HYDROLYSIS OF DIETHYL BENZOYLPHOSPHONATE IN AQUEOUS AND HYDROCHLORIC-ACID SOLUTIONS - POSTULATION OF A STABLE PENTAVALENT PHOSPHORUS-CONTAINING INTERMEDIATE

The hydrolysis of diethyl benzoylphosphonate (1) in aqueous acid solutions was found to be pseudo first order in the substrate, by monitoring the π → π band. Products of hydrolysis were stoichiometric amounts of benzoic acid and diethyl hvdrogenphosphonate. Correction was made for the absorption of benzoic acid. The reaction rate was retarded as the pH of the medium was lowered by adding hydrochloric acid (pH 2-4). The product of the pseudo-first-order rate constant and H3O+ ion concentration in solution was found to be reasonably constant [(1.2 ± 0.1) × 10-8 mol s-1 L-1]. Analysis of the 31P N MR. UV. and Raman spectral data of solutions of 1 revealed the formation of stable hydrates of 1, formed by a fast process. At very low-pH (~1 N HCl), cleavage of the phosphonyl group occurred. A mechanism has been proposed in which 1 could exist in equilibrium with the two hydrates, a phosphonyl hydrate and a carbonyl hydrate. The C-P bond cleavage could occur by the attack of 2 or 3 equiv of H3O on 1 in the rate-determining step, followed by an intramolecular proton abstraction by the phosphonyl oxygen atom in a fast step. The stable intermediate species proposed were reasonable precursors for the products resulting from phosphonyl ester cleavage. 31P NMR signals for the above products were observed in highly acidic solutions of ester 1. The experimental activation parameters [Δ/≠ = 20 ± 2 kcal/mol, ΔS≠ = -52 ± 7 eu (for 3 equiv of water in the ratedetermining step)] and the solvent isotopic effect kH2O/kD = 1.6 as measured in solution were in conformity with the proposed mechanism. © 1979, American Chemical Society. All rights reserved.