Triphenylmethane dyes. Theoretical considerations

The Basic Series.—In general, aromatic aldehydes condense with aromatic amines in the presence of zinc chloride to form triphenylmethane derivatives (0. Fischer) phenols and phenyl ethers behave similarly in the presence of concentrated sulphuric acid (Baeyer). The products formed are the leuco-compounds of well-known dyes. 

When pure, these leuco-compounds dissolve in acids or alkalis to 

The formation of the acid dye bmzaurin takes place in a quite analogous manner. p-Tetramethyldiaminobenzophenone (Michler s ketone), which is prepared technically from dimethylaniline and phosgene, can, like benzaldehyde, be condensed with dimethylaniline in the presence of phosphorus oxychloride. In this case, however, it is not a methane derivative, a leuco-compound, which is obtained, but, as the equation shows, the carbinol stage is at once reached. As will soon be shown, however, this stage in acid solution is equivalent to the formation of the dye. 

Michler s ketone is converted into the carbinol base of malachite green by the action of phenyl magnesium bromide (the Grignard reaction) 

The technical preparation of crystal violet and of its methyl-free parent substance, parafuchsine, almost the oldest of the triphenylmethane dyes, is not so easily explained. As is well known, in this process aniline and p-toluidine are united by oxidation in an acid melt. (In the preparation of fuchsine itself, which contains a methyl group attached to one of the benzene rings, o-toluidine is an additional ingredient.) Although all the phases of this important synthesis have not yet been experimentally established, we may nevertheless explain it on the basis of a dehydrogenation similar to that involved in the formation of malachite green. Moreover, the union of several molecules of base proceeds exactly according to the principle on which indamines are formed (p. 321) (Bucherer). 

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