Hydrolysis study of the organophosphorus pesticide diazinon in strong acid solution and in alkaline solution in the presence of cyclodextrins

This thesis deals with the hydrolysis of diazinon at 25°C under two conditions: in acid (1--14M H2SO4) and at pH 12 in the presence of cyclodextrins (CDs).; In the acid study, pKa values were determined for the conjugate acids of diazinon and its hydrolysis products. Product analysis by 31P NMR revealed that the only phosphorus containing hydrolysis product was O,O-diethyl phosphorothioic acid. A pH-rate profile for hydrolysis was produced and the data were treated by the excess acidity method. A-1 and A-2 mechanisms did not fit but a non-acid catalyzed mechanism did fit the data. However, the mechanistic pathway has not conclusively been determined.; In the study of CDs, the binding constants (Kb) between diazinon and the CDs were determined by NMR (1H and 31P) and from kinetics using the Lineweaver-Burk equation. The kinetics also yielded kc, the rate constant for hydrolysis of CD bound pesticide. In general it was found that CDs with low Kb values catalyzed the reaction whereas CDs with large Kb values inhibited reaction. This suggests that strong binding places the pesticide too far in the CD cavity to undergo interaction with hydroxy groups on the CD rim, whereas pesticides with weaker binding are included less deeply and can react with the CD rim by way of specific or general base catalysis. Computer modeling (molecular dynamics running MM2) found that the orientation of binding with alpha- and beta-CD was with the isopropyl group of the pyrimidinol ring positioned parallel to the cavity opening whereas with gamma-CD it was positioned perpendicular to the cavity.; It is proposed that CDs could be used to treat contaminated soils. First, CDs could be applied to increase aqueous solubility of the pesticide and remove soil bound pesticide in a pump and treat method. Alternatively, CDs which catalyze the reaction could be added to soil and simply degrade the pesticide in situ, leaving behind the much less toxic hydrolysis products and any residual CD, a glucose based molecule which can be biodegraded to harmless products in the soil.