Decomposition of Paraoxon and Parathion by Amines, HOO-and OH-Ions: Reaction Mechanism and Origin of the α-Effect

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 Br nsted-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 Br nsted-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.