triphenylmethane

Triphenylmethane
IUPAC name	Triphenylmethane
Other names	1,1',1"-Methylidynetrisbenzene
Identifiers
CAS number	519-73-3 Y
EC number	208-275-0
SMILES	c1ccccc1C(c2ccccc2)c3ccccc3
Properties
Molecular formula	C19H16
Molar mass	244.33 g/mol
Density	1.014 g/cm3
Melting point	92-94 °C
Boiling point	359 °C
Solubility in water	insoluble
Hazards
MSDS	External MSDS
R-phrases	R36 R37 R38
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C₆H₅)₃CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane has the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence. A trityl group in organic chemistry is a triphenylmethyl group Ph₃C, e.g. triphenylmethyl chloride — trityl chloride.

Preparation

Triphenylmethane can be synthesized by Friedel-Crafts reaction from benzene and chloroform with aluminium chloride catalyst:

3 C₆H₆ + CHCl₃ → Ph₃CH + 3 HCl

Alternatively, benzene may react with carbon tetrachloride using the same catalyst to obtain the trityl chloride-aluminium chloride adduct, which is hydrolyzed with dilute acid:^[1]

3 C₆H₆ + CCl₄ + AlCl₃ → Ph₃CCl·AlCl₃

Ph₃CCl·AlCl₃ + HCl → Ph₃CH

Synthesis from benzylidene chloride, prepared from benzaldehyde and phosphorus pentachloride, is used as well.

Acidity

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. However, delocalization does not occur simultaneously over all phenyl rings, due to sterics: each phenyl ring forces the other two out of coplanarity to form a three-vaned fan, such that delocalization only occurs if the sp² orbital of the anionic carbon is properly aligned with one of the aromatic systems. 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. The sodium salt can be prepared also from the chloride:^[2]

(C₆H₅)₃CCl + 2 Na → (C₆H₅)₃CNa + NaCl

Before the popularization of butyllithium and related strong bases, trityl sodium was often used as a strong, non-nucleophilic base.

Triarylmethane dyes

Examples of triarylmethane dyes are bromocresol green:

or malachite green:

References