Studies on hydrolysis of methyl parathion with Zr(IV), Hf(IV) and Cu(II) catalysts in acidic aqueous solutions

Abstract

Hydrolysis of O,O-dimethyl O-p-nitrophenyl phosphoro-thionate (Methyl Parathion) has been studied in acidic medium with catalysts such as Zr⁴⁺, Hf⁴⁺ and Cu²⁺ cations. In the case of Zr⁴⁺ and Hf⁴⁺ cations, the rate maxima were observed around pH 2.0. Since the metal bound water molecules (M-OH₂) of octameric metal-complex are known to exist in equilibrium with metal-hydroxide ion (M-OH⁻), OH⁻ nucleophiles are abundantly available from these cations even in acidic aqueous solutions. Therefore, an interaction of sulphur of P=S group with metal ion and an intramolecular nucleophilic attack of metal bound OH⁻ at phosphorus atom results in P-O bond cleavage to liberate p-nitrophenol. The 3.2×10⁴ fold rate enhancement with respect to uncatalyzed hydrolysis at pH 6.0 for Cu²⁺ catalysis, has been attributed to high reactivity of hexa-aquo copper complex (i.e., [Cu(H₂O)₆]²⁺), which is known to predominate at around pH 6.0.