Theoretical investigation of the neutral hydrolysis of diethyl 4-nitrophenyl phosphate (paraoxon) in aqueous solution

Abstract

In this work the neutral or spontaneous hydrolysis of paraoxon, one of the most popular organophosphate pesticides, in aqueous solution was investigated at the DFT and MP2 levels of theory, using a combination of local solvation of the phosphoryl group with explicit water molecules, and treating the long range solvent effects using continuum solvation model. In contrast to the alkaline hydrolysis, the neutral hydrolysis takes place in two steps, through an A + D mechanism, with formation of a pentacoordinate phosphorane intermediate. The reaction has activation free energies of 31.8 and 1.9 kcal mol for the first and second steps, respectively, and has an overall reaction free energy of -9.3 kcal mol, computed at the MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d) level of theory. The reaction proceeds through a sequence of proton transfer processes from the attacking water molecule and ends with the protonation of the nitrophenolate leaving group. Explicit description of the local solvating water molecules is essential to describe the proton transfer processes along the reaction coordinate and to stabilize the pentacoordinate intermediate formed. The neutral hydrolysis is very slow and has an overall rate constant of 3.05 × 10 s, computed at the MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d) level of theory. This result, in conjunction with the sensitivity of the rate constant to the experimental conditions, indicates that the hydrolysis of paraoxon in aqueous solution can be even slower than predicted experimentally.

Nucleophilic Neutralization of Organophosphates: Lack of Selectivity or Plenty of Versatility?

“…H 2 O [38] À 1.54 100 % P No 7 years À OH [38] 15.54 100 % P No 4 min IMZ [38] 7.00 100 % P Yes 8 min NH 2 OH [70c] 5.96 100 % P No 3 min BHA [50] 9. H 2 O [97] À 1.54 100 % P No > 20 years e À OH [55] 15.54 100 % P No 46 s IMZ [55] 7.00 100 % P Yes 77 h NH 2 OH [75] 5.96 100 % P No 5 min Piperidine [51] 10.82 100 % P No 2 h 1-formyl piperazine [51] 7. 63 100 % P No 32 h 4-hydroxy-pyridine [51] 11.5 100 % P No 27 min 3,4-dimethyl pyridine [51] 5.…”

“…[50] For the least reactive of all P=O organophosphates, Paraoxon (Table 4), its spontaneous hydrolysis is estimated in over 20 years. [97] All nucleophiles shown evidence a selectivity towards the phosphorus and only IMZ is catalytic. [55] As with DEDNPP, the reactions of secondary alicyclic amines and substituted pyridines with Paraoxon are suggested as non-catalytic.…”

“…Therefore, in 4). [97] The latter can be explained by a catalytic effect of [Ch][AA] in those mixtures with a low amount of ILs, through hydrogen bonding interaction that stabilizes the TS for the nucleophilic attack of water to the phosphoryl center of Paraoxon, as shown in Figure 30.…”

Amidoxime-functionalized bead cellulose for the decomposition of highly toxic organophosphates

“…q is the organophosphate or 4-nitrophenol concentration in the time t divided by the initial concentration of organophosphate, q N is the residual fraction of organophosphate at the end of reaction, and k obs is the overall (observed) rate constant It was assumed that the hydrolysis of organophosphate pesticides follows a two-step mechanism, similar to that previously reported for paraoxon methyl. 41 The rst step of the reaction, associated with the transition state TS1, is an attack of the water molecule opposite of the phosphoester bond with a simultaneous proton transfer from the attacking water onto the axial sulphur or oxygen atom, which results in a pentacoordinate phosphorane intermediate state INT. The second step of the reaction is the breakage of the phosphoester bond and a proton transfer from the axial sulphur or oxygen to the leaving oxygen atom of the original phosphoester bond.…”

Density functional theory explorations of parathion and paraoxon hydrolysis as a function of the underlying alkaline environment

“…While other phosphate esters, such as diester phosphorus compounds, exist naturally, tri-ester phosphorous centers are generally synthesized. 5,6 The introduction of the sulfur group leads to the thio effect, which is when the hydrolysis rate constant changes due to the replacement of an oxygen group by a sulfur group. [7][8][9] The thio effect has the potential to change the reaction mechanism, requiring a detailed analysis of parathion (which contains a P]S group) with respect to its oxonate counterpart, paraoxon (see Fig.…”