Aromatic compounds, toxicity

In order to achieve high yields, the reaction usually is conducted by application of high pressure. For laboratory use, the need for high-pressure equipment, together with the toxicity of carbon monoxide, makes that reaction less practicable. The scope of that reaction is limited to benzene, alkyl substituted and certain other electron-rich aromatic compounds. With mono-substituted benzenes, thepara-for-mylated product is formed preferentially. Super-acidic catalysts have been developed, for example generated from trifluoromethanesulfonic acid, hydrogen fluoride and boron trifluoride the application of elevated pressure is then not necessary. 

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom.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. 

The usual aromatic bromination are performed by free bromine in the presence of a catalyst, most often iron. However, liquid bromine is not easy to handle because of its volatile and toxic character. On the other hand, alumina-supported copper(II) bromide can be treated easily and safely as a solid brominating reagent for aromatic compounds. The advantages of this procedure using the solid reagent are simple workups, mild conditions, and higher selectivities. Products can be isolated in good yield by simple filtration and solvent evaporation, and no extraction steps are required. 

The signiflcance of toxic metabolites is important in diverse metabolic situations (a) when a pathway results in the synthesis of a toxic or inhibitory metabolite, and (b) when pathways for the metabolism of two (or more) analogous substrates supplied simultaneously are incompatible due to the production of a toxic metabolite by one of the substrates. A number of examples are provided to illustrate these possibilities that have achieved considerable attention in the context of the biodegradation of chlorinated aromatic compounds (further discussion is given in Chapter 9, Part 1) ... 

Daly JW, DM Jerina, B Witkop (1972) Arene oxides and the NIH shift the metabolism, toxicity and carcinogenicity of aromatic compounds. Experientia 28 1129-1149. 

Jacob J, Kaechee W, and Wagstaffe PJ (1984) Polycyclic aromatic compounds of environmental and occupational importance - their occurrence, toxicity and development of high purity certified reference materials. Part I. Fresenius J Anal Chem 317 101-114,... 

Superheated water at 100°-240 °C, with its obvious benefits of low cost and low toxicity, was proposed as a solvent for reversed-phase chromatography.59 Hydrophobic compounds such as parabens, sulfonamides, and barbiturates were separated rapidly on poly(styrene-divinyl benzene) and graphitic phases. Elution of simple aromatic compounds with acetonitrile-water heated at 30°-130 °C was studied on coupled colums of zirconia coated with polybutadiene and carbon.60 The retention order on the polybutadiene phase is essentially uncorrelated to that on the carbon phase, so adjusting the temperature of one of the columns allows the resolution of critical pairs of... 

It appeared to the author some years ago that, irrespective of the mechanism of the toxic action of DDT, there might be a correlation of structure and toxicity in analogous compounds. Hammett (13) has shown that the rate and equilibrium constants of over 50 side-chain reactions of meta and para substituted aromatic compounds may be correlated with the so-called substituent constant a, according to the equation log k — log k0 = pa, where k and k0 are rate (or equilibrium) constants for substituted and unsubstituted compounds, respectively, p is the reaction constant giving the slope of the linear relationship, and a is the substituent constant, which is determined by the nature and... 

About 100 gal of process wastewater is typically generated from 1 t of coke produced.15 These wastewaters from byproduct coke making contain high levels of oil and grease, ammonia nitrogen, sulfides, cyanides, thiocyanates, phenols, benzenes, toluene, xylene, other aromatic volatile components, and polynuclear aromatic compounds. They may also contain toxic metals such as antimony, arsenic, selenium, and zinc. Water-to-air transfer of pollutants may take place due to the escape of volatile pollutants from open equalization and storage tanks and other wastewater treatment systems in the plant. 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

