Bromobenzenes, chlorination

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... 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

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] ... 

Stock and Baker2 5 9 measured the relative rates of chlorination of a number of halogenated aromatics in acetic acid containing 20.8 M H20 and 1.2 M HC1 at 25 °C and the values of the second-order rate coefficients (103Ar2) are as follows p-xylene (11,450), benzene (4.98), fluorobenzene (3.68), chlorobenzene (0.489), bromobenzene (0.362), 2-chlorotoluene (3.43), 3-chlorotoluene (191), 4-chloro-toluene (2.47), 4-fluorotoluene (9.70), 4-bromotoluene (2.47). Increasing the concentration of the aromatic, however, caused, in some cases, a decrease in the rate coefficients thus an increase in the concentration of chlorobenzene from 0.1 M to 0.2 M caused a 20 % decrease in rate coefficient, whereas with 4-chloro-and 4-bromo-toluene, no such change was observed. 

Besides benzyl chloride, methyl- and/or chlorine-substituted benzyl chlorides, phenethyl chloride, etc. are also successfully employed to give 2 -diaralkylaminofluorans in excellent yield. However, aryl halides such as chlorobenzene and bromobenzene hardly enable the reaction, though aryl iodides such as iodobenzene give 2 -diarylaminofluorans in low yield. 

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... 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

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. 

Most of the side reactions have already been discussed in Section 26.1.3.3. (dediazoniation in organic solvents). Chlorinated aliphatic solvents, such as 1,2-dichloroethane and dichloro-methane, lead to an extensive formation of chloroaromatics, and aromatic solvents, even halogenated examples, can be arylated to some extent by arenediazonium tetrafluoroborates (vide supra). For example, during dediazoniation of benzenediazonium tetrafluoroborate in toluene, chlorobenzene, bromobenzene or anisole, 3-5 % of substituted biphenyls Ph-C6H4-X (X = Me, Cl, Br, OMe) are formed together with 0.5-0.8% of fluorobiphenyls.5 Fluorobiphenyls are formed through an ionic pathway (only 2- and 4-isomers are formed) whereas chlorobiphenyls result from a radical process (X = Cl, 2-/3-/4-isomer 26 47 27).243... 

The relative reactivity of heteroaromatic compounds and their reactivity relative to the benzene system have not been extensively studied, but the reactivity sequences (decreasing) 2-chloroquinoline > 2-bro-mopyridine > bromobenzene,65 2-bromopyridine > 3-bromopyridine > 4-bromopyridine65 and the reactivity order I > Br > Cl > F65-106 allow a relative assessment of reactivity to be made. The relative ease of displacement of chlorine in differing environments is clearly shown in the dichloro compounds (28) and (29), in which the indicated chlorines are cleanly and preferentially replaced.104... 

The decompositions of bromobenzene [717] and chlorobenzene ions [716] have been studied by the special PIPECO experiment using variable source residence times. In the case of chlorobenzene, increasing the residence time from 0.7 to 8.9 ps resulted in a shift (kinetic shift) in the breakdown curves by 0.4 eV. Detailed analysis of the effects of varying residence time provided information on the k(E) vs. E curve in the vicinity of 104—106 s-1. The k(E) vs. E curve obtained differed significantly [by almost an order of magnitude in k(E) at some energies] from the curve reported in the earlier PIPECO study of metastable ions [22], The initial analysis [716] placed the critical energy for chlorine loss at 3.40 0.05 eV, but this has subsequently been revised to 3.19 0.02 eV [717]. The transition state was found to be loose . 

No information was located regarding toxic interactions of 1,1-dichloroethane with other xenobiotics. Evidence exists to indicate that 1,1- dichloroethane is detoxified by glutathione (Colacci et al. 1985). Thus, it is likely that other substances that deplete glutathione stores such as other chlorinated hydrocarbons (e.g. 1,1-dichloroethane and 1,2-dichloroethane), acetaminophen, and bromobenzene may enhance the toxicity of 1,1- dichloroethane. Substances that alter the activity of the microsomal enzymes that are responsible for the metabolism of 1,1 -dichloroethane may also affect the toxicity of this chemical. For example, it has been shown that ethanol increases the metabolism of 1,1 -dichloroethane in vitro (Sato et al. 1980). 

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