Replacement of the Diazo Group by Chlorine

Introduction. In the preceding experiment it was shown that an aqueous solution of a diazonium salt treated with a solution of an alkali iodide gives iodobenzene. The replacement of the diazo group, N2X, by chlorine or bromine atoms involves the catalytic action of copper salts. Thus, benzenediazonium chloride warmed with a solution of cuprous chloride and hydrochloric acid gives chlorobenzene with cuprous bromide, bromobenzene is formed. The use of the cuprous salt is known as the Sandmeyer method. Instead of cuprous salts, finely divided copper may be used according to Gatterman s method. 

The catalytic action of the cuprous salts is assumed to depend on the formation of addition compounds which aid in the elimination of nitrogen and direct the halogen to the nucleus. In the Sandmeyer method the diazonium salt solution is added to a freshly prepared solution of the cuprous halide at a temperature below 10°. The mixture is then warmed until the evolution of nitrogen ceases. In some cases better yields are obtained if the diazonium solution is added slowly to a boiling solution of the cuprous salt. In the Gatterman method the desired diazonium salt is prepared. In the case of bromide and iodide, the diazonium sulfate is prepared and is mixed with an excess of potassium bromide or iodide. Finely divided copper is added and the mixture is warmed until the evolution of nitrogen ceases. The finely divided copper may be prepared by adding zinc dust to copper sulfate solution. The object of this experiment is to illustrate both methods. For semimicro work either chlorobenzene or o-chlorotoluene may be used. 

2 g of sodium chloride in 15 ml of water. Heat to effect solution, Add slowly a solution of 2 g of sodium bisulfite and 1 g of sodium hydroxide in 6 ml of water. Cool in tap water to about 20 and allow to stand for five minutes. Wash with water by decantation. Pour off most of the water, and add 3 ml of concentrated hydrochloric acid and 2 ml of water. Transfer to an Erlen-meyer flask, and add all at once the diazonium salt solution. Allow the mixture to stand for five minutes, and then heat gradually to 50-60 . Cool the solution, and remove the dark layer of chlorobenzene which separates out by means of a separatory tube. Extract the aqueous layer with 8 ml of ether, and unite the ether extract with the crude oily layer. Wash the mixture with 5 ml of water and 1 ml of sodium hydroxide solution and again with water. Dry with 0.5 g of calcium chloride, and distill. Collect the fraction which boils at 130-135 . The yield is about 2 g. 

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The present experiment illustrates the replacement of the diazo group by iodine, and Experiment 55 the use of cuprous halides in the replacement of the diazo group by chlorine. 

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