Photodegradation of the carbamate insecticide pirimicarb

In order to study the photoreactivity of pirimicarb (2-(dimethylamino)-5,6-dimethyl-pyrimidin-4-yl-N-dimethylcarbamate ) on plant surfaces, model experiments with organic solvents were performed. Pirimicarb decomposed readily when irradiated (lambda > 280 nm) in the presence of the selected model solvents to form a number of products. Half-lives were in the order isopropanol < cyclohexane < cyclohexene (up to a maximum of 140 min). In the presence of all three solvents photooxidation predominated leading to N-formylpirimicarb and, in a second step, to N-desmethylpirimicarb as well as to further oxygenated products with the carbamate moiety intact. Photolysis in cyclohexene resulted in photomineralisation as the main degradation pathway. Irradiation of pirimicarb in the presence of isopropanol also yielded an addition product of the parent compound with the secondary alcohol.