Substituent Effects in the Gas Phase: 1-Substituted Allyl Anions

A series of 1-substituted allyl anions containing first-row, second-row, and resonance-stabilizing substituents has been examined in the gas phase with a variable-temperature flowing afterglow device. Several of these ions are weakly bound, undergo electron detachment readily, and can only be studied at subambient temperatures. The reactivity and thermodynamic properties of these species are reported. 3-Fluoropropene, 3-methoxypropene, 3-(dimethylamino)propane, and propene all have identical acidities. Electron-withdrawing and π-donating substituents (F, OCH₃, and N(CH₃)₂) do not destabilize allyl anion by increasing its basicity but rather by decreasing its electron-binding energy. There is, however, a linear correlation between acidity and electron affinity. Ab initio calculations reveal that the geometries of the allylic anions vary considerably as a result of a number of compensating factors, including inductive, resonance, and polarization effects. Consequently, linear free energy relationships are not applicable, and different substrates may respond to the same substituents in very different ways.