Reactions involving replacement of the diazo group

Diazonium salts undergo a large number of reactions in which the diazo group is lost as molecular nitrogen and is replaced by a variety of other groups (e.g. OH, I, Br, Q, F, CN, N02, S02H, Ar and H) which become attached to the aromatic ring. This section describes the experimental conditions necessary to effect such conversions. 

When a solution of a diazonium salt is heated, nitrogen is evolved and the diazo group is replaced by a hydroxyl group in an SN1 type of displacement reaction. 

The diazonium sulphate is used in preference to the diazonium chloride, since the presence of chloride ions gives rise to small quantities of the aryl chloride as a by-product. The solution must be acidic in order to avoid the coupling reaction between unreacted diazonium salt and the phenol (see Section 6.7.2, p. 946). For the preparation of phenols and cresols, the aqueous solution of the diazonium compound is warmed to about 50 °C at higher temperatures the reaction may become unduly vigorous and lead to appreciable quantities of tarry compounds. For certain substituted amines, a higher temperature (e.g. boiling 40 60% sulphuric acid) is necessary to decompose the diazonium salt 

When an aqueous solution of an aryldiazonium salt is treated with an equivalent of potassium iodide and warmed on a water bath, the aryl iodide is formed in good yield (e.g. iodobenzene, Expt 6.70). 

The aryl chloride is formed when the diazonium-copper(i) chloride complex decomposes by a radical mechanism summarised below. Copper catalyses this decomposition because it can undergo interconversion between the +1 and + 2 oxidation states as a result of electron transfer. 

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