Replacement of hydrogen by halogen in aromatic compounds general

Replacement of hydrogen by halogen in aromatic compounds general 

The ease with which aromatic compounds, and heterocyclic ones that resemble aromatic ones, are halogenated in the nucleus depends greatly on the substituents already attached to the ring. If these are halogen, COOH, S03H, or N02 a catalyst must usually be used, whereas OH, O-Alkyl, and NH2, for instance, favor nuclear halogenation so much that it can be effected even without a catalyst. 

Indirect routes must be used to introduce halogen into the meta-position of aromatic compounds containing substituents of the first class or into the orthojpara-positions of those with substituents of the second class. 

Thus 1,3,5-tribromobenzene is prepared from 2,4,6-tribromoaniline, which is very easily obtained by leading a stream of air saturated with bromine vapor into an aqueous solution of aniline hydrochloride the tribromoaniline is diazotized and boiled with ethanol, as described in detail in Organic Syntheses.371 

6-Tribromobenzoic acid is similarly obtained from 3-amino-2,4,6-tribromobenzoic acid the NH2 group is replaced by H when the amino acid is diazotized in the presence of hypophosphorous acid. For details see Organic Syntheses 12 

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