Dinitrochlorobenzene from chlorobenzene

In the preparation of 2,4-dinitrochlorobenzene from chlorobenzene, the excess nitric acid and sulfuric acid must be washed from the product. Which would you select for this purpose aqueous sodium hydroxide or aqueous sodium bicarbonate Why ... 

Equip a 1 Utre three-necked flask or a 1 litre bolt- head flask with a reflux condenser and a mercury-sealed stirrer. Dissolve 50-5 g. of commercial 2 4-dinitro-l-chlorobenzene in 250 ml. of rectified spirit in the flask, add the hydrazine solution, and reflux the mixture with stirring for an hour. Most of the condensation product separates during the first 10 minutes. Cool, filter with suction, and wash with 50 ml. of warm (60°) rectified spirit to remove unchanged dinitrochlorobenzene, and then with 50 ml. of hot water. The resulting 2 4-dinitrophenylhydrazine (30 g.) melts at 191-192° (decomp.), and is pure enough for most purposes. Distil oflF half the alcohol from the filtrate and thus obtain a less pure second crop (about 12 g.) recrystallise this from n-butyl alcohol (30 ml. per gram). If pure 2 4-dinitrophenylhydrazine is required, recrystallise the total yield from n-butyl alcohol or from dioxan (10 ml. per gram) this melts at 200° (decomp.). 

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. 

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. 

Intermediates derived from 2-chloro- and 4-chloronitrobenzene. These two products are produced simultaneously when chlorobenzene is nitrated with mixed acid (HNO3, H2O, H2SO4, 35 12 53). The 4-chloro compound predominates (65%), the remainder being the 2-chloro derivative. The isomers are separated by a combination of vacuum fractional distillation and fractional crystallization. On further nitration of either isomer 2,4-dinitrochlorobenzene is the product 1,4-dichlorobenzene gives 2,5-dichlo-... 

How to prepare 2,4-dinitrochlorobenzene Chlorine is o,/ -directing, and deactivating, so we should nitrate chlorobenzene. Chlorobenzene is readily prepared from benzene itself, so the synthesis is now complete (Figure 13.56). 

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