Environmental Fate and Transformations of Aniline-Based Herbicides with Special Reference to Chlorpropham

Organic chemistry, Agriculture

Zaater, Mohammed Fehmi Ali

University:University of Glasgow

关键词: Organic chemistry, Agriculture;   

Others  :  http://theses.gla.ac.uk/78112/1/11007399.pdf

来源: University of Glasgow

【 摘 要 】

The work described in this thesis is principally an investigation into the fate and possible transformations of the aniline-based herbicides or phenylamides with particular reference to chlorpropham in potatoes. This xenobiotic chemical also known as isopropyl N-3-chlorophenylcarbamate or CIPC, has been widely used since the early 50's as a selective herbicide. It is also used on staple food-potatoes, which are often eaten without peeling, to suppress sprouting during storage. Special attention has been focussed on three aspects including: (1) Analysis of the peel of laboratory and commercially treated and stored potatoes for residual chlorpropham and three of its potential metabolites, (2) Photolysis of chlorpropham in aqueous and organic solvents, and (3) The effect of horseradish peroxidase, HRP, system on chlorpropham and related anilines. A comprehensive review of the literature pertaining to the environmental fate of phenylamides revealed that chemicals belonging to this group all have in common an amide linkage which Is susceptible to hydrolysis at least enzymatically especially where no organophosphorus or methylcarbamate insecticides exist. Hydrolysis of the aniline-based herbicides produces aniline or substituted aniline(s), which are very unstable and may undergo various competing reactions with the possibility of forming ultimately more or less toxic derivatives than their precursors. In chapter three analytical methods with high precision and good overall recoveries were developed which allowed simultaneous determination of chlorpropham and three suspected metabolites viz. 3,3-dichloroazobenzene, 3,3-dichloroazoxybenzene and 4-methoxy-chlorpropham. Some effort was given to synthesising and identifying these metabolites. Quantification of the chlorpropham was made possible by GC-ECD or GC-FID, while for the azo and azoxy derivatives quantification was best made only by GC-ECD and for the methoxy metabolite by GC-FID. The minimum detection limits were 4, 8, 8 and 20 ng g-1 for the chlorpropham, the azo, azoxy and methoxychlorpropham respectively. Analysis results revealed on the one hand the existence of significant amounts of chlorpropham in potato peel, on the other hand, that none of the analysed peel samples from laboratory and commercially treated and stored potatoes contain any of the questionable metabolites within the detection limits already stated even after long periods of storage. The photolysis investigation in chapter four demonstrated that chlorpropham is a photolabile chemical especially under the uv-lamp. The rate of its phototransformation followed first order kinetics with the highest rate at the lowest concentration, and was affected by the nature of the solvent in the order, water>hexane>methanol. The principal pathways of chlorpropham photolysis in the various media were reductive dechlorination to the more stable propham, solvolysis and coupling or dimerization with the formation of chlorpropham-propham or propham-propham biphenyl dimers. Finally, the enzymatic work in this study clarified that chlorpropham did not respond to the peroxidase even after a long period of incubation, similar to the behaviour of nitroaniline and sulfanilamide, probably because chlorpropham hydrolyses too slowly if at all in the buffer media of the enzyme system providing not sufficient chloroaniline to react with the enzyme. The susceptibility of other tested anilines to react with HRP especially where no efficient reducing agent such as Na2S2O4 or ascorbic acid was present, correlated in a positive manner with the electron densities on the nitrogen atoms. Susceptibilities were in the order: P-anisidine> 2-amino-p-chlorophenol>3-chloro-p-anisidine>> 3-chloroaniline. A complex mixture of products and oligomers such as substituted acylanilide, azo, azoxy, anilino azo, diphenylamine, benzoquinone and phenazine type compounds were successfully isolated from the reaction media by thin layer chromatography and eventually identified by GC-MS and/or mass spectroscopy. The mass spectra of forty compounds are reported. In the light of these findings, together with the controversial and/or undiscovered toxicity of chlorpropham and many of the identified products, it may be adviseable to stay alert to the possibility that such metabolites may contaminate the environment and to suggest that chlorpropham treated potatoes should as a first precaution be peeled before consumption or processing.