Replacement of NH2 in aromatic compounds by Cl or Br Schwechten reaction

The Sandmeyer procedure and its Gattermann variant (see page 259) often give poor yields when applied to diphenylamines, naphthylamines, or phen-anthrenamines, but good results can be obtained by the method worked out by Schwechten1239 as a variant of the Schiemann reaction (thermal decomposition of diazonium fluoroborates, see below). 

Adding aqueous solutions of equal amounts of HgCl2 and KC1 or of HgBr2 [or Hg(N03)2] and KBr to solutions of the diazotized amine precipitates complex salts according to reaction (a) or (b) above. These salts are washed with acetone or with methanol and ether or simply dried in the air, then mixed with twice the weight of KC1, NH4C1, or KBr and 

The decomposition may be effected by cautious warming in a round-bottomed flask under an air-condenser, in a wide hard-glass tube, or even in an evaporating dish. The last of these techniques was used for preparation of 1-, 2-, and 3-chlorophenanthrene, and for the corresponding bromo compounds,1241 the amines having been diazotized at 0° by nitrosyl-sulfuric acid (prepared from NaN02 and a 2 1 mixture of concentrated H2S04 and water) to which pyridine solutions of the bases were added. 

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