Colour in relation to chemical constitution of the organic and inorganic salts of isonitroso-pyrazolones and isooxazolones

Abstract

(1) Like violuric acid and diphenyl-violuric acid, isonitroso-3-methyl-isooxazolone and isonitroso-3-phenyl-isooxazolone have been found to form very intense,i.e., purple and magenta coloured salts with organic and inorganic bases, while the substances themselves are almost colourless and dissolve in water and organic solvents to colourless solutions. (2) This behaviour of the above-mentioned compounds on salt formation has been shown to be due to a fundamental change in the constitution of the molecules from oximino-ketonic to nitroso-enolic forms with consequent changes in the molecular strain and therefore the absorption of these substances. (3) The change of colour of these substances from colourless to purple or magenta is sufficiently sharp and intense for them to act as excellent indicators. (4) The validity of the hypothesis with regard to the colour phenomena of the above-mentioned isonitroso-iso-oxazolones is borne out by taking into consideration the organic and inorganic salts of isonitroso-3-methyl-pyrazolone and isonitroso-3-phenyl-pyrazolone. In these substances there is an alternative more acidic but less strained oximino-enolic configuration possible of existence, and consequently the salts of these isonitroso-pyrazolones have less strained structures and their colours also are far less intense than the corresponding salts of the isonitroso-isooxazolones mentioned before, in spite of all of them possessing very closely analogous molecular structures. The behaviour of all these substances therefore lends a further additional proof to the correctness of the "Theory of colour on the basis of molecular strain" advanced by Dutt.