Table 2. Reaction mechanisms in direct photolysis of pesticide in water.
| Photoreactions | Bond / mechanism | Pesticide class a) | |
|---|---|---|---|
| Photo-induced hydrolysis | sulfonylurea (C–S, N–S), organophosphorus (P–O) | ||
| chlorinated triazine herbicide (C–Cl) | |||
| Homolytic cleavage of | C–O(S) | Norrish type-I | pyrethroid a), thioester, O-aryl N-alkylcarbamate |
| C(=O)X (X=O, N, S) | β-cleavage | pyrethroid (type-II) | |
| C–N | Norrish type-I | aryl amide a), O-alkyl N-arylcarbamate a), benzoylurea b) | |
| Norrish type-II | aryl amide | ||
| Homolytic bond cleavage | C–O (ether) c) | diphenyl ether, phenoxyacetic acid | |
| C–S | sulfonamide, thioether c) | ||
| N–O | oxime ether | ||
| N–S | sulfonamide | ||
| Dehalogenation | homolytic | organochlorine (DDT, methoxychlor) | |
| heterolytic | 2- or 3-halogenated phenol d), aniline & their derivatives | ||
| chloroacetanilide d), 4-halogenated aromatics e) | |||
| Denitration, deamination | C–N | neo-nicotinoid, triazinone herbicide | |
| Isomerization | geometrical | pyrethroid, alkene, oxime ether | |
| valence | pyridine, pyridazine & isoxazole rings | ||
| Cyclization | C–C f) | pesticides having aromatic rings adequately oriented | |
| C–O g) | diamide insecticide, benzoylcyclohexanedione herbicide | ||
| C–N | dinitroaniline herbicide | ||
| Rearrangement | photo-Fries a) | O-alkyl N-arylcarbamate, O-aryl N-alkylcarbamate, phenylurea | |
| photo-Claisen | phenoxyacetic acid a), aryl benzyl ether | ||
| Beckmann | oxime ether | ||
| thiono–thiolo a) | organophosphorus | ||
a) Minor pathway in direct aqueous photolysis under sunlight. b) Norrish type II process followed by disproportionation is alternatively proposed. c) Heterolytic bond cleavage is an alternative mechanism. d) Homolytic bond cleavage is an alternative mechanism. e) Formation of carbene is proposed for some compounds. f) Electrocyclic or oxidative process. g) SN(ET)Ar* mechanism is most probable.