Oh

Figure 6.22 Selective reduction of haloaromatic ketones in the multiphasic system using Pt/C.

x = ci, Br R = Me, Et, Ph x = ci, Br R = Me, Et, Ph

Figure 6.23 Selective reduction of haloaromatic ketones in the multiphasic system using Pt/C.

known as dioxins). They can be generated by incineration processes, high-temperature transformation of organic compounds, and in some cases they are residues of the agrochemical sector (e.g., insecticides such as DDT, or defoliants such as agent orange), or from other applications (PCB as dielectric medium in electric transformers). These molecules accumulate in the environment and in the fatty tissues of animals, and tend to concentrate along the food chain, posing a serious hazard for man. Elimination of this class of compounds by incineration is the cheapest method, and widely employed; it is, however, undesirable, as there is the risk of generating even more toxic PCDDs and PCDFs in the process. The alternative is reductive dehalogenation. The multiphasic catalytic hydrodehalo-genation method described in the previous paragraph was immediately recognized as an efficient method for the detoxification of PCBs.44 It was then applied to environmental samples of PCDDs and PCDFs extracted from fly ashes of a municipal solid-waste incinerator (MSWI).

The mild (50°C, 1 atm) catalytic multiphasic methodology can reduce the toxicity equivalent (TEQ) of a sample from a total of 572pgTEQ/mL to less than

Figure 6.24 Hydrodechlorination of an environmental sample of dioxins: TEQ vs. time.

3 pgTEQ/mL (the detectability limit as determined by HRMS) in 3 hours reaction time (Figure 6.24).52 Since the large excess of charcoal present as Pd/C may have simply adsorbed the dioxins without reaction, a series of experiments were conducted in order to ensure that the PCDD/Fs were being completely dechlori-nated. Later the multiphasic methodology was employed to hydrodechlorinate other hazardous chlorinated organic compounds (Figure 6.25).

Lindane, a widespread insecticide constituted of technical mixtures of hexa-chlorocyclohexane (HCH), was dechlorinated using Pd/C, Pt/C, or Raney-Ni. The product using the multiphasic system was benzene, obtained within 1 hour.58 The base initially promoted HCl elimination from HCH to yield trichlorobenzene, which then underwent the usual hydrodechlorination reaction.

Dieldrin—which belongs to the 12 POPs banned by the Stockholm convention and is in the same class of other pesticides named "drins," such as aldrin and endrin—possesses six aliphatic chlorine atoms on a polycyclic skeleton. The mul-tiphasic dechlorination, in the presence of A336, isooctane, aqueous KOH, Pd/C, and hydrogen, proceeded with a different selectivity and degree of dechlorination, depending on the choice of catalyst system, and base concentration. It always required the base and was favored by the presence of A336. It produced a mixture of products derived from the subsequent removal of chlorines, up to a small percentage of monochlorinated derivative.62

DDT—[1,1' -bis-(4-chlorophenyl)-2,2,2-trichloroethane]—could be hydrode-chlorintated completely, both with Pd/C and Raney-Ni, provided A336 was present. The aliphatic chlorines reacted faster than the aromatic ones, the first via base promoted elimination of HCl.62

Finally, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T: agent orange) and pentachlorophenol (PCP)—two acidic polychlorinated pesticides—were also investigated. In these cases, the presence of A336 contributed to the solubility of

Figure 6.25 Chemical structures of some toxic chlorinated organics.

Figure 6.25 Chemical structures of some toxic chlorinated organics.

the reagent in the organic phase, by forming the ion pair between the anion of the reagent and the ammonium cation.63

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