Ionization of the PH Bond in Diethyl Phosphonate

Article abstract of DOI:10.1021/ja00299a027

Diethyl phosphonate is oxidized by halogens in a reaction subject to general base catalysis when the halogen concentration is sufficiently high.Under these conditions the reaction is zero order in iodine or bromine and the Broensted exponent, β, is, contrary to earlier reports, about 0.7.When the halogen concentration is quite low, but still within the range of the spectrophotometric analysis used, a third-order rate law is obeyed and the product of the ionization constant of the phosphonate and the rate constant for the reaction of the anion with halogen (which is probably diffusion controlled) can be derived.This gives an acid dissociation constant of 2.5*10^-15.When the diethyl phosphonate is exchanged with D2O, a substantial kinetic isotope effect is observed for the oxidation, again contrary to expectation from previous reports on a similar reaction.This reaction is thus closely analogous to the ionization of hydrogen α to a carbonyl group or a nitro group in all of which the proton transfer is slower than the diffusion-controlled rate in both directions.