Chlorine of chlorobenzenes

The formation of benzyne as an intermediate was first postulated by Roberts et al. (1953) for the amino-de-chlorination of chlorobenzene (see Roberts, 1990, p. 110). 

Recently Mikae et al. (66) showed that NaY zeolite impregnated with NaCl solution can be used as a para selective catalyst for chlorination of chlorobenzene, which is indeed another example of zeolite modified advantageously by means of monolayer dispersion of salts. 

The electrophilic aromatic substitution of aryl halides takes place less readily than with benzene (electron-withdrawing effect), but occurs at the ortho and para positions (the lone pairs on the halogen assist in delocalizing the positive charge in the intermediate). Further chlorination of chlorobenzene, in the presence of aluminium or iron trichlorides, gives 1,4-dichlorobenzene and some 1,2-dichlorobenzene. Nitration normally occurs to give the 2- and 4-nitro- and 2,4-dinitrochlorobenzenes (Scheme 4.13). 

The aim of the present study is to enhance the selectivity for para-products using zeolite catalysts. The present paper describes the results of the chlorination of chlorobenzene, toluene, 1,2-dichlorobenzene and naphthalene using zeolites as catalysts and CI2 as the chlorinating agent. 

More progress has been reported on the halogenation of aromatic compounds over zeolite catalysts. Jang et al. [55] studied the vapor phase catalytic chlorination of chlorobenzene using solid-acid catalysts such as silica-alumina, alumina, zeolites and modified clay (bentonite) impregnated with FeClj Dichlorobenzene selectivity was higher over the zeolite catalysts. 

Draw all possible contributing structures to the carbocation intermediate in the chlorination of chlorobenzene. Explain why the major products are 0- and p-dichlorobenzene. Note p-Dichlorobenzene is produced commercially this way, for use against clothes moths.)... 

Does chlorination of chlorobenzene require the use of a Lewis acid Explain why or why not ... 

Unlike the case of benzene in which ionization involves loss of a tt electron from the ring electron impact induced ionization of chlorobenzene involves loss of an elec tron from an unshared pair of chlorine The molecular ion then fragments by carbon-chlorine bond cleavage... 

The peak at m/z 77 m the mass spectrum of chlorobenzene m Figure 13 41 is attributed to this fragmentation Because there is no peak of significant intensity two atomic mass units higher we know that the cation responsible for the peak at m/z 77 cannot contain chlorine... 

A solution to the question of the mechanism of these reactions was provided by John D Roberts m 1953 on the basis of an imaginative experiment Roberts prepared a sample of chlorobenzene m which one of the carbons the one bearing the chlorine was the radioactive mass 14 isotope of carbon Reaction with potassium amide m liquid... 

Dinitrochlorobenzene can be manufactured by either dinitration of chlorobenzene in filming sulfuric acid or nitration ofy -nitrochlorobenzene with mixed acids. Further substitution on the aromatic ring is difficult because of the deactivating effect of the chlorine atom, but the chlorine is very reactive and is displaced even more readily than in the mononitrochlorobenzenes. 

CgH Cl, is produced commercially in the Hquid phase by passing chlorine gas into benzene in the presence of molybdenum chloride at 30—50°C and atmospheric pressure. This continuous process yields a 14 1 ratio of chlorobenzene to j -dicblorobenzene [106-46-7J, The reaction of iodine with... 

The chlorination of benzene can theoretically produce 12 different chlorobenzenes. With the exception of 1,3-dichlorobenzene, 1,3,5-trichlorobenzene, and 1,2,3,5-tetrachlorobenzene, all of the compounds are produced readily by chlorinating benzene in the presence of a Friedel-Crafts catalyst (see Friedel-CRAFTS reactions). The usual catalyst is ferric chloride either as such or generated in situ by exposing a large surface of iron to the Hquid being chlorinated. With the exception of hexachlorobenzene, each compound can be further chlorinated therefore, the finished product is always a mixture of chlorobenzenes. Refined products are obtained by distillation and crystallization. 

Chlorobenzenes were first synthesized around the middle of the nineteenth century the first direct chlorination of benzene was reported in 1905 (1). Commercial production was begun in 1909 by the former United Alkali Co. in England (2). In 1915, the Hooker Electrochemical Co. at Niagara EaUs, New York, brought on stream its first chlorobenzenes plant in the United States with a capacity of about 8200 metric tons per year. 

All of the chlorobenzenes are now produced by chlorination of benzene in the Hquid phase. Ferric chloride is the most common catalyst. Although precautions are taken to keep water out of the system, it is possible that the FeCl3H20 complex catalyst is present in most operations owing to traces of moisture in benzene entering the reactor. This FeCl3H20 complex is probably the most effective catalyst (13). 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) > 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the... 

With 77 % aqueous acetic acid, the rates were found to be more affected by added perchloric acid than by sodium perchlorate (but only at higher concentrations than those used by Stanley and Shorter207, which accounts for the failure of these workers to observe acid catalysis, but their observation of kinetic orders in hypochlorous acid of less than one remains unaccounted for). The difference in the effect of the added electrolyte increased with concentration, and the rates of the acid-catalysed reaction reached a maximum in ca. 50 % aqueous acetic acid, passed through a minimum at ca. 90 % aqueous acetic acid and rose very rapidly thereafter. The faster chlorination in 50% acid than in water was, therefore, considered consistent with chlorination by AcOHCl+, which is subject to an increasing solvent effect in the direction of less aqueous media (hence the minimum in 90 % acid), and a third factor operates, viz. that in pure acetic acid the bulk source of chlorine ischlorineacetate rather than HOC1 and causes the rapid rise in rate towards the anhydrous medium. The relative rates of the acid-catalysed (acidity > 0.49 M) chlorination of some aromatics in 76 % aqueous acetic acid at 25 °C were found to be toluene, 69 benzene, 1 chlorobenzene, 0.097 benzoic acid, 0.004. Some of these kinetic observations were confirmed in a study of the chlorination of diphenylmethane in the presence of 0.030 M perchloric acid, second-order rate coefficients were obtained at 25 °C as follows209 0.161 (98 vol. % aqueous acetic acid) ca. 0.078 (75 vol. % acid), and, in the latter solvent in the presence of 0.50 M perchloric acid, diphenylmethane was approximately 30 times more reactive than benzene. 

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. 

Figure 8.9 Dynamics of STM-driven desorption and dissociation of chlorobenzene at Si(lll) (7 x 7) (a) before and (b) after a desorption scan the circles indicate the positions of chlorobenzene molecules before and after desorption (c) appearance of a chlorine adatom formed by dissociation of chlorobenzene with corresponding 3D image (d) measured rates of desorption and dissociation as a function of tunnelling current for a sample bias of + 3 V. (Reproduced from Ref. 26).

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... 

With the exception of chlorobenzene and 1,2-dichloroethane, halocarbon solvents are unsuitable diluents, as carbon tetrachloride and chloroform may react violently with alkylalumimum derivatives. The hazards of individually mixing 7 alkyla-luminiums with 7 chlorinated solvents have been assessed comparatively. Most of a series of cyclic coordination complexes between triethylaluminium and a-iminoketones decomposed violently when dissolved in halogenated solvents. 

Apart from the need for isotopic enrichment and synthesis there are other problems in applying whole molecule mass spectrometry to measure isotope ratios. Assume, for example, that we want to determine isotopic composition of chlorine from the spectrum of chlorobenzene presented in Fig. 7.7. The peaks at 114 and... 

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