Dyestuffs Leuco forms

If amido- or hydroxyl-groups are introduced into triphenylmethane, in certain positions, colourless compounds, the leuco-derivatives of dyestuffs, are formed. 

Besides the reduced forms of reversible systems, e.g. hydro-quinones, -phenylendiamines, leuco forms of dyestuffs, hydrazo-benzenes, N-phenylhydroxylamines etc. oxidation waves have been described so far only for the derivatives containing an enediol group (e.g. ascorbic acid). The system formed during the oxidations of 5-hydroxychromane and of 5-hydroxycoumarane is similar to a quinoid system. 

A further dyestuff derivative of diamidotriphenylmethane is Quinoline Green [1], obtained by action of tetramethyldiamido-benzophenone, its chloride, or its hydrol on quinoline. Tn the last case a leuco-compound is formed. 

The Pyronines are a series of red basic dyestuffs of comparatively recent introduction. They are derivatives of diphenylmethane, and resemble the rhodamines in shade. A typical pyronine may be prepared as follows —Dimethylmetamidophenol is condensed with formaldehyde, and the resulting dioxytetramethyldiamido-diphenylmethane is treated with strong sulphuric acid, whereby one molecule of water is split off, and a leuco-base, tetramethyl-diamidodiphenylmethane-oxide, is formed, according to the equation —... 

As this compound is also formed by methylation of the leuco-compound of Lauth s violet, the relationship between the two dyestuffs is demonstrated. 

It is prepared by oxidation of dimethylparaphenylenediamine-thiosulphonic acid (see Methylene Blue) with ethyl-methylaniline, and boiling the resulting insoluble green compound with zinc-chloride solution. A leuco-compound is formed, and is converted into dyestuff by oxidation. The commercial product forms a reddish-brown powder. 

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