246 THE PREPARATION OF TINSYM METRICAL BIARYLS A fairly large container, such as a large wide-mouthed bottle, should be used for the diazo reaction in order to provide for the increase in volume thrpugh foaming of the mixture. The aqueous solution of sodium hydrox-ide (or sodium acetate) is added dropwise to the vigorously stirred mix-ture of the diazonium salt and aromatic liquid, which is kept at 5-10° throughout the reaction. Usually the mixture is stirred for an hour or two at this temperature after the addition has been completed; occa-sionally, the mixture is then allowed to warm up to room temperature and stirring is continued for a few hours longer. The layers are then separated and the product is isolated from the organic liquid. When it is difficult to remove traces of azo compounds from the biaryl by distillation or recrystallization, chemical means may be employed. Colorless 4-bromobiphenyl was obtained by treating an alcoholic solu-tion of the yellow product with a little zinc dust and hydrochloric acid. 60
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