Intrinsic acidity and electrophilicity of gaseous propargyl/allenyl carbocations

The ion/molecule chemistry of four representative propagyl/allenyl cations 1–4 of the general formula R1CH+-CC-R (a) ↔ R1CHCC+-R (b), that is, the reactive C3H3+ ions of m/z 39 from EI of propargyl chloride (H2C+-CC–H, 1a), isomeric C4H5+ ions of m/z 53 from EI of 3-butyne-2-ol (2a, H2C+-CC-CH3) and 2-butyne-1-ol (CH3-CH+-CC–H, 3a), and Ph-C3H2+ ions of m/z 115 from 3-phenyl-2-propyn-1-ol (H2C+-CC-Ph, 4a) was studied via pentaquadrupole mass spectrometry. With pyridine, proton transfer was observed as the predominant process for 1 and the sole reaction channel for the isomeric 2 and 3, whereas 4 reacted preferentially by adduct formation. These outcomes were rationalized using DFT calculations from isodesmic proton transfer reactions. Similar reaction tendencies were observed with acetonitrile and acrylonitrile, with adduct formation appearing again as a minor pathway for 1, 2 and 3, and as a major reaction channel for 4. With 1,3-dioxolane, hydride abstraction was a dominant reaction, with proton transfer and adduct formation competing as side reactions. With 2,2-dimethyl-1,3-dioxolane, an interplay of reactions including methyl anion abstraction, proton transfer, hydride abstraction and adduct formation were observed depending on the ion structure, with 4 reacting again mainly by adduct formation. Proton transfer was also observed as a dominant process in reactions with ethanol for 1, 2 and 3, with 4 being nearly unreactive whereas no adduct formation was observed for any of the carbocations studied. Limited reactivity was exhibited by these ions in cycloaddition reaction with isoprene.