Aromatic Colourless Systems

On the Theory of Dyes.—In carbon compounds absorption in the visible part of the spectrum, i.e. subjective colour, is conditioned by the presence of a so-called chromophoric group in the molecule. The nitroso-group is strongly chromophoric, the nitro-group much less so, whilst the azo-group is quite considerably chromophoric, but only in aromatic systems. Azomethane is colourless. 

Qualitative spot tests for aldehydes, in the presence of ketones, are generally only reliable for water-soluble compounds. This problem can be overcome by the use of 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (Purpald , Aldrich Chemical Company) in the presence of Aliquat (Scheme 5.27). Under aerial oxidation, the initially formed colourless cyclic adduct changes colour through red to purple. The colourless cyclic aminal can also be formed by ketones, but only the adducts derived from the aldehydes are oxidized to the purple bicyclic aromatic system [28]. Weakly electrophilic aldehydes, e.g., 4-methoxybenzaldehyde, reacts slowly, but will give the positive coloration upon gentle heating to ca. 70°C for one or two minutes. 

The chemical features of this system were established more than sixty years ago by Pratt and Perkins who found that TTP 4) crystallizes from ethanol or glacial acetic acid as thin colourless plates but from aromatic solvents, such as benzene, toluene, the xylenes or from solutions containing aromatic molecules as long yellow... 

There are two main types of indicator. One involves a complex of a metal ion, which changes colour when the oxidation state of the metal ion changes. The iron-1,10-phenanthroline system is a typical example, the iron(//) chelate (ferroin) being red, the iron(//7) chelate (ferrion) being essentially colourless (actually very pale blue). The transition potential is about 1,1 V, so the indicator is very suitable for use in titrations with cerium(Ty) in sulphuric acid. The second type of indicator comprises various types of organic compound (aromatic amines, triphenylmethane dyestuffs, for example) which can be oxidised and reduced reversibly, and change colour on doing so. 7V,iV -Diphenylbenzidine is a typical example. 

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