diethoxypropylphosphonate (I) have been studied in aqueous dioxan solutions, at different acid concentrations and temperatures. The reactions follow an A1 type mechanism with the inductive effect of the phosphonated substituent having only a minor influence. The measured second-order rate constant at 30° is 0.032 l mol^–1 s^–1. Plots of log k_obs. against log C_H3O⁺ are not linear. The deuterium solvent isotope effect k_D2O/k_H2O is 2.32. The energy of activation is 92 kJ mol^–1 and the entropy of activation of 21 J mol^–1 K^–1. General acid catalysis was not observed. An extremely rapid decrease in the rates of acid-catalysed hydrolysis was observed in going from compound (I) to diethyl 2,2-diethoxyethylphosphonate, (II), and to diethyl diethoxymethylphosphonate, (V). Compound (V) does not undergo regular acetal acid-catalysed hydrolysis.