Triphenylmethane

Chembox new
ImageFile = Triphenylmethane.png ImageSize =
IUPACName = Triphenylmethane
OtherNames = 1,1',1"-Methylidynetrisbenzene
Section1 = Chembox Identifiers
CASNo = 519-73-3
EINECS = 208-275-0
PubChem =
SMILES = c1ccccc1C(c2ccccc2)c3ccccc3
Section2 = Chembox Properties
Formula = C₁₉H₁₆
MolarMass = 244.33 g/mol
Appearance =
Density = 1.014 g/cm³
MeltingPt = 92-94 °C
BoilingPt = 359 °C
Solubility = insoluble
Section3 = Chembox Hazards
RPhrases = R36 R37 R38
ExternalMSDS = [http://ptcl.chem.ox.ac.uk/MSDS/TR/triphenylmethane.html External MSDS] | MainHazards =
FlashPt =
Autoignition =

Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C₆H₅)₃CH. This colorless solid is soluble in nonpolar organic solvents, but not 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: [OrgSynth | title = Triphenylmethane | author = J. F. Norris | collvol = 1 | collvolpages = 548 | prep = cv1p0548]

: 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. 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: [OrgSynth | author = W. B. Renfrow Jr and C. R. Hauser | title = Triphenylmethylsodium | collvol = 2 | collvolpages = 607| year = 1943 | prep = CV2P0607] :(C₆H₅)₃CCl + 2 Na → (C₆H₅)₃CNa + NaClBefore 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