Triphenylmethane

Triphenylmethane, or triphenyl methane, is a hydrocarbon. It is a colorless to light brown powder. It is the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence. It is insoluble in water and soluble in chloroform.

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The pKa of the hydrogen on the central carbon is around 31. Triphenylmethane is significantly more acidic than most other hydrocarbons because the planar trityl anion is stabilized by extensive delocalization over three phenyl rings. The trityl anion absorbs strongly in the visible region, making it red. This colour can be used as an indicator when maintaining anhydrous conditions with calcium hydride; the hydride reagent reacts with water to form solid calcium hydroxide, while it is also a strong enough base to generate the trityl anion. If the hydride is used up then the solution will turn colourless.

Triphenylmethane can be synthesized by Friedel-Crafts reaction from benzene and chloroform with aluminium chloride catalyst. Synthesis from benzylidene chloride, prepared from benzaldehyde and phosphorus pentachloride, is used as well.

Triarylmethane dyes

Examples of triarylmethane dyes are bromocresol green:

or malachite green: