Base-Initiated Aldol Condensations in the Gas Phase

The enolate anion CH₂=CHO⁻ reacts with acetaldehyde in the gas nhase at ca. 0.1 Torr to produce a “stable” adduct that is amenable to study by collisional activation and tandem mass spectrometry (MS/MS). The activated adduct and a reference ion HCOCH₂CH(CH₃)O⁻ both decompose by elimination of methane and water and by a retro reaction to re-form CH₂=CHO⁻. Although the kinetic energy releases associated with the decompositions and the charge reversed spectra of the adduct and reference are nearly identical, the extent of water and methane loss is considerably attenuated for the ionmolecule reaction adduct. These two losses are assigned to be characteristic of a species of tetrahedral geometry. The adduct undergoes these losses less abundantly because, in addition to existing as a tetrahedral complex, it also is formed as ion-dipole and proton-bound species. The enolate of acetone reacts in a similar way with neutral acetone to give also three adducts: ion-dipole, proton-bound, and tetrahedral complexes.