Problems benzene chlorination

Pibouleau et al. (1988) provided a more flexible representation for the synthesis problem by replacing the single reactor unit by a cascade of CSTRs. They also introduced parameters for defining the recovery rates of intermediate components into the distillate, the split fractions of top and bottom components that are recycled toward the reactor sequence, as well as parameters for the split fractions of the reactor outlet streams. A benzene chlorination process was studied as an example problem for this synthesis approach. In this example, the number of CSTRs in the cascade was treated as a parameter that ranged from one up to a maximum of four reactors. By repeatedly solving the synthesis problem, an optimum number of CSTRs was determined. 

In earlier editions of the Eniyclopedia there have been articles covering the properties, manufacture, capacities, etc, of polychlorinated biphenyls (PCBs), chlorinated naphthalenes, benzene hexachloride, and chlorinated derivatives of cyclopentadiene. These materials are no longer in commercial use because of their toxicity. However, they stiU impact on the chemical industry because of residual environmental problems. Their toxicity and environmental impact are discussed (see Cm.OROCARBONSANDCm.OROHYDROCARBONS, TOXIC aromatics). 

Because HCl is constandy present in most parts of the equipment, corrosion is always a potential problem. Chlorine and benzene, or any recycled material, must be free of water to trace amounts to prevent corrosion and deactivation of the catalyst. The reactor product contains HCl and iron. In some plants, the product is neutralized with aqueous NaOH before distillation. In others, it is handled in a suitably-designed distillation train, which includes a final residue from which FeCl can be removed with the high boiling tars. 

Evaluating Lethal Body Burdens of Chlorinated Benzenes in Fish Problem... 

Chlorobenzene is produced by the electrophilic chlorination of benzene in the liquid phase96,97 catalyzed by FeCl3 at 25-50°C. As a result of corrosion problems and deactivation of the catalyst, dry reactants must be used. A few percent of dichlorobenzene is formed as byproduct. 

The partial rate factors for the substitution reactions of biphenyl, with the exception of a few observations, are on a firm experimental basis. The chlorination of biphenyl was examined on several occasions (de la Mare et al., 1958a Beaven et al., 1961 Mason, 1959 Dewar and Mole, 1957). There are significant differences in the reported values for the rate relative to benzene. A recent careful examination of the products (Beaven et al., 1961) indicated the formation of 2- and 4-chloro-biphenyl in 76.5% yield with 17.5% of the residual chlorine consumed via addition processes. The partial rate factors presented in the table are corrected on this basis. Two early studies of the nitration of biphenyl with acetyl nitrate in acetic anhydride yield rate data in poor agreement (Dewar et al., 1956 Simamura and Mizuno, 1957). A recent re-examination of the problem (Billings and Norman, 1961) yielded partial rate factors (ofh = 36.4 = 32.6) confirming the results... 

Regardless of one s position in this debate, it is apparent that the use of compounds that show persistence in the environment must be carefully examined and monitored. Any compound that is persistent and hydrophobic (lipophilic or fat soluble) will be subject to the process of biomagnification and may present special problems. In addition to DDT and other chlorinated pesticides, another example is provided by the polychlorinated biphenyls (PCBs). These compounds have two benzene rings bonded together, with varying numbers of chlorines substituted on the rings. One example is provided by the following structure ... 

Note that this is the para para (abbreviated as p,p ) isomer. The term para means that the chlorine atoms are positioned on the benzene ring across from the linkage to the rest of the molecule. DDT won Paul Muller the Nobel Prize in medicine in 1948 for malaria control because DDT killed the mosquitoes that transmitted this disease. At least 2 billion kilograms of DDT has been used worldwide since about 1940. Because of problems with calcium metabolism in birds (egg shell thinning), DDT has been banned in most industrialized countries since about 1970-1975. By the loss of HC1, p,//-DDT degrades relatively quickly to DDE (see below), which is almost permanently stable in the environment. DDT, however, is still produced and used to control malaria in developing countries because it is cheap to manufacture and simple to use. 

The feed rate of the dry chlorine is 1.4 lb mole of chlorine/(h) (lb mole of initial benzene charge). The following rate constants are estimated values for the catalyst used at 55°C (assumed for this problem) ... 

Chlorinated benzenes are widely used in industry and are sometimes encountered in drinking-water from surface sources. They usually give rise to taste and odour problems at concentrations below the health-based guideline value, where one has been proposed. 

Many chromium oxidants suffer from problems of stability, light sensitivity or acidity, but tetrakis(py-ridine)silver dichromate is stable, nonphotosensitive, nonhygroscopic and a neutral oxidant It can be used to oxidize allylic and benzylic alcohols selectively in benzene. Unfortunately, it cannot be used in chlorinated solvents because it decomposes in these solvents. 

The same type of problem can result from the contamination of the contained liquid with a powerful solvent. In at least one case, small amounts of chlorinated benzene in waste hydrochloric acid caused swelling and penetration of the membrane and carried the acid through the membrane into the vessel... 

Problem 11.5 Certain activated benzene rings can be chlorinated by hypo-chlorous acid, HOC), and tlrs reaction is catalyzed by In light of the above discussion, can you suggest a possible function of ... 

Problem 16.2 How many products might be formed on chlorination of o-xylene (n-dimethyl-benzene), nnxylene, and />xylene ... 

Historically, several processes have been developed to an industrial scale to produce phenol, including (i) sulfonation of benzene and alkali fusion of the benzene sulfonate (ii) chlorination of benzene and hydrolysis of chlorobenzene (iii) the cumene process (Section 13.2) (iv) toluene oxidation to benzoic add and subsequent oxidative decarboxylation of the latter to phenol and (v) dehydrogenation of cyclohexanol-cyclohexanone mixtures. Today, however, only the cumene process and the toluene oxidation are still run on an industrial scale, all the other processes having been given up due to economic reasons or environmental problems. 

PROBLEM 23.1 Consider all the isomers of C7H7CI containing a benzene ring and write the structure of the one that has the weakest carbon-chlorine bond as measured by its bond dissociation energy. 

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