The structure of the products of reaction of 2‐oxo‐3‐dicyanomethylidene‐2,3‐dihydroindole (1a) and 2‐oxo‐3‐cyanoethoxycarbonylmethylidene‐2,3‐dihydroindole (1b) with the pyrazolin‐5‐one derivatives 2a,b could be established via 13C nmr and high resolution 1H nmr to be the spiropyranylindolone derivatives 5a‐d.
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In conformity with the assumption that light fastness depends upon the electron density of the azo centre, it is now shown that condensation of the aldehyde function in p‐aryl azo salicylaldehydes with hydroxylamine, semicarbazide, aniline, phenylhydrazine, 2,4‐dinitrophenylhydrazine and ethylene diamine causes in most cases an increase in light fastness on nylon 6 as a result of the decreased electron attracting properties of the functions as compared with the aldehyde function. Aftertreatment of dyeings from arylazo salicylaldehydes or arylazo acetophenone with ethylene diamine causes a similar increase. Further confirmation of this observation is obtained in case of monoazo dyestuffs derived from salicylaldehyde and derivatives which contain the sulphonic acid group when applied as acid dyestuffs on Nylon 6. Application of the latter dyes on wool revealed an analogous tendency only in case of dyes derived from 2‐naphthylamine‐4, 8‐disulphonic acid.
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