Unravelling the reactivity of bifenazate in water and on vegetables: Kinetics and byproducts

In this study, we aimed to better understand the transformation mechanisms of bifenazate, a biphenyl hydrazine derivative insecticide poorly studied up to now. For this, we compared its reactivity in the dark and under simulated solar light irradiation in different media (water, non-aqueous polar solvent, surface of apolar wax films, skin of vegetable). In air-saturated pH = 5.7 water, bifenazate underwent both autoxidation in the dark (t_1/2 = 34 h) and photolysis (t_1/2 = 17 h). In an aprotic polar solvent such as acetonitrile, bifenazate was stable in the dark but was quickly photodegraded in the presence of oxygen (t_1/2 = 2 h). The phototransformation of bifenazate was due to the oxidation of excited states by oxygen and to the cleavage of the NN bond, while the autoxidation in water started by the initial oxidation of the molecule by oxygen and involved the superoxide anion as chain carrier. On paraffinic wax film, photodegradation (t_1/2 = 365 h) and dark autoxidation (t_1/2 = 1600 h) were very slow. On green pepper skin, bifenazate disappeared both in the dark (t_1/2 = 34 h) and through photolysis (t_1/2 = 23 h) at rates close to those measured in water. This shows that on green pepper skin, bifenazate is affected by water contained in the vegetable and possibly released by transpiration. Bifenazate diazene was the major degradation product in all studied conditions. Minor byproducts were detected too. They depended on the experimental conditions showing that degradation pathways are governed by the nature and properties of the medium. In particular, on green pepper one found byproducts generated in acetonitrile and on wax by photolysis and in water by autoxidation. This finding highlights the need for a better model than wax to mimic photolysis on plant surfaces.