PREPARATIONS 191 extended to the preparation of bromobenzene and of cyanobenzene or benzo-nitrile. Thus an aqueous solution of benzenediazonium chloride on treatment with a solution of cuprous bromide in hydrobromic acid gives nitrogen and bromobenzene: alternatively, on treatment with a solution of potassium cupro-cyanide, K 3 [Cu(CN) 4 ], it gives nitrogen and cyanobenzene (or benzonitrile), C 6 H 5 CN. No similar catalyst is required for the preparation of iodobenzene (p. 184). Potassium cupro-cyanide is the most convenient form in which cuprous cyanide can be used in Sandmeyer's Reaction. It is prepared by adding an excess of potassium cyanide to copper sulphate solution, whereby the cupric cyanide which is formed immediately breaks down to give cuprous cyanide and cyanogen, and the cuprous cyanide then dissolves in the excess of potassium CuSO 4 + zKCN = Cu(CN) 8-f K 2 SO 4 2Cu(CN) 2 = Cu 2 (CN) 2 + C 2 N 8 6KCN + Cu 2 (CN) 2 = 2K 8 [Cu(CN) 4 ] cyanide to give potassium cupro-cyanide. Required:
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