Triarylmethane dyes, intermediates

Unlike triarylmethane dyes, comparatively little work has been done with diaryl heteryl-, and aryl diheteryl methane compounds. Analogous to triarylmethanes, triheterylmethane dyes are also prepared using POCl3 and a ketone. The intermediate leuco compounds (similar to 65, see Scheme 8) are not isolated40 in the case of triheterylmethane leuco dyes. 

Dyes in general and triarylmethane dyes in particular are rarely subjected to chemical processing once they have been formed. The introduction of substituents is usually carried out during the manufacture of the intermediates where the position and number of the groups introduced may be more precisely controlled. Dyes are sometimes exposed to oxidizing and reducing conditions during application and afterward. 

During the 1860s, the manufacture of intermediates based on aniline and its congeners laid the foundations of the synthetic dyestuffs industry. These intermediates were used mainly in the production of triphenylmethane and azo dyes. At the end of the 20th century, the two most important dye classes were azo and anthraquinone vat dyes, though triphenylmethane (more correctly triarylmethane) dyes were, and remain, significant contributors to the overall output. Competition from fiber-reactive dyes has reduced demand for vat dyes. 

The presence of at least two sulphonic acid groups in the triarylmethane ring system permits the derivatives to be applied as acid dyes. Although sulphonic acid groups are often present in the intermediates used, acid dyes can also be obtained by direct sulphonation of the basic dye itself or at the leuco stage. 

The iV-subsii lined anilines, available since the mid-1860s, became important intermediates in the manufacture of both the new triarylmethane and the azo colorants. /v,/v-Dimethylanilinc (32) was used by Caro to synthesize the thiazine dye methylene blue (33), the first colorant for which a patent was granted under the new German patent law (Scheme 11). In 1877, Otto Fischer achieved the synthesis from benzaldehyde and 32... 

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