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Ullmann reaction[′əl·mən rē‚ak·shən]
a method of obtaining binuclear and polynuclear aromatic compounds by heating aryl halides (ArX, where X = CI, Br, I) at 100°-360°C with powdered copper. One example of the Ullmann reaction is the synthesis of diphenyl from iodobenzene:
2C6H5I + Cu → C6H5 - C6H5 + Cul2
Iodine derivatives enter into the Ullmann reaction more readily. An increased yield is made possible by the activation of copper, for example, with iodine in acetone, and by the application of a solvent—dimethylformamide. It is assumed that the Ullmann reaction proceeds according to a free-radical mechanism. Discovered by F. Ullmann in 1896, the reaction is used, for example, in the manufacture of vat dyes.