Chlorinated phenols mechanisms

Careful control of the conditions of synthesis of chlorinated phenols will reduce the formation of dioxins, and this is now the rule for avoiding their formation. Likewise, macro-contaminants, such as inactive isomers, are now recognized as unwanted components of chemicals that may cause harm in the environment or to humans. However, a good understanding of the mechanism of toxic action and how the chemical interacts with the receptor is needed to be able to recognize that toxicity is isomer-specific and to then use this in the intelligent design of low-risk products. 

Tang and Huang (1996a) reached two conclusions. First, hydroxyl radicals will attack unoccupied sites of the aromatic ring second, chlorine atoms will be released from the chlorinated aliphatic intermediates instead of the aromatic ring. When chlorinated phenols are present in the system, the following reaction mechanisms can be assumed ... 

Dickson LC, Karasek FW. 1987. Mechanism of formation of polychlorinated dibenzo-p-dioxins produced on municipal incinerator fly ash from reactions of chlorinated phenols. J Chromatogr 389 127-137. 

Finally, Sotelo et al. [179], while studying the ozonation of resorcinol and phloroglucinol, two precursors of trihalomethanes (THM) during water chlorination, found some polar intermediates that confirmed the proposed phenol mechanism reported elsewhere [42]. From the identified intermediates it was deduced that ozonation of phenols yields more oxygenated compounds that eventually could be removed in biological steps. 

In addition to transformation by corrodable metals (such as Fe° and Zn°), bimetallic combinations of a catalytic metal with a corrodable metal (such as Pd/Fe or Ni/Fe) have also been shown to transform a variety of contaminants. In most cases, rates of transformation by bimetallic combinations have been significantly faster than those observed for iron metal alone [26,96,135-139]. Not only have faster transformation rates been observed with bimetallic combinations, but, in some cases, transformation of highly recalcitrant compounds, such as polychlorinated biphenyls (PCBs), chlorinated phenols, and DDT has been achieved [24,140,141]. The mechanism responsible for the enhanced reactivity with bimetallic combinations is still unclear however, it has been suggested that electrochemical effects, catalytic hydrogenation, or intercalation of H2 may be responsible. A likely limitation to the full-scale application of bimetallic combinations to groundwater remediation is deactivation of the catalytic surface either by poisoning (e.g., by sulfide) or by formation of thick oxide films [136,142,143]. 

Chlorinated phenols are common environmental pollutants, introduced as pesticides and herbicides. Studies have been carried out on the potential use of radiation to destroy these compounds as a means of environmental cleanup . While these studies were concerned with mechanisms (and are discussed in the chapter on transient phenoxyl radicals), other studies involved large-scale irradiation to demonstrate the decomposition of phenol in polluted water . Continuous irradiation led to conversion of phenol into various degradation products (formaldehyde, acetaldehyde, glyoxal, formic acid) and then to decomposition of these products. At high phenol concentrations, however, polymeric products were also formed. 

CPA (4-chlorophenoxyacetic acid, X) and other phenoxy herbicides were photolyzed to produce the corresponding chlorinated phenol as a major product (39, 40) (Figure 4). Isolation of the formate ester of the phenol—e.g., 4-chlorophenyl formate, XI—indicated the operation of an oxidative mechanism analogous to that proposed for naphthalene-acetic acid. The ester was converted to the phenol by either photohydrolysis or elimination of carbon monoxide. Here water could serve simultaneously as cage for the solvated electron, as reaction medium, and as a reagent. 

The mechanism by which pesticides exert their toxic effects on mammals has been characterized for only a few groups of compounds [17,18]. For example, the mechanism for organophosphorus and carbamate insecticides involves inhibition of cholinesterase also, nitrophenols and higher chlorinated phenols are inhibitors for oxidative phosphorylation [10]. Fat-soluble substances (e.g., organochlorines such as DDT, HCH, and other persistent substances) accumulate in the body and, when stored in fatty tissues, cannot be... 

Pesticides are beUeved to be carcinogenic by multiple mechanisms including genotoxicity, tumor promotion, immunotoxicity, and hormonal action. This subject is addressed in some detail in an excellent review paper on pesticides and cancer [80], Exposures to individual pesticides have been associated with some cancers. For example, phenoxyacetic acids, of which 2,4-D is the most widely used and researched, and chlorinated phenols carry risks of developing soft tissue sarcoma and non-Hodgkin s lymphoma [82, 83], It should be noted, however, that the authors of... 

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