Estudo cinético e computacional da solvólise de organofosforados em formamida

Abstract

Phosphate triesters are synthetic organophosphorus compounds widely used as pesticides because of their high toxicity and stability. Studies involving the solvolysis of organophosphates have attracted great interest due to the need for toxicity attenuation of obsolete stocks of pesticides and also to understand its reactivity. In this work the degradation of the activated triester diethyl 2,4-dinitrophenylphosphate (DEDNPP) and the pesticide diethyl-4-nitrophenylphosphate (Paraoxon) in formamide (FMD) and N, N-dimethylformamide (DMF) were evaluated by means of spectrophotometric kinetic studies (UV-Vis). Results showed higher solvolysis rates of DEDNPP and Paraoxon in FMD than in DMF, attributed to the absence of steric hindrance by methyl groups in the unsubstituted amide. It was verified that the degradation reaction in FMD is 102 and 103-fold faster than the spontaneous hydrolysis of DEDNPP and Paraoxon, respectively. Moreover, FMD promotes the degradation of DEDNPP catalytically in the presence of small amount of water as confirmed by 31P nuclear magnetic resonance analysis. Preliminary theoretical calculations using the density functional theory were performed in order to investigate the reaction mechanism between FMD and DEDNPP. Calculations indicated much higher energy barrier than observed experimentally (40.1 and 21.2 kcal mol-1, respectively) suggesting more solvent molecules are involved in the transition state structure. On the other hand, considerably low energy barrier was observed for the reaction of iminoalcohol (iminol) tautomer with DEDNPP when compared to experimental results, which suggests iminol is not significantly involved in the reaction. Thus, amide solvents appear as potential promoters of the degradation of phosphate triesters as the process, which consists only of the use of a binary mixture of simple and low-cost solvents, is shown to be authentically catalytic.