Photolysis of dicamba (3,6-dichloro-2-methoxybenzoic acid) in aqueous solution and dispersed on solid supports

By:Jean-Pierre Aguer,1 Frédéric Blachère,1 Pierre Boule,1 Sandrine Garaudee,2 and Chantal Guillard2

Dicamba (3,6-dichloro-2-methoxybenzoic acid) was exposed to UV light in aqueous solution. It was also irradiated in the solid phase without support or adsorbed on laponite (a synthetic clay) and ferric oxide.
Two main photoproducts (1) and (2) were identified in irradiated aqueous solutions. Both involve the substitution of chlorine by OH. The unexpected product (2) is formed through a kinetic reaction of primary product; it results from an oxidation and it is not formed in the absence of oxygen.
A huge number of intermediate products were simultaneously formed when dicamba is irradiated in the solid phase in presence of laponite. They correspond to methylation, decarboxylation, chlorination, dechlorination, and hydroxylation reactions. The latter is probably due to the presence of water in clays, whereas methylation, chlorination and decarboxylation have been attributed to the cage effect and to an energy transfer process. Without support or with ferric oxide, no decarboxylation occurs. The nature of the inorganic support also modify the kinetics of dicamba photolysis.




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