Degradation of MCPA by ozone and light

The degradation of dilute aqueous solutions of the herbicide, MCPA (4-chloro-2-methyl-phenoxyacetic acid), by ozone has been examined in the presence and absence of very low energy u.v. light (> 300 nm) to identify early benzenoid intermediates. In the dark, ozonolysis proceeded rapidly, and few aromatic products were detected; under irradiation, MCPA loss occurred even more rapidly, and a series of benzenoid intermediates could be isolated, including 4-chloro-2-methyl-phenol, its formate ester, 5-chlorosalicylaldehyde and 5-chlorosalicyclic acid. Isolation of 5-chloro-3-methylbenzene-1,2-diol from dark ozonolysis suggests the operation of two distinct ozonolysis pathways: ring-hydroxylation and cleavage by molecular ozone in the dark, and side-chain oxidation by hydroxyl radicals under irradiation.