Abstract
The photochemical behaviour of many aromatic pesticides (mainly herbicides) are compared and the main reactions are separated in three different classes: 1- reactions involving carbon-halogen bond; 2 - other reactions involving the aromatic ring; 3 - reactions of the aliphatic moiety. It appears that the nature of the substituents and their relative positions on the ring play a major role in the orientation of the reaction. The molecular and ionic forms of ionisable molecules may have different photochemical behaviour. A wavelength effect is observed with some compounds. The case of mecoprop [2-(4-chloro-2-methylphenoxy)-propanoic acid] is presented as an example.The irradiation of herbicides in aqueous solution may lead to the formation of photochemical intermediates more toxic to microorganisms than the initial substrate.