An experimental study of the reactivity and relative basicity of the methoxide anion in the gas phase at room temperature, and their perturbation by methanol solvation

Abstract

Flowing afterglow measurements at 296 ± 2 K are reported which explore three aspects of the gas-phase acid–base chemistry of the methoxide anion. Firstly, the intrinsic reactivity of this ion has been determined from measurements of rate constants for solvent-free proton-transfer reactions with molecules more acidic than methanol including CH2=C=CH2, C6H5CH3, C2H5OH, C2H2, CH3CN, CH3COCH3, CH3CHO, CH3NO2, and HCN. Secondly, equilibrium constant measurements have been performed for solvent-free proton-transfer reactions which provide a gas-phase scale of acidities for these molecules relative to the acidity of methanol. Finally, rate constants were measured for the reactions of these acids with methoxide ions solvated with up to three molecules of methanol. The results establish trends in reactivity as a function of step–wise solvation when relative acidity is preserved and when a reversal occurs in the relative acidity upon solvation.

The Ionic Hydrogen Bond

“…Solvation effects can compress, and sometimes reverse, the relative acidities and basicities. This was shown in early studies on the acidities of alcohols and the basicities of amines by Brauman and Blair, , Kebarle, McIver and co-workers, and Bohme and co-workers. ,, …”

“…This was shown in early studies on the acidities of alcohols and the basicities of amines by Brauman and Blair, 248,249 Kebarle, 5 McIver and co-workers, 250 and Bohme and co-workers. 11,251,252 Since these early measurements, the gas-phase acidities and basicities of thousands of compounds were compiled. 27,30,128,253 These data allow an analysis of structural effects on molecular acidities and basicities, and comparison with solution quantifies the solvation effects.…”

An experimental study of the influence of hydration on the reactivity of the hydroxide anion in the gas phase at room temperature

“…Also hydration of the OH-was observed to promote the reverse of reaction [7]. Measurements of the equilibrium constant for reaction [7] for 11 = I , 2, and 3 provided insight into the shift of the position of equilibrium for this reaction with hydration (6).…”

“…Measurements of the equilibrium constant for reaction [7] for 11 = I , 2, and 3 provided insight into the shift of the position of equilibrium for this reaction with hydration (6). Reaction [7] is rapid, k > lo-' cm' molecule-' s-I, for 17 = 0 to 3 and the product ions quickly undergo secondary reactions with CH30H which displace H,O in the solvation shell as is indicated by reaction [8]. In this study the measurements were repeated with OD-.…”

“…The differences in gas-phase acidity indicated in Fig. 6 represent average values derived from the absolute gas-phase acidities based on pulsed-1CR (16) and flowing afterglow measurements (7,17,18). Hydration of the hydroxide ion acts to decrease the reaction exoergicity and can render the reaction endoergic unless it is offset by the hydration energy of the conjugate base that is produced (and the hydration energy of possible hydrated neutral products).…”