Chlorobenzene, bromination

Interestingly, the isomer ratio in chlorobenzene bromination is constant through the reaction when the process is catalyzed by Fe(III) but is decreasing with conversion when catalyzed by Na—Y zeolite. This suggests a change in the catalyst structure as the reaction... 

Halogenation Bromine reacts with benzene in the presence of iron(lll) bro mide as a catalyst to give bromobenzene Chlorine reacts similarly in the presence of iron(lll) chloride to give chlorobenzene... 

Chlorine or bromine react with benzene in the presence of carriers, such as ferric halides, aluminum halides, or transition metal halides, to give substitution products such as chlorobenzene or bromobenzene [108-86-17, C H Br occasionally para-disubstitution products are formed. Chlorobenzene [108-90-7] ... 

Chlorine and bromine add to benzene in the absence of oxygen and presence of light to yield hexachloro- [27154-44-5] and hexabromocyclohexane [30105-41-0] CgHgBr. Technical benzene hexachloride is produced by either batch or continuous methods at 15—25°C in glass reactors. Five stereoisomers are produced in the reaction and these are separated by fractional crystallization. The gamma isomer (BHC), which composes 12—14% of the reaction product, was formerly used as an insecticide. Benzene hexachloride [608-73-17, C HgCl, is converted into hexachlorobenzene [118-74-17, C Clg, upon reaction with ferric chloride in chlorobenzene solution. 

Chlorination is cariied out in a manner similar- to bromination and provides a ready route to chlorobenzene and related ar-yl chlorides. Fluorination and iodination of benzene and other arenes are rarely perfor-med. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are nor-mally cariied out by way of functional group transformations of arylffluines these reactions will be described in Chapter 22. 

The order in reactivity in the Y-zeolite catalyzed bromination found is toluene > benzene > fluorobenzene > chlorobenzene > bromobenzene... 

The structural range of industrially important representatives of these groups is enormous, and includes chlorobenzenes (solvents), polychlorinated biphenyls (PCBs) (hydraulic and insulating fluids), and polybrominated biphenyls and diphenyl ethers (flame retardants). There is widespread concern over both the persistence and the potential toxicity of all these compounds, and sites that have become contaminated during their production represent a threat both to the environment and to human health. Pathways for the aerobic bacterial degradation of chlorobenzenes and chlorobiphe-nyls, and their brominated analogs have been discussed in Chapter 9, Part 1. 

Chlorobromobenzene has been prepared by the diazotiza-tion of o-bromoaniline followed by replacement of the diazonium group by chlorine 1 by the elimination of the amino group from 3-chloro-4-bromoaniline 2 by the chlorination of bromobenzene in the presence of thallous chloride,3 aluminum chloride,4 or ferric chloride 4 by the bromination of chlorobenzene without a catalyst6 or in the presence of aluminum,4 iron,4 or ferric bromide 6 by the diazotization of o-chloroaniline followed by replacement of the diazonium group with bromine 4,6 and from o-chlorophenylmercurie chloride by the action of bromine.7... 

Benzene, toluene, ethylbenzene and chlorobenzene have been shown to be suitable substrates under these conditions, and the reaction rates are 50 to 70 times faster than for uncatalysed reactions. This reaction can be further enhanced by use of methanol as a co-catalyst, which allows bromination of anilines in quantitative yield with complete selectivity for the para- isomer [54],... 

The difference between bromine and chlorine as the substituents is slight. I.R. measurements (Tamres, 1952) of the displacement of the OD-valency vibration of CH3OD dissolved in benzene derivatives also show only a slight difference between bromobenzene and chlorobenzene. 

Bromine (dry gas) Bromine (liquid) Bromobenzene Butanol Butyl acetate Butylamine Butylchloride Butyric acid Calcium chloride Carbon tetrachloride Castor oil Cellosolve Cellosolve acetate Chlorine (dry gas) Chlorine water Chloroacetic acid Chlorobenzene Chloroform Chlorosulfonic acid Chromic acid Citric acid Colza oil Copper sulfate Cyclohexane Cyclohexanol Cyclohexanone... 

In the presence of anhydrous Lewis acid (e.g. FeCls or FeBrs), benzene reacts readily with halogens (bromine or chlorine) to produce halobenzenes (bromobenzene or chlorobenzene). Fluorine (Fy reacts so rapidly with benzene that it requires special conditions and apparatus to carry out fluorination. On the other hand, iodine (I2) is so unreactive that an oxidizing agent (e.g. HNO ) has to be used to carry out iodination. 

Miller and Walling1 6 have shown that both bromine and sulfonyl chloride groups can be displaced during the photochemical reaction of chlorine with p-bromobenzenesulfonyl chloride and that a similar displacement of the sulfonyl group of benzenesulfonyl chloride occurs to give chlorobenzene. 

Exercise 22-14 Aluminum chloride is a much more powerful catalyst than ferric bromide for bromination of benzene. Would you expect the combination of aluminum chloride and bromine to give much chlorobenzene in reaction with benzene Explain. 

The 3-(-butoxycarbonylbenzene oxide 104 has been prepared by dehydro-bromination of either the saturated dibromo epoxide 105 or the unsaturated monobromo epoxide 106, S Similarly, 3-chlorobenzene oxide (107) is prepared by dehydrobromination of the dibromochloro epoxide 108,5 5 and 4-... 

The most obvious example of such behavior is for molecules which contain chlorine or bromine. The two isotopes of chlorine occur naturally in die ratio of 35C1 37C1 = 100 32.7 (3.058 1). A molecule such as chlorobenzene would... 

SECOND-ORDER RATE COEFFICIENTS (l.mole-. SeC ) FOR THE BROMINOLYSIS OF TETRAALKYLTINS BY BROMINE IN SOLVENT CHLOROBENZENE AT 20 °C 3... 

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