KINETICS AND MECHANISTIC ASPECTS OF THE HYDROLYSIS REACTION OF CYCLIC PHOSPHONODITHOITES

Abstract

The hydrolysis of the cyclic phosphonodithioite C₆H₅ P(SCH(CO₂CH₂CH₃CH(CO₂CH₂CH₃)S) in aqueous acetonitrile follows a rate law that is first order in the concentration of C₆H₅ P(SCH(CO₂CH₂CH₃)CH(CO₂CH₂CH₃)S), and independent of the water concentration. The values of E_a , ΔH* and ΔS* are 10.4 k cal mol⁻¹, 9.6 k cal mol⁻¹ and -45 cal mol⁻¹ respectively. The isotope effect for D₂O as compared to H₂O is 2.5. A mechanism is proposed whereby a pre-equilibrium is established between the phosphonodithioite and water, with a subsequent proton transfer from water to the tricoordinate phosphorus occurring in a subsequent step. The slow step involves hydrolytic cleavage of the first phosphorus-sulfur bond. The compound 2-nitro-5-thiocyanatobenzoic acid is used to provide supporting evidence for the formation of an intermediate thiol.

Key Words:

• Phosphonodithioite
• hydrolysis
• mechanism