1-Phenylethylium cation

Nikolayev and collaborators have restudied the polymerisation of styrene by trifluoroacetic acid and H(CF2)6COOH. Limiting yields were often obtained, coinciding with the total consumption of the acid, and polymerisation could be reinitiated at this point by addition of further acid. It seems obvious from the ensemble of their results that although the ester is inactive in the absence of free acid, the active species are probably ester molecules solvated by the add, and not carbenium ions. The dielectric constant in these reactions was in fact low and it is difficult to envisage that the 1-phenylethylium cation could survive in these media. 

Fig. 5.P18b. Reaction profile for ionization and protonation routes to 1-phenylethylium cation. Relative energies are in kcal/mol. Reproduced from the Bulletin of the Chemical Society of Japan., 71, 2427 (1998).

Bond length Protonated Alcohol TSl Phenylethylium Cation-HjO Complex TS2 Styrene-H3O+ Complex Styrene... 

Simpler procedures are of course available for the preparation and characterisation of carbenium ions in solution, particularly for the more stable ones. Concentrated sulphuric acid was extensively used as protogenic medium before the superacid mixtures were shown to be superior, but many of the spectroscopic assignements in those earlier studies were later proved erroneous, particularly in the case of such reactive entities as the 1-phenylethylium ion Model monomers which cannot polymerise because of steric hindrance can generate fairly stable carbenium ions by interacting with Lewis or Br nsted acids in normal cationic polymerisation conditions. Thus, 1,1-diphenylethylene and its dimer, and 1,1-diphenylpropene give rise to typical visible absorption bands from which the concentration of the corresponding diphenyl-methylium ions can be accurately calculated. As for carbenium ions capable of forming stable salts, their synthesis and characterisation is obviously easy. 

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