Enhanced hydrolysis of a phosphonate ester by mono-aquo metal cation complexes

The hydrolysis of the ester 2,4-dinitrophenyl- ethyl methylphosphonate has been examined by both stop-flow spectrophotometric and pH-stat techniques. These reactions have been carried out in the presence of several nucleophiles including simple non-labile (w.r.t. substitution) mono-aquo metal ion complexes. Comparison of reaction rates of the metal complexes with sterically hindered organic nucleophiles has led to the conclusion that the metal ions function predominantly as general base catalysts in dilute aqueous solution. Reaction rates for the various nucleophiles studied are tabulated together with solvolysis constants for hydroxide ion and water at 35 °C and I=0.1 mol dm⁻³ (KNO₃). These later two values are respectively 32.7 mol⁻¹ dm³ s⁻¹ and 1.37 x 10⁻⁴ s⁻¹. A Brönsted β value of 0.52 for the phosphonate ester studied has also been derived.