trityl

Triphenylmethane or triphenyl methane (sometimes also known as Tritan), is the hydrocarbon with the formula (C₆H₅)₃CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane is 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) and the triphenylmethyl radical (trityl radical).

Preparation

Triphenylmethane was first synthesized in 1872 by the German chemist August Kekulé and his Dutch student Antoine Paul Nicolas Franchimont (1844–1919) by heating diphenylmercury (Hg(C₆H₅)₂, ''Quecksilberdiphenyl'') with benzal chloride (C₆H₅CHCl₂, ''Benzylenchlorid'').

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 triphenylmethyl chloride–aluminium chloride adduct which is then treated with diethyl ether for 24 hours at room temperature and hydrolyzed with concentrated hydrochloric acid:
:3 C₆H₆ + CCl₄ + AlCl₃ → Ph₃CCl·AlCl₃
:Ph₃CCl·AlCl₃ + Et₂O + HCl → Ph₃CH

It can also be synthesized from benzylidene chloride, which is prepared from benzaldehyde and phosphorus pentachloride.

Reactions of C-H bond

The Ph₃C-H bond is relatively weak, with a bond dissociation energy (BDE) of 81 kcal/mol, or about 24 kcal/mol less than methane. Correspondingly, triphenylmethane is mildly acidic, with a p''K''_a of 33.297.

Triphenylmethane is significantly more acidic than most other hydrocarbons because the charge is delocalized over three phenyl rings. Steric effects however prevent all three phenyl rings from achieving coplanarity simultaneously. Consequently diphenylmethane is even more acidic, albeit only slightly, because in its anion the charge is spread over two phenyl rings at the same time.

The trityl anion is isolable in crown ethers:
:
Its sodium salt can be prepared from the chloride:
:(C₆H₅)₃CCl + 2 Na → (C₆H₅)₃CNa + NaCl

The use of tritylsodium as a strong, non-nucleophilic base has been eclipsed by the popularization of butyllithium and related strong bases.

The unmodified anion is red, and can be used as an indicator in acid–base titrations. Derived substances have proven useful as chemical dyes.

Triarylmethane dyes

Examples of triarylmethane dyes are bromocresol green:
:

And the nitrogen-bearing malachite green:
:

Trityl group

Protecting group

The triphenylmethyl substituent, also called trityl after a 1927 suggestion by Helferich et al., is widely used in organic chemistry. Trityl serves as a protecting group for alcohols.
: protection (requires proton acceptor): Ph₃CCl + ROH → Ph₃COR + HCl
: deprotection: Ph₃COR + HBr → ROH + Ph₃CBr

Platform for unusual functional groups

Trityl derivatives of reactive functional groups are often crystalline and in some cases sterically stabilized relative to less bulky derivatives. Three such derivatives are ''S''-nitrosotriphenylmethanethiol (Ph₃CSNO), tritylsulfenyl chloride (Ph₃CSCl), and trityl sulfenamide (Ph₃CSNH₂).

See also

* Tetraphenylmethane
* Triphenylmethanol
* Triphenylmethyl chloride
* Triphenylmethyl hexafluorophosphate
*Triphenylmethyl radical

References

{{Reflist

Aromatic hydrocarbons
Phenyl compounds
Substances discovered in the 19th century