4-bromonitrobenzene

Place 60 g. of o-ohloronitrobenzene and 75 g. of clean dry sand in a 250 ml. flask equipped with a mechanical stirrer. Heat the mixture in an oil or fusible metal bath to 215-225° and add, during 40 minutes, 60 g. of copper bronze or, better, of activated copper bronze (Section 11,50,4) (1). Maintain the temperature at 215-225° for a further 90 minutes and stir continuously. Pour the hot mixture into a Pjrox beaker containing 126 g. of sand and stir until small lumps are formed if the reaction mixture is allowed to cool in the flask, it wiU set to a hard mass, which can only be removed by breaking the flask. Break up the small Jumps by powdering in a mortar, and boil them for 10 minutes with two 400 ml. 

The experimental conditions for conducting the above reaction in the presence of dimethylformamide as a solvent are as follows. In a 250 ml. three-necked ilask, equipped with a reflux condenser and a tantalum wire Hershberg-type stirrer, place 20 g. of o-chloronitrobenzene and 100 ml. of dimethylformamide (dried over anhydrous calcium sulphate). Heat the solution to reflux and add 20 g. of activated copper bronze in one portion. Heat under reflux for 4 hours, add another 20 g. portion of copper powder, and continue refluxing for a second 4-hour period. Allow to cool, pour the reaction mixture into 2 litres of water, and filter with suction. Extract the solids with three 200 ml. portions of boiling ethanol alternatively, use 300 ml. of ethanol in a Soxhlet apparatus. Isolate the 2 2- dinitrodiphenyl from the alcoholic extracts a.s describe above the yield of product, m.p. 124-125°, is 11-5 g. 

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The protonation of the triplet jtjt state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a S y23Ar mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stem-layer. 

Photolytic. A carbon dioxide yield of 19.7% was achieved when bromobenzene adsorbed on silica gel was irradiated with light (X >290 nm) for 17 h (Freitag et al., 1985). Irradiation of bromobenzene in air containing nitrogen oxides gave phenol, 4-nitrophenol, 2,4-dinitrophenol, 4-bromophenol, 3-bromonitrobenzene, 3-bromo-2-nitrophenol, 3-bromo-4-nitrophenol, 3-bromo-6-... 

Bromination of nitrobenzene. Bromination of nitrobenzene can be effected by addition of KBrO., (slight excess) to the arene in aqueous H2S04 (1 1 v/v) at a rate such that the temperature docs not exceed 35". The 3-bromonitrobenzene thus obtained (76 91% yield) is contaminated with a slight amount of dibromonitroben-... 

The S 2 Ar mechanism is characterized by direct interaction between the aromatic halide in its excited state (in many cases a triplet state) and the nucleophile, leading to a o-complex. In its most simple form, the mechanism may be summarized as in equations 154 and 155. The photosubstitution of bromide by chloride in 3-bromonitrobenzene in... 

Fig. 29.10 H-NMR spectra of (a) methylbenzene (b) methoxybenzene (c) nitrobenzene (d) 1,4-dimethylbenzene (e) 4-methoxynitrobenzene (f) 4-amino-3-bromonitrobenzene (NH2 protons not shown).

Because 70 and 72 are generated much more slowly than 67, the major product is 3-bromonitrobenzene (56). Such an intermediate is very unstable, is difficult to form with a high activation energy barrier, and will react more slowly than benzene and much more slowly than derivatives bearing an electron-releasing group. Further, the arenium ion arising from attack at the ortho and para carbons is more stabilized and will form much more slowly than the less destabilized meta arenium ion, so the major product is meta. 

For an example of applying this capability in synthetic strategy, consider the preparation of 3-bromobenzenamine. Direct bromination of benzenamine (aniline) leads to complete ortho and para substitution (Section 16-3) and is therefore useless. However, bromination of nitrobenzene allows preparation of 3-bromonitrobenzene, which can be converted into the required target molecule by reduction. The outcome is a benzene derivative in which two ortho, para directors emerge positioned meta to each other. 

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