The chemistry of some basic triphenylmethane dyes

Abstract

The effect of the nature of the terminal groups on some properties of triphenylmethane dyes has been investigated. Two series of dyes have been synthesised: compounds with two identical terminal groups, analogues of Malachite Green, and
those with three terminal groups, the Crystal Violet series. N,N-dimethylamino--, N,N-diethylamino-, pyrrolidino-,piperidino
and morpholino- grou'ps have been slected as terminal groups in order to give a wide variation in both structural and
basic characteristics.
Spectral data for both series have been obtained in 98% acetic acid. The variations mAa of the major absorption band and in the plC of the aminoftnyl moiety of the dyes have been discussed mnterms of the donor ability of the terminal group and have been related to the stability of the cationic species.
In the Crystal Violet series, addition of water to a solution of the dye in 98% acetic acid led to the appearance of a second band at shorter wavelength for dyes containing the more basic diethylamino- and piperidino- terminal groups. This is consistent with protonation of these groups, effectively converting the Crystal Violet type into the corresponding Malachite Green dye. Furthermore, the red shift of the major absorption band, relative to the parent compound, is in accord with the presence of an electron-withdrawing group in the 4-position of the phenyl ring. Substituent constants have been obtained for the -diethylanilinium and piperidinium groups from the appropriate }lammett plot.
The alkaline hydrolisis of Malachite Green and related dyes has been investigated. The observed trend in k 2 , the second order rate constant, correlates well with the relative abilities of the terminal groups to stabilise the cationic charge. The values obtained for Malachite Green and Brilliant Green show good agreement with values obtained in previous studies. The kinetic parameters,AHt and4St , have been obtained for each dye and variations in these values have been assessed. The implications of the unusual values of aC for the morpholine dye have been considered.
Morpholine Green has been found to undergo an extremely rapid hydrolysis and this necessitated a modification of the usual technique used in the kinetic experiments. The kinetic data for this dye, relative to other dyes, have been discussed in terms of the effect of the oxygen atom in the morpholine. group.