Formic acid is more acidic than benzoic acid.
The acidity of a carboxylic acid is primarily determined by the stability of its conjugate base (the carboxylate ion) formed after deprotonation. A more stable conjugate base indicates a stronger acid. Acidity is commonly quantified by the pKa value; a lower pKa signifies a stronger acid.
Here's a comparison of their pKa values:
| Acid | Chemical Formula | pKa Value |
|---|---|---|
| Formic Acid | HCOOH | 3.77 |
| Benzoic Acid | C₆H₅COOH | 4.202 |
As the table shows, the pKa value of formic acid is lower than that of benzoic acid, clearly indicating that formic acid is a stronger acid than benzoic acid.
Why Formic Acid is More Acidic
The difference in acidity between formic acid and benzoic acid can be attributed to the nature of the substituent attached to the carboxyl group and its influence on the stability of the conjugate base:
-
Formic Acid (HCOOH):
- The carboxyl carbon in formic acid is bonded to a single hydrogen atom. A hydrogen atom has a negligible inductive or resonance effect on the carboxyl group.
- When formic acid loses its proton, it forms the formate ion (HCOO⁻). The negative charge on the formate ion is effectively delocalized through resonance between the two oxygen atoms.
- Furthermore, the small size of the hydrogen atom allows the formate ion to be very efficiently solvated by water molecules. This strong solvation significantly stabilizes the formate ion in aqueous solution, contributing to formic acid's higher acidity.
-
Benzoic Acid (C₆H₅COOH):
- In benzoic acid, a phenyl group (C₆H₅-) is attached to the carboxyl carbon. The phenyl group affects the stability of the benzoate ion (C₆H₅COO⁻) in a complex way:
- Inductive Effect: The phenyl group, due to its sp2 hybridized carbons, exhibits a weak electron-withdrawing inductive effect (-I effect). This effect would generally help stabilize the negative charge on the carboxylate group, which should increase acidity.
- Subtle Resonance Effect: Despite the inductive withdrawal, the pi electrons of the phenyl ring can also engage in a subtle electron-donating resonance effect (+R effect) towards the carbonyl carbon. This electron donation can increase the electron density around the carboxylate oxygens, slightly destabilizing the negative charge on the benzoate ion. This counteracts the stabilizing inductive effect.
- Steric and Solvation Effects: The relatively bulky phenyl group can also hinder the efficient solvation of the carboxylate group by surrounding water molecules compared to the smaller formate ion. Reduced solvation means less stabilization of the benzoate ion in solution.
- In benzoic acid, a phenyl group (C₆H₅-) is attached to the carboxyl carbon. The phenyl group affects the stability of the benzoate ion (C₆H₅COO⁻) in a complex way:
In summary, while the phenyl group in benzoic acid does have a weak electron-withdrawing inductive effect, this is outweighed by a subtle electron-donating resonance interaction and potentially less effective solvation due to its bulk. These factors collectively lead to the benzoate ion being less stable than the formate ion, making formic acid the stronger acid.