One of numerous examples of LOX-catalyzed cooxidation reactions is the oxidation and demethylation of amino derivatives of aromatic compounds. Oxidation of such compounds as 4-aminobiphenyl, a component of tobacco smoke, phenothiazine tranquillizers, and others is supposed to be the origin of their damaging effects including reproductive toxicity. Thus, LOX-catalyzed cooxidation of phenothiazine derivatives with hydrogen peroxide resulted in the formation of cation radicals [40]. Soybean LOX and human term placenta LOX catalyzed the free radical-mediated cooxidation of 4-aminobiphenyl to toxic intermediates [41]. It has been suggested that demethylation of aminopyrine by soybean LOX is mediated by the cation radicals and neutral radicals [42]. Similarly, soybean and human term placenta LOXs catalyzed N-demethylation of phenothiazines [43] and derivatives of A,A-dimethylaniline [44] and the formation of glutathione conjugate from ethacrynic acid and p-aminophenol [45,46],... 

Tarhanen, J., J. Koistinen, J. Paasivirta, P.J. Vuorinen, J. Koivusaari, I. Nuuja, N. Kannan, and R. Tatsukawa. 1989. Toxic significance of planar aromatic compounds in Baltic ecosystem — new studies on extremely toxic coplanar PCBs. Chemosphere 18 1067-1077. 

The carcinogenicity of polycyclic aromatic compound-rich tyre extender oils has lead to the proposal of a legislative ban on their use in Europe. The suitability of naphthenic oils as non-toxic plasticisers in tyre treads is discussed and results are presented of experimental studies of the use of these plasticisers in SBR, EPDM, sulphur-cured EPDM and peroxide-cured EPDM. Despite their low aromatic content, the naphthenic plasticisers are shown to give good results in SBR, probably as a result of the contribution to solvent characteristics of the naphthenic molecular structure. The use of naphthenic oils is expected to increase worldwide as they are said to be one of the best alternatives to aromatic extracts with regard to solvent properties, compatibility, performance and availability. 

One further compound should be mentioned in this connexion, namely, p-fluorophenylacetic acid (XXII), which has the carbon skeleton of the highly toxic 5-fluoropentanecarboxylic acid (XXIII). It seemed unlikely that (XXII) could be broken down in vivo to fluoroacetic acid, and as expected it was non-toxic. It should be mentioned, however, that aromatic compounds are capable of certain types of oxidative breakdown in the animal body. Jaffe,1 for example, isolated small quantities of muconic acid from the urine of dogs and rabbits which had received considerable quantities of benzene. 

In Australian tenebrionid beetles, defensive compounds and their patterns seem to be of only low chemotaxonomic value. However, the aforementioned aromatic compounds are restricted to the genus Tribolium. Abdominal defensive compounds were used as chemosystematic characters in order to construct a phylogenetic tree for the genus Tribolium [330]. The defensive secretion of adults of Tenebrio molitor was shown to contain toluquinone 7 and m-cresol 89 [333]. The quantification of benzoquinones in single individuals of Tribolium castaneum at different days after adult eclosion indicates that the amount of toxic quinone only shows a maximum subsequent to cuticle sclerotization. Obviously, there is a need for an adequate cuticular barrier for self-protection from these defensive compounds [334]. 

Diesel-like products (jet fuel, diesel. No. 2 fuel oil, kerosene) are moderately volatile products that can evaporate with no residue. They have a low-to-moderate viscosity, spread rapidly into thin slicks, and form stable emulsions. They have a moderate-to-high (usually, high) toxicity to biota, and the specific toxicity is often related to type and concentration of aromatic compounds. They have the ability to penetrate substrate, but fresh (unoxidized) spills are nonadhesive. 

Short-term (acute) hazards of lighter, more volatile, and water-solnble aromatic compounds (such as benzenes, toluene, and xylenes) include potential acnte toxicity to aquatic life in the water column (especially in relatively confined areas) as well as potential inhalation hazards. However, the compounds that pass through the water column often tend to do so in small concentrations and/or for short periods of time, and fish and ofher pelagic or generally mobile species can often swim away to avoid impacfs from spilled oil in open waters. Most fish are mobile and it is not known whether or not they can sense, and thns avoid, toxic concentrations of oil. 

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