The Dechlorination of Polychlorinated Biphenyls by UV-irradiation. VIII. Reactions of 2,3- and 3,4-Dichlorobiphenyl in a 2-Propanol Solution

Abstract

The photodechlorinations of 2,3-dichlorobiphenyl (2,3-DCB) and 3,4-dichlorobiphenyl (3,4-DCB) in 2-propanol gave 3-chlorobiphenyl and 4-chlorobiphenyl respectively. The quantum yields of monochlorobiphenyl formation were 0.2 for 2,3-DCB and 0.06 for 3,4-DCB, independent of the substrate concentration and the wavelength of the exciting light. On the basis of the results obtained in quenching experiments using cis-1,3-pentadiene, it was concluded that 2,3-DCB was photodechlorinated exclusively in the excited singlet state, while 3,4-DCB was photodechlorinated in both excited singlet and triplet states. In the case of 3,4-DCB, the relative contributions of excited singlet and triplet states to the total dechlorination were evaluated as 0.8 and 0.2 respectively. The participation of the excited triplet state in the photodechlorination of 3,4-DCB was ascertained both by quenching reactions with biacetyl as a quencher and by photosensitized dechlorinations with acetone and acetophenone as sensitizers.

Degradation pathways of PCBs upon UV irradiation in hexane

“…Also, the results shown in Fig. 1 are in agreement with those described in previous reports, that the or~ho chlorines are lost preferentially when ortho and other positions are substituted by chlorines [19,20].…”

Degradation pathways of PCB upon gamma irradiation.

“…Most of the congeners for which levels were enhanced upon irradiation were apparently produced through the loss of an ortho chlorine atom from their immediate precursor. This degradation pathway is similar to that observed for UV irradiation of PCB, a process that generates similar reaction intermediates, namely, PCB radicals (2,3).…”

Borohydride, micellar, and exciplex-enhanced dechlorination of chlorobiphenyls

“…In each case, the average of duplicate or triplicate injections was used in the quantitation of the percent reaction. Temperature programming allowed excellent separation of the many isomers and congeners involved in this work, as exemplified by Figure 1 which shows the separation of a mixture of 2-chlorobiphenyl (1), 4chlorobiphenyl (2), 2,4-dichlorobiphenyl (3), 2,4,6-trichlorobiphenyl (4), 2,2',4,6-tetrachlorqbiphenyl (5), 2,2',3',4,5-pentachlorobiphenyl ( 6), 2*2',3,4,5,6-hexachlorobiphenyl (7), 2,2,,3,4,4',5,,6-heptachlorobiphenyl (8), 2,2',3,3,,5,5',6,6/-octachlorobiphenyl (9), 2,2',3,3',4,4/,5,6,6'-nonachlorobiphenyl (10), and 2,2',3,3,,4,4/,5,5',6,6'-decachlorobiphenyl (11) to demonstrate the chromatographic separation.…”

“…However, the utility of photolysis for destruction of PCB's is limited by two major disadvantages. First, the "quantum efficiency" (the statistical number of molecules which react per photon of light absorbed) is extremely low for many congeners (6)(7)(8)(9)(10)(11). The quantum yield is usually particularly low in those compounds lacking an ortho chlorine but bearing meta or para substituents and also many of the highly chlorinated biphenyls.…”