Chlorobenzene, bromination chlorination

Palladium- (and nickel-) -catalysed coupling reactions proceed normally on halodiazines and the equivalent triflates, the most significant feature being the enhanced reactivity relative to chlorobenzenes of chlorine at position a and 7 to a nitrogen, just as in pyridine chemistry. In some particularly activated caes, this extra activation is sufficient to overcome the normally higher reactivity of bromine, but not... 

Palladium- (and nickel-) catalysed coupling reactions proceed normally on halo-diazines, the most significant feature, as with pyridines, being the enhanced reactivity, relative to chlorobenzene, of chlorine at positions a and y to a nitrogen, but it is important to recognise that this activation does not overcome the higher intrinsic reactivity of bromine and iodine at any position." ... 

Chlorobenzene and bromobenzene, for example, undergo nitration at a rate approximately 30 times slower than benzene. The relative percentages of monosubstituted products that are obtained when chlorobenzene is chlorinated, brominated, nitrated, or sulfonated are shown in Table 15.1. 

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. 

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... 

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. 

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... 

Suzuki, H. and Mori, T. Ozone-mediated nitration of chlorobenzene and bromo-benzene and some methyl derivatives with nitrogen dioxide - High ort/io-Directing trends of the chlorine and bromine substituents, J. Chem. Soc., Perkin Trans. 2, 479-484, 1994. 

When Cl, Br, or I is the substituent, there is a size mismatch, and therefore a poor overlap, between the 2p orbitals from the carbon atoms and the p orbitals from the halogen (3p for chlorine, 4p for bromine, and 5p for iodine). This size mismatch is clearly illustrated by comparing the reactivities of aniline and chlorobenzene chlorine and nitrogen have approximately the same electronegativity, but aniline is much more reactive than chlorobenzene because of the better overlap between the carbon and nitrogen 2p orbitals. 

Phenylarsenic oxychloride, CgHsAsOClj, may be prepared by treating the above tetrachloride with the requisite amount of water, or in quantitative yield by the action of chlorine on phenylarsenoxide. It is a white, crystalline substance, melting at about 100 C., fumes slightly in air, dianging to the acid. It is readily soluble in water, the acid crystallising from the solution. Heated at 120 C. it is decomposed into chlorobenzene and arsenious oxychloride. In a similar manner phenylarsenic oxybronvide is produced by the action of bromine on phenylarsenoxide. 

Introduction. In the preceding experiment it was shown that an aqueous solution of a diazonium salt treated with a solution of an alkali iodide gives iodobenzene. The replacement of the diazo group, N2X, by chlorine or bromine atoms involves the catalytic action of copper salts. Thus, benzenediazonium chloride warmed with a solution of cuprous chloride and hydrochloric acid gives chlorobenzene with cuprous bromide, bromobenzene is formed. The use of the cuprous salt is known as the Sandmeyer method. Instead of cuprous salts, finely divided copper may be used according to Gatterman s method. 

At low (xrnversion the only products detected result from the ipso substitution of chlorine for bromine. Fluorine was completely unreacrive. Equimolar amounts of chlorobenzene and of 3-hromofluorobenzene were obtained at low conversion. At longer contact times traces of by-products (fluorobenzraie and b izene) were found and some Cl and Br were lost, probably in tte firrm of Qa and of Bra (brown effluents). 

The reaction of benzene with bromine or chlorine in the presence of a Lewis acid catalyst (such as FeBr3, FeCl3 or A1C13) leads to the formation of bromobenzene or chlorobenzene, respectively. The Lewis acid, which does not have a full outer electron shell, can form a complex with bromine or chlorine. This polarises the halogen-halogen bond (making the halogen more electrophilic), and attack occurs at the positive end of the complex. 

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