DIAZO-COMPOUNDS resemble Azo Compounds (see COAL-TAR COLORS), in contain ing the group —N,—, but with the difference that in the former two organic radicals are united to the characteristic group, as R.Ni.R, instead of one as in the latter.
These compounds were discovered in 1858 by Griess, who obtained them by treating aro matic amino-compounds with nitrous acid at low temperatures. (See AMINE). Following up his discovery he experimented with them in reaction with many substances, among these the amido-compounds, from which combinations were developed a series of brilliant direct dyes for animal fibres. The present economic value of the diazo-compounds lies in their availability as intermediates in the manufacture of dye stuffs, and in this function they are not usually separated from the solutions in which they are formed, but are promptly converted into the dyes by a further operation.
When isolated they are colorless crystals, which dissolve readily in water, sparingly in alcohol, and not at all in ether. They are in general unstable, and when dry are explosive.
They are prepared by dissolving a molecular part of the chosen amine in water with from two to three parts of hydrochloric acid, and then adding slowly, while stirring a solution of one molecular part of sodium nitrate, keeping the apparatus cold— at 35° to 40° F. When
the mixture shows a slight excess of nitrous acid, the diazotisation is complete. This solu tion is then added to a solution of one molecu lar part of some amine or phenol, stirring constantly. The dye thus formed in some cases precipitates, in others it has to be separated out by the addition of sodium chloride or hydro chloric acid. The benzcnoid and naphthal enoid amines are those most commonly diazo tised on a large scale for commercial uses. Diazo-compounds are also produced in a dry or paste form for the use of dyers in the pro duction of the so-called "ingrain" azo-colors, which are developed directly on the textile fibres. Consult Beacall, T., 'Dyestuffs and Coal Tar Products' (London 1915) ; Cain, J. C., 'The Chemistry of the Diazo-Compounds) (London 1908) ; Sidgwick, N. V., 'The Organic Chemistry of Nitrogen' (Oxford 1910).