Hydrodechlorination

Hydrodechlorination has long been recogni2ed as an important chemical transformation. However, the en2ymatic hydrodechlorination of atra2ine (3) by soil microorganisms has also been demonstrated (eq. 15) (21). 

Hydrodechlorination is a common reaction of chlorinated pesticides such as atrazine (eq. 15), alachlor, and metolachlor (2) (eq. 16). These reactions are catalyzed primarily by transition metals or by soil surfaces (clays or humic substances). 

Chloroform can be manufactured from a number of starting materials. Methane, methyl chloride, or methylene chloride can be further chlorinated to chloroform, or carbon tetrachloride can be reduced, ie, hydrodechlorinated, to chloroform. Methane can be oxychlorinated with HCl and oxygen to form a mixture of chlorinated methanes. Many compounds containing either the acetyl (CH CO) or CH2CH(OH) group yield chloroform on reaction with chlorine and alkali or hypochlorite. Methyl chloride chlorination is now the most common commercial method of producing chloroform. Many years ago chloroform was almost exclusively produced from acetone or ethyl alcohol by reaction with chlorine and alkali. 

Hydrogenation of Carbon Tetrachloride. Carbon tetrachloride can be hydrogenated, ie, hydrodechlorinated, to chloroform over a catalyst (25,26) or thermally (27). Although there are no industrial examples of this process at this time, it will receive more attention as more carbon tetrachloride becomes available as the CFC-11 and -12 markets decline (see, Chlorocarbons and chlorohydrocarbons, carbon tetrachloride). Chloroform can be further hydrodechlorinated to methylene chloride (28,29). 

Table 8.1 Comparison of Ni and Pd catalysed hydrodechlorination of aryl chlorides...

The exact nature of the catalytically active Ni species in these reactions is yet to be conclusively established. Hydrodechlorination proves optimal with a NHC Ni ratio of 2 1 suggesting that 14-electron Ni(NHC)2 is involved, whereas the 1 1 NHC Ni ratio necessary for hydrodefluorination implies that it is the 12-electron mono-carbene adduct Ni(NHC) which is catalytically active [10]. Smdies by Matsubara et al. revealed that treatment of NKacac) with either one or two equivalents of IMes HCl 1 or SlMes HCl 2 in the presence of NaOHu formed the mono-NHC complex Ni(NHC)(acac)j which, upon reduction with NaH in the presence or absence of carbene, formed Ni(NHC)2 [11]. Density functional theory (DFT) calculations suggest that the strength of the Ni-NHC bond (ca. 50 kcal/mol) makes... 

Very recently, efforts have turned towards the use of [PdCii -CjHjjCKIPr)] 5 for the hydrodechlorination of polychlorinated phenyl substrates [14], In these cases, [PdCri -CjHjjCKIPr)] 5 proved to be less active than the non-allyl containing dimer [Pd( J,-Cl)Cl(IPr)]2 7 for the complete hydrodechlorination of 1,2,4,5-tetrachlo-robenzene to benzene, with conversions of 40% and 95% respectively found at 80°C and 0.02 mol% catalyst loading in the presence of KO Bu/ PrOH. 

The potential of the dimeric system 7 is perhaps best exemplified by its ability to bring about the total hydrodechlorination of decachlorobiphenyl, a member of the notorious family of polychlorinated biphenyls or PCBs (Persistent Organic Pollutants, POPs), in the presence of the inexpensive base NaOH (Table 8.3). 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

Due to electronic deactivation of the substrate, hydrodechlorination of the dichlorophenol is discriminated in comparison to the monosubstituted substrate. 

Figure 13. Specific 2-CP (open symbols) and 2,4-DCP (solid symbols) hydrodechlorination rate constant K) as a function of the average Ni particle diameter ( nO for reaction over Ni catalysts prepared via impregnation with nitrate (0,0), deposition-precipitation (A,A) and impregnation with nickel ethanediamine ( , ) r= 423K reaction data refer to aqueous solutions. (Reprinted from Reference [147], 2003, with permission from Royal Society of Chemistry).

An important application of carbon-skeleton gas chromatography is the simplification of the analysis of complex samples such as polychlorinated biphenyls, polybrominated biphenyls and polychloroalkanes [709-711], These complex mixtures of halogenated isomers produce multiple peaks when separated by gas chromatography, making quantitation difficult. The isomers have identical carbon skeletons, resulting in a very simple chromatogram after hydrodechlorination. 

Treatment of 8-fluoro-4-hydroxy-l/7-[l,2,4]triazino[4,5- ]quinoline-l,6(2/7)-dione 72 with POCl3 readily and selectively gave compound 103 which was subsequently hydrodechlorinated affording 104 (Scheme 6) <2003JHC789>. 

Scheme 4.30 Hydrodechlorination of chlorobenzene using active flow cells in a continuous-flow microwave process.

HyChlor Formerly called DHC. A catalytic hydrodechlorination process which converts organic chlorides to hydrogen chloride and saturated hydrocarbons. The UOP HyChlor process recovers and recycles both the organic and inorganic reaction products in order to minimize waste disposal requirements and maximize yield. Used for treating wastes from the production of chlorinated petrochemicals such as vinyl chloride. Developed by UOP but not commercialized as of 1992. 

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

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. The base initially promoted HCl ehmination from HCH to yield trichlorobenzene, which then underwent the usual hydrodechlorination reaction. 

As already shown in paragraph Section 6.2.2, the multiphasic conditions for hydrodechlorination, are also active for hydrogenation reactions, such as was the case of haloaromatic ketones, which could selectively be reduced to the alcohol." This reaction was investigated from the kinetic standpoint,... 

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