Chlorinated dimers present in several technical chlorophenols used as fungicides.

The presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin in 2,4,5-trichlorophenoxy acid est-ers was shown to originate from 2,4,5-tri-chlorophenol used in industrial processes. The dimerization is thought to occur when the phenol is produced from the corresponding tetrachlorobenzene by the action of alkali. Logically all products originating from chlorinated benzenes treated with alkali are possible sources of chlorinated dibenzo-p-dioxins (chlorinated dioxins). We have therefore investigated the presence of chlorinat-ed dioxins in some pentachlorophenol formulations (1). The first result obtained was that the content of the expected octachloro-dibenzo-p-dioxin in one formulation was 1000 ppm (1 g/kg). However, the phenol-free extract was treated with diazomethane and reinjected into the gas chromatograph. Despite the fact that a dioxin cannot react with diazomethane, the octachlorodioxin content decreased to 50 ppm. At the same time, two new peaks appeared in the chro-matogram. The compounds corresponding to these peaks were identified by means of mass spectrometry to be the methyl ethers of two isomeric nonachlorohydroxydiphenyl ethers. The phenol corresponding to one of these ethers was later on proved to produce the octachlorodioxin in the gas chromatograph-ic column. Thus the first (high value) of octachlorodioxin was the sum of the true dioxin and its precursor, the hydroxydi-phenyl ether. This compound was consequently given the name of predioxin. In the other diphenyl ether the hydroxy groups were not in a position allowing ring closure, and it was therefore given the name of iso-predioxin. The highest values found in the formulations investigated are given in parentheses in eq. (1). In addition to these three substances, also present was an analogous series originating from the reaction of one molecule of pen-tachlorophenol with one molecule of the impurity tetrachlorophenol. The corresponding values for the above mentioned formulation were D7, 870 ppm; PD7, 1300 ppm; and iso-PD7, 1200 ppm. The total content of these dimeric substances was thus 10 g/kg.