Triphenylmethane
Template:Chembox 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).
PreparationEdit
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).<ref>Aug. Kekulé and A. Franchimont (1872) "Ueber das Triphenylmethan" (On triphenylmethane), Berichte der deutschen chemischen Gesellschaft, 5 : 906–908.</ref>
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:<ref>Template:Cite journal</ref>
- 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 bondEdit
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.<ref>Template:Cite journal</ref> Correspondingly, triphenylmethane is mildly acidic, with a pKa of 33.297.<ref>Template:Cite journal</ref>
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:<ref>Template:OrgSynth</ref>
- (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 dyesEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Examples of triarylmethane dyes are bromocresol green:
And the nitrogen-bearing malachite green:
Trityl groupEdit
Protecting groupEdit
The triphenylmethyl substituent, also called trityl after a 1927 suggestion by Helferich et al.,<ref>Template:Cite journal</ref> is widely used in organic chemistry. Trityl serves as a protecting group for alcohols.<ref>Template:Cite journal</ref>
- protection (requires proton acceptor): Ph3CCl + ROH → Ph3COR + HCl
- deprotection: Ph3COR + HBr → ROH + Ph3CBr
Platform for unusual functional groupsEdit
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).<ref>Template:Cite journal</ref>
See alsoEdit
- Tetraphenylmethane
- Triphenylmethanol
- Triphenylmethyl chloride
- Triphenylmethyl hexafluorophosphate
- Triphenylmethyl radical