HALOGENATION OF AROMATIC HYDROCARBONS

The halogen carriers or aromatic halogenation catalysts are usually all electrophilic reagents (ferric and aluminium haUdes, etc.) and their function appears to be to increase the electrophilic activity of the halogen. Thus the mechanism for the bromination of benzene in the presence of iron can be repre-sfflited by the following scheme  

The base [FeBr4] facilitates the elimination of a proton from the carbonium ion (I). 

The reaction must be carried out in the absence of direct sunlight, since sunlight causes direct addition of the halogen to the hydrocarbon, particularly if the latter is warm benzene, for example, yields the hexahalide  

Comparable results are not obtained with the less reactive iodine, because the hydrogen iodide formed tends to reduce the iodo compound and a condition of equilibrium is produced  

However, if an oxidising agent (fuming nitric acid or sodium persulphate) is present to destroy the hydrogen iodide as it is formed, the equilibrium is displaced and the iodo compound may be conveniently prepared, for example  

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The direct halogenation of aromatic hydrocarbons in the laboratory is feasible. Chlorination or bromination of benzene gives one monohalogen, CoHisX, and by proper control of temperature and concentration of the halogen, the formation of dihalogen com-poimds, C6H4X2, can be kept at a minimum ... 

The solubility parameter is in the range 18.4-19 MPa and the polymer is predictably dissolved by halogenated and aromatic hydrocarbons of similar solubility parameter. Stress cracking can occur with some liquids. 

Bromobenzene. -The replacement of hydrogen by the halogens Cl and Br, in the nucleus of aromatic hydrocarbons, is assisted by the presence of a halogen carriei, the action of which lias been referred to in the Note on the piepaiations of chlor- and bioin-acetic acids, p. 252. Iodine, iion, iron and alupiinium chlorides and bromides, the aluminium-mercury... 

Gibson DT, JR Koch, CL Schuld, RE Kallio (1968) Oxidative degradation of aromatic hydrocarbons by microorganisms. II. Metabolism of halogenated aromatic hydrocarbons. Biochemistry 7 3795-3802. 

Edwards EA, Liang LN, Grbic-Galic D. 1993. Anaerobic microbial transformation of aromatic hydrocarbons and mixtures of aromatic hydrocarbons and halogenated solvents. Stanford, CA Environmental Engineering and Science program. Department of Civil Engineering. 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

The emission of a complete set of personal computers and monitors are described by Nakagawa et al. (2003). Several VOC like benzene, toluene, etc. were identified and quantified. The results are shown in Table 17.3. The emission rates of aliphatic hydrocarbons, terpenes, esters, ketones, alcohols and halogens were not found to be significantly different for PCs with CRT and TFT monitors. In the case of aromatic hydrocarbons the emission rates were higher if a PC with CRT monitor was used. The same was found for aldehyde emissions but the differences in emission rates were lower. The separate test CRT monitor and the associated computer in this study proved that the monitor was the main source of chemical emissions. 

C. M. Suter and A. W. Weston, Direct Sulfonation of Aromatic Hydrocarbons and Their Halogen Derivatives, Org. React. 1946, 3, 141-197. 

Haglund P, AlsbergT, Bergmann A, Jansson B (1987), Chemosphere 16 2441-2450.. .Analysis of halogenated polycyclic aromatic hydrocarbons in urban air, snow and automobile exhaust" Hilker DR, Aldous KM, Smith RM, O Keefe PW, Gierthy JF, Jurusik J, Hibbins SW, Spink D, Parillo RJ (1985), Chemosphere 14 1275-1284.. .Detection of sulfur analog of 2,3,7,8-TCDD in the environment"... 

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