Organic reaction mechanisms

In addition to the more sophisticated studies about multiply charged ions, transition-metal catalysis etc., also classical organic chemistry still offers some formidable challenges. An example in this respect is protonated benzene, for which experimental evidence has suggested some kind of "memory effect" in that the incoming proton is distinguished from the hydrogen atoms already present in the ring, which was interpreted as evidence for the existence of a π-complex. This suggestion has prompted immediate objections by various theoreticians, because it questions the well-established existence of the cyclohexadienylium ion (i.e. the π-complex of a proton with benzene). By preparative synthesis of appropriate precursor molecules in the laboratory in conjunction with dedicated photoionization experiments, we were able to resolve the previously existing discrepancies between experiment and theory.

Sketch of the proton-transfer experiments performed to probe the existence of a memory effect in protonated benzene. The key step is the selective production of a certain tautomer by a combination of threshold photoionization and selective isotope labeling. For details, see: Journal of Physical Chemistry A (2004).

Another example is the unexpected ion chemistry of the formal methanol adduct of Michler's hydrol blue. Upon addition of methanol, the free carbenium ion (1⁺) affords the protonated ether (2⁺) as an intermediate which than, in solution, rearranges to the energetically more stable form of a protonated aniline (3⁺). In the gas phase, however, the interconversion of 2⁺ and 3⁺ is quasi irreversible, and upon collision-induced dissociation (CID) not even a trace of the reverse reaction (3⁺ → 2⁺) is observed. Interestingly, it takes only a single molecule of an adequate base to catalyze the conversion of 3⁺ into the reactant ion 1⁺. Moreover, this process is overall exothermic because the carbenium ion 1⁺ can profit from stabilization by two donor-substituted arene substituents. This particular kind of ion/molecule reactions is referred to as "proton shuttle catalysis".