Nitration of chlorobenzenes

A mixture of the two mononitro-chlorobenzenes is prepared by nitration of chlorobenzene. Further nitration of the mixture or of either of the mononitro-compounds gives 2,4-dinitrochlorobenzene, m.p. 5 C, b.p. 315"C. 

In acetic acid the rates of nitration of chlorobenzene and bromo-benzene were fairly close to being first order in the concentration of aromatic, and nitration fully according to a first-order law was observed with O, m-, and/i-dichlorobenzene, ethyl benzoate and 1,2,4-trichloro-benzene. 

Further evidence that the nitronium ion was not the electrophile in the uncatalysed reaction, and yet became effective in the catalysed reaction, came from differences in the orientation of substitution. The nitration of chlorobenzene in the uncatalysed reaction yielded only 43 % of the para compound, whereas, when the catalysed reaction was made important by adding some nitric acid, the ratio of substitution was that usually observed in nitration involving the nitronium ion ( 5.3.4). In the case of the uncatalysed reaction however, the reaction was complicated by the formation of nitrophenols. 

Using the partial rate factors for nitration of chlorobenzene, Ridd and de la Mare calculated the relative rates of nitration of the dichlorobenzenes, with respect to /)-dichlorobenzene, with the results shown below. Also given are results based on more recent nitrations in mixed... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

Nitration of chlorobenzenes, mosdy monochlorobenzene in the United States, with nitric acid has wide industrial appHcations. 

Figure 16.15 Carbocation intermediates in the nitration of chlorobenzene. The ortho and para intermediates are more stable than the meta intermediate because of electron donation of the halogen lone-pair electrons.

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

Halogen substituents withdraw electron density from the aromatic nucleus but direct olp-through resonance effects. The result is that halobenzenes undergo nitration with more difficulty relative to benzene. The nitration of chlorobenzene with strong mixed acid gives a mixture of 2,4- and 2,6-isomeric dinitrochlorobenzenes in which the former predominates." The nitration of 2,4-dinitrochlorobenzene to 2,4,6-trinitrochlorobenzene (picryl chloride) requires an excess of fuming nitric acid in oleum at elevated temperature. Both are useful for the synthesis of other polynitroarylene explosives but only 2,4-dinitrochlorobenzene finds industrial importance (Sections 4.8.1.2 and 4.8.1.3). 

Dinitrochlorobenzene is an industrially important chemical synthesized from the nitration of chlorobenzene with mixed acid. The halogen atom of 2,4-dinitrochlorobenzene is activated by two o/jt-nitro groups and is particularly reactive. Consequently, 2,4-dinitrochlorobenzene is used as a cheap and readily available starting material for the synthesis of many explosives. 

Orientation Similarly, groups already present on the benzene ring direct the orientation of the new substituent to ortho, para or meta positions. For example, nitration of chlorobenzene yields ort/io-nitrochlorobenzene (30%) and para-nitrochlorobenzene (70%). 

Dinitro-1-chlorobenzene or l,3-Dinitro-4 -chlorobenzene. Three polymorphs exist orthorhmb crysts or alpha(stabile form), mp 53.4°, bp 315°, d 1.697 at 22°/4° vsol in hot ale, eth, benz or CS2 si sol in ale insol in w can be prepd by nitration of chlorobenzene with mixed acid(Refs 1, 2a, 5, 9, 10) rhmb crysts or beta(labile form), mp 4 3°, d 1.680 at 20°/4° solubility same as alpha can be prepd by rapid cooling of the alpha form(Ref 3) and gamma, mp 27°(Refs 1 10). Halpern(Ref 8) reported an expln occurred during distillation of 2 4 -DNCB at 1 mm press... 

The nitration of chlorobenzene is easier than the nitration of benzene and more difficult than the nitration of toluene. Trinitro-chlorobenzene (picryl chloride) can be prepared on the plant scale by the nitration of dinitrochlorobenzene, but the process is expensive of acid and leads to but few valuable explosives which cannot be procured more cheaply and more simply from dinitrochlorobenzene by other processes. Indeed, there are only two important explosives, namely TNB and hexanitrobiphenyl, for the preparation of which picryl chloride could be used advantageously if it were available in large amounts. In the laboratory, picryl chloride is best prepared by the action of phosphorus pentachloride on picric acid. 

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. 

Similar results have been obtained in the nitration of chlorobenzene, as shown in Table 4. 

The nitration of chlorobenzene was first mentioned in 1862 by Riche [6], Later the reaction was studied by Sokolov [7] and Holleman and de Bruyn [8], Junglleisch [9] was the first to describe the method of preparation of chlorodinitrobenzene from o- or p- chloronitrobenzene. Laubenheimer [10] investigated the products of the nitration of m- chloronitrobenzene, and Ostromyslenskii [11] determined the structure of the products obtained by Jungfleisch, i.e. of 1,2,4- and 1,2,6-chloro-dinitrobenzenes. Finally in 1894 the Griesheim factory reported [12] on a method of preparing chlorotrinitrobenzene by direct nitration of chlorobenzene. 

The o- and p- isomers can be obtained during the nitration of chlorobenzene. The higher the nitration temperature, the more o- isomer is formed. 

Among the six chlorodinitrobenzenes known, the 1,2,4- and 1,2,6-isomers are the most important as they are the principal products of the nitration of chlorobenzene. l-Chloro-2,4-dinitrobenzene results from the nitration of o- and p- chloronitrobenzenes and l-chloro-2,6-dinitrobenzene from the o- isomer. Apart from these, the 1,3,4-isomer, which forms in the nitration of m- chloronitrobenzene, is of some importance. 

The nitration of chlorobenzene to chlorodinitrobenzene without separating the isomers of chloronitrobenzene is accomplished in two successive stages in the same nitrator. 

The following diagram represents the complete process of nitration of chlorobenzene to trinitro derivatives ... 

Reaction at the meta position gives a sigma complex whose positive charge is not delocalized onto the halogen-bearing carbon atom. Therefore, the meta intermediate is not stabilized by the halonium ion structure. The following reaction illustrates the preference for ortho and para substitution in the nitration of chlorobenzene. 

Zeolites have been used before in the vapour phase nitration of aromatic compounds using nitrogen dioxide.20 However, the conditions were harsh and there was no regioselectivity. Initially, therefore, we attempted to reproduce the mild conditions of Suzuki for nitration of chlorobenzene. Liquid N2O4 (approx. 10 ml) was condensed into a trap at -78 °C and was then warmed to 0 °C. Fe(acac)3 (0.355 g) and chlorobenzene... 

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