Origin of the acidity enhancement of formic acid over methanol: resonance versus inductive effects.

Density functional theory calculations were employed to study the relative contribution of resonance versus inductive effects toward the 37 kcal/mol enhanced gas-phase acidity (DeltaH degrees (acid)) of formic acid (1) over methanol (2). The gas-phase acidities of formic acid, methanol, vinyl alcohol (5), and their vinylogues (6, 8, and 9) were calculated at the B3LYP/6-31+G level of theory. Additionally, acidities were calculated for the formic acid and vinyl alcohol vinylogues in which the formyl group and the vinyl group, respectively, were perpendicular to the rest of the conjugated system. Comparisons among these calculated acidities suggest that inductive effects are the predominant effects responsible for the enhanced acidity of formic acid over methanol, accounting for between roughly 62% and 65% of the total enhanced acidity; the remaining 38% to 35% of the acidity enhancement appears to be due to resonance effects. Further comparisons suggest that resonance effects are between roughly 58% and 65% of the 26 kcal/mol calculated acidity enhancement of vinyl alcohol over methanol, and the remaining 42% to 35% are due to inductive effects.