1. _Replacement of -NH2 by -OH_:--The amine is diazotized and the aqueous solution of the diazonium salt is heated, nitrogen being eliminated and a phenol formed.
2. _Replacement of -NH2 by halogens and by the -CN and -CNO groups_:--The diazonium salt is warmed with an acid solution of the corresponding cuprous salt (T. Sandmeyer, _Ber._, 1884, 17, p. 2650), or with copper powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892, 25, p. 1074). In the case of iodine, the substitution is effected by adding a warm solution of potassium iodide to the diazonium solution, no copper or cuprous salt being necessary; whilst for the production of nitriles a solution of potassium cuprous cyanide is used. This reaction (the so-called "Sandmeyer" reaction) has been investigated by A. Hantzsch and J. W. Blagden (_Ber._, 1900, 33, p. 2544), who consider that three simultaneous reactions occur, namely, the formation of labile double salts which decompose in such a fashion that the radical attached to the copper atom wanders to the aromatic nucleus; a catalytic action, in which nitrogen is eliminated and the acid radical attaches itself to the aromatic nucleus; and finally, the formation of azo compounds.
3. _Replacement of -NH2 by -NO2_:--A well-cooled concentrated solution of potassium mercuric nitrate is added to a cooled solution of benzene diazonium nitrate, when the crystalline salt 2C6H5N2·NO3, Hg(NO2)2 is precipitated. On warming this with copper powder, it gives a quantitative yield of nitrobenzene (A. Hantzsch, _Ber._, 1900, 33, p. 2551).
4. _Replacement of -NH2 by hydrogen_:--This exchange is brought about, in some cases, by boiling the diazonium salt with alcohol; but I. Remsen and his pupils (_Amer. Chem. Journ._, 1888, 9, pp. 389 et seq.) have shown that the main product of this reaction is usually a phenolic ether. This reaction has also been investigated by A. Hantzsch and E. Jochem (_Ber._, 1901, 34, p. 3337), who arrived at the conclusion that the normal decomposition of diazonium salts by alcohols results in the formation of phenolic ethers, but that an increase in the molecular weight of the alcohol, or the accumulation of negative groups in the aromatic nucleus, diminishes the yield of the ether and increases the amount of the hydrocarbon formed. The replacement is more readily brought about by the use of sodium stannite (P. Friedlander, _Ber._, 1889, 22, p. 587), or by the use of a concentrated solution of hypophosphorous acid (J. Mai, _Ber._, 1902, 35, p. 162). A. Hantzsch (_Ber._, 1896, 29, p. 947; 1898, 31, p. 1253) has shown that the chlor- and brom- diazoniumthiocyanates, when dissolved in alcohol containing a trace of hydrochloric acid, become converted into the isomeric thiocyanbenzene diazonium chlorides and bromides. This change only occurs when the halogen atom is in the ortho- or para- position to the -N2- group.