tritylium

Tritylium, or the triphenylmethyl cation, is the carbocation Ph3C+. It is a tertiary carbocation whose positive charge is stabilized by resonance with the π systems of the three adjacent phenyl rings. Although the charge is formally on the central carbon, it is delocalized over the three benzenoid rings, giving the trityl cation unusually high stability for a carbocation, especially in strongly acidic media or when balanced by a weakly coordinating counteranion.

Generation and isolation: The cation is typically generated in situ by ionization of triphenylmethyl chloride (trityl chloride) under Lewis acidic conditions, such as with AlCl3 or FeCl3, or in strong superacids. In non-nucleophilic solvents, it can be isolated as salts, for example [Ph3C]+ SbF6−, [Ph3C]+ BF4−, or [Ph3C]+ PF6−, which stabilize the cation with weakly coordinating anions.

Reactivity and applications: As a powerful electrophile, the tritylium cation can transfer the bulky trityl group to nucleophiles. It is the basis for the trityl protecting group used in organic synthesis to mask alcohols and amines; installation is carried out with trityl chloride or related reagents and base, and deprotection proceeds under acidic conditions to reveal the unprotected functionality. The trityl group is valued for stability under base and many reaction conditions, and for its acid-labile removal.

Related compounds and terminology: The chemistry centers on the triphenylmethyl cation and its salts, and the protective group derived from it, commonly abbreviated as Tr or Trt in protecting-group schemes.