Textboo 6C.21 (b)

[Chanel Mao 3D]

6C.21 Suggest an explanation for the different strengths of acetic acid and formic acid.

The textbook states that formic acid is stronger than acetic acid. Can someone explain why the single H atom in formic acid (HCOOH) accounts for higher acidity strength than the CH3 group in acetic acid (CH3COOH)?

[Eva Becker]

Hi! The methyl (CH3) group in acetic acid is contributing more electron density to the COOH group. This makes it harder for the OH bond in acetic acid to break. Since formic acid doesn't have that methyl group to stabilize and weaken the acid, it is stronger.

[Tae Pasawat 2A]

Hi! The methyl (CH3) group in acetic acid is contributing more electron density to the COOH group. This makes it harder for the OH bond in acetic acid to break. Since formic acid doesn't have that methyl group to stabilize and weaken the acid, it is stronger.

Hi Eva!

I just wanted to clarify, the methyl CH3 group is an electron-donating species that destabilizes the anion that would form if acetic were to donate its proton. What we want to look for is an electron drawing molecule that will allow the H to leave more easily. Since the CH3 is donating electrons, it makes it hard for the H in the carboxyl group to leave because the partially positive charge on the hydrogen is drawn to the donated electron from CH3. THe H in the formic acid (instead of the CH3 group) does not donate an electron.

Moreover, acetic acid cannot form a resonance if it looses its H like formic acid. Resonance stabilizes the structure and makes proton donating more favorable in formic acid, making it a stronger acid.