Triarylmethane dye

Triarylmethane dyes are synthetic organic compounds containing triphenylmethane backbones. As dyes, these compounds are intensely colored. They are produced industrially as dyes.^[1]

Families

Triarylmethane dyes can be grouped into families according to the nature of the substituents on the aryl groups. In some cases, the anions associated with the cationic dyes (say crystal violet) vary even though the name of the dye does not. Often it is shown as chloride.

Methyl violet dyes

Methyl violet dyes have dimethylamino groups at the p-positions of two aryl groups.

• Methyl violet dyes
• 
  Methyl violet 2B
• 
  Methyl violet 6B
• 
  Methyl violet 10B

Fuchsine dyes

Fuchsine dyes have primary or secondary amines (NH₂ or NHMe) functional groups at the p-positions of each aryl group.

• Fuchsine dyes
• 
  Pararosaniline
• 
  Fuchsine (hydrochloride salt)
• 
  New fuchsine (As chloride)
• 
  Fuchsine acid

Phenol dyes

Phenol dyes have hydroxyl groups at the p positions of at least two aryl groups.

• Phenol dyes
• 
  Phenolphthalein
• 
  Phenol red
• 
  Chlorophenol red
• 
  Cresol red
• 
  Bromocresol purple
• 
  Bromocresol green

Malachite green dyes

Malachite green dyes are related to the methyl violet dyes, except that they contain one phenyl (C₆H₅) group.

• Malachite green dyes
• 
  Malachite green
• 
  Brilliant Green (dye)
• 
  Brilliant Blue FCF, a common food colorant.

Victoria blue dyes

Victoria blue dyes are related to the methyl violet dyes, except they contain one naphthylamino group. Variation is found is dimethylamine vs diethylamino substituents on the phenyl rings and variations of the secondary amine on the naphthyl group.

• Victoria blue dyes
• 
  Victoria Blue B
• 
  Victoria Blue FBR
• 
  Victoria blue BO
• 
  Victoria Blue FGA
• 
  Victoria blue 4 R
• 
  Victoria blue R

Bridged arenes

Where two of the aryl groups are bridged by a heteroatom, these triarylmethane compounds may be further categorized into acridines (nitrogen-bridged), xanthenes (oxygen-bridged), and thioxanthenes (sulfur-bridged).

Synthesis

The amine-containing dyes are often prepared from Mischler's ketone or its diethylamino analogue. In this way, the third aryl group is readily differentiated. The Friedel–Crafts alkylation reaction is a popular method to prepare many of the phenolic derivatives:

Applications

In addition to their dominant use as dyes, many of these dyes react reversibly with acid and base, and thus serve as pH indicators.^[1]

References

1. Gessner, Thomas; Mayer, Udo (2005), "Triarylmethane and Diarylmethane Dyes", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a27_179