Phototransformation de l’acide 5-chlorosalicylique en solution aqueuse et sur cuticule foliaire simulee.

Abstract

The photochemical reactivity of 5-chlorosalicylic acid (Cl-C6H3-OH,COOH; 5-ClSA), an elicitor of natural plant defense mechanisms, was examined under different experimental conditions. In neutral aqueous solution and under the action of a simulated solar irradiation (suntest), the monoanionic form (5-ClSA‒) is phototransformed relatively quickly, dechlorination being the predominant primary step. This leads mainly to the respective conjugated bases of 2,5- dihydroxybenzoic acid (DHBA, P3), by photohydrolysis, and 2-carboxy-3-hydroxy-1,4- benzoquinone (P1), by photooxidation. A hydroxylation compound of 5-ClSA‒ (5ClSA-OH) and several dimeric compounds are also formed, but in smaller proportions. The phototransformation of DHBA, experimented separately, leads to the formation of dihydroxycyclopentadiene acid, through a very exceptional ring-contraction reaction. By fusion with P1, DHBA generates a polysubstituted biphenyl and by molecular association with the same P1, a quinhydrone-like complex, via an intermolecular charge transfer, a phenomenon that is just as rarely observed as the contraction of the cycle. About 40 photoproducts, comprising from 19 to 01 C atoms (formaldehyde), diversely oxygenated, comprising aromatic, aliphatic, polycyclic, monocyclic or acyclic compounds, in trace or ultra-trace concentrations, have been identified, revealing the nature of the detectable successive photoproducts, up to quasi-mineralization. The latter was confirmed by measuring the residual content of Total Organic Carbon. The production and involvement of hydroxy radicals (OH•), produced by ""light-induced secondary OH precursors "", in the mineralization of 5-ClSA, has been experimentally proven. The mechanism of photolysis of 5-ClSA, established through the identification of its photoproducts, has been refined by the identification of transient species, at the top of which is the triplet of 5-ClSA, a fleeting species holding nevertheless a central role. The solvated electron, the 2-carboxy-4-chlorophenoxyl radical and the carbene ""4-oxocyclohexa-2,5- dienylidene"" complete the picture. Singlet oxygen (1O2), the corollary of the triplet, was evidenced by phosphorescence. On a leaf model, 5-ClSA, without additives, resists to phototransformation. The presence of a surfactant, added in commercial formulations, considerably accelerates the phototransformation, while orienting it towards reduction. Salicylic acid (SA), the only photoproduct that accumulates, is also a mediator in the response of plants to pathogens.