Aromatic halides, toxicity

Palladium catalyzes the carbonylation of allylic, vinylic, benzylic, and aromatic halides in alcohols to form esters under conditions similar to those required by the nickel carbonyl catalyst (11). The palladium-catalyzed reaction offers the advantage of not requiring the use of highly toxic and volatile nickel carbonyl, and perhaps higher catalyst activity, although accurate comparisons have not been made. Like the nickel reaction, the palladium reaction... 

The chlorinated phenols, partieularly pentachlorophenol and the trichloro-phenol isomers, are signifieant hazardous wastes. These compounds are biocides that are used to treat wood to prevent rot by fungi and to prevent termite infestation. They are toxic, causing liver malfunction and dermatitis. However, contaminant polychlorinated dibenzodioxins ( dioxin ) may be responsible for some of the observed effects. Pentachlorophenol and other aromatic halides and aromatic hydrocarbons used as wood preservatives are encoimtered at many hazardous-waste sites in wastewaters and sludges. 

The reduction reaction of an aromatic halide is also useful for destruction of highly toxic polyhalogenated aromatic hydrocarbons (PCBs) (Safe and Hutzinger, 1987). 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom. Although they have important uses, many haloalkanes are highly toxic and a threat to the environment. The haloalkane 1,2-dichlorofluoroethane, CHC1FCH2C1, is an example of a chlorofluorocarbon (CFC), one of the compounds held responsible for the depletion of the ozone layer (see Box 13.3). Many pesticides are aromatic compounds with several halogen atoms. 

In comparison to PTFE, PVF is easily processable using a variety of techniques used for most thermoplastic materials. It offers good flame retardancy, presumably due to the formation of hydrogen fluoride that assists in the control of the fire. Thermally induced formation of hydrogen fluoride is also a negative factor because of its toxicity. As in the case of PVC, elimination of the hydrogen halide (HF) promotes formation of aromatic polycyclic products that are toxic. 

Several solvent uses have been proposed. Dimethyl sulfate has been used as a solvent for the study of Lewis acid—aromatic hydrocarbon complexes (148). It also is effective as an extraction solvent to separate phosphoms halide—hydrocarbon mixtures and aromatic hydrocarbons from aliphatics, and it acts as an electrolyte in electroplating iron (149—152). The toxicity of dimethyl sulfate precludes its use as a general-purpose solvent. 

Aromatic carboxylic acids, a,/f-unsaturated carboxylic acids, their esters, amides, aldehydes and ketones, are prepared by the carbonylation of aryl halides and alkenyl halides. Pd, Rh, Fe, Ni and Co catalysts are used under different conditions. Among them, the Pd-catalysed carbonylations proceed conveniently under mild conditions in the presence of bases such as K2CO3 and Et3N. The extremely high toxicity of Ni(CO)4 almost prohibits the use of Ni catalysts in laboratories. The Pd-catalysed carbonylations are summarized in Scheme 3.9 [215], The reaction is explained by the oxidative addition of halides, and insertion of CO to form acylpalladium halides 440. Acids, esters, and amides are formed by the nucleophilic attack of water, alcohols and amines to 440. Transmetallation with hydrides and reductive elimination afford aldehydes 441. Ketones 442 are produced by transmetallation with alkylmetal reagents and reductive elimination. 

A9.6.4.7 The Nordic Council of Ministers issued a report (Pederson et al, 1995) entitled Environmental Hazard Classification, that includes information on data collection and interpretation, as well as a section (5.2.8) entitled QSAR estimates of water solubility and acute aquatic toxicity . This section also discusses the estimation of physicochemical properties, including log Kow For the sake of classification purposes, estimation methods are recommended for prediction of minimum acute aquatic toxicity, for ...neutral, organic, non-reactive and non-ionizable compounds such as alcohols, ketones, ethers, alkyl, and aryl halides, and can also be used for aromatic hydrocarbons, halogenated aromatic and aliphatic hydrocarbons as well as sulphides and disulphides, as cited in an earlier OECD Guidance Document (OECD, 1995). The Nordic document also includes diskettes for a computerized application of some of these methods. 

Acylation of aromatic compounds is a widely used reaction for the production of fine chemicals. The classic methodology for acylation is Friedel-Crafts, this uses stoichiometric amounts of Lewis acids, such as metal halides, but some Bronsted acids, such as polyphosphoric and sulphuric acid, can also be used as catalysts. Moreover, the reaction is carried out homogeneously and the catalyst must be destroyed to obtain the final products, generating a great amount of corrosive and toxic waste. 

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