Decomposition of paraoxon and parathion by amines, HOO- and oh- Ions: Reaction mechanism and origin of the α-Effect

Abstract

The second-order rate constants have been measured spectrophotometrically for the reactions of paraoxon 1 and parathion 2 with a series of alicyclic secondary amines, OH- and HOO- ions in H2O at 25.0 ± 0.1 °C. A linear Brønsted-type plot with βnuc = 0.40 was obtained for the reactions of 1 with amines and OH-. The reaction has been concluded to proceed through a concerted mechanism. HOO- deviates positively from the linear Brønsted-type plot, implying that the α-effect is operative. The magnitude of the α-effect (kHOO-/kOH-) was found to be ca. 55 for the reaction of 1 and 290 for that of parathion 2, indicating that HOO- is highly effective in decomposition of the toxic phosphorus compounds although it is over 4 pKa units less basic than OH-. Among the theories suggested as origins of the α-effect (e.g., TS stabilization through an intramolecular Hbonding interaction, solvent effect, and polarizability effect), polarizability effect appears to be the most important factor for the α-effect in this study, since the polarizable HOO- exhibits a larger α-effect for the reaction of the more polarizable substrate 2.