Bromination of nitrobenzene

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-... 

A nitro substituent is deactivating and meta-directing. The most stable cyclohexadienyl cation formed in the bromination of nitrobenzene is... 

Answer By bromination of nitrobenzene via procedure V-9. Thus, tile synthesis of XJI-g becomes... 

Other reactions go the same way so that bromination of nitrobenzene gives meta-bromo-nitrobenzene in good yield. The combination of bromine and iron powder provides the necessary Lewis acid (FeBr3) while the high temperature needed for this unfavourable reaction is easily achieved as the boiling point of nitrobenzene is over 200 °C. 

Bromo-4-nitrocumene, inter alia, is obtained similarly.617 The classical bromination of nitrobenzene by bromine and iron powder is recorded in detail in Organic Syntheses.618... 

Bromination of nitrobenzene is remarkably good, considering the unreactivity of nitrobenzene in electrophilic aromatic substitution. One recipe uses iron powder and bromine at 140 °C and gives 74% of the meta product. We shaU need these reactions in the next section. 

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. 

Write the structure(s) of the major product(s) that you expect from each of the following electrophilic aromatic substitutions, (a) Snlfonation of methoxybenzene (anisole) (b) bromination of nitrobenzene (c) nitration of benzoic acid (d) Friedel-Crafts acetylation of chlorobenzene. 

Bromination of benzene followed by nitration gives a mixture of o- and /-bromonitro-benzene. The meta isomer is not formed. However, nitration gives nitrobenzene, whose substituent directs subsequent electrophiles to the meta position. Thus, bromination of nitrobenzene followed by reduction of the product gives w-bromoanihne. 

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