Riboflavin photosensitized oxidation of 2,4-dichlorophenol: assessment of possible chlorinated dioxin formation

Dimeric products are formed by riboflavin-sensitized photooxidation of 2,4-dichlorophenol. The products of this reaction were examined to determine whether chlorinated dibenzo-p-dioxins could be formed from chlorophenols in water by the action of light of wavelengths greater than 280 nanometers. Dimers are formed by union of phenoxy radicals through carbon-carbon or carbon-oxygen bonds. 4,6-Dichloro-2-(2,4-dichlorophenoxy) phenol was obtained in greater quantity than other dimers. Products were characterized by combined gas chromatography and mass spectrometry. Chlorinated dibenzo-p-dioxins which could result from ring closure of a 2-phenoxyphenol derivative were not detected in the products of photolysis. The failure to detect chlorinated dibenzo-p-dioxins may result from the rapid photolytic breakdown of the lower chlorinated dibenzop-dioxins. Under environmental conditions, dioxins are unlikely products of the lower chlorinated phenols or phenoxyalkanoic acids.