Why is triflic acid stronger than sulfuric acid?

Triflic acid (CF₃SO₃H) is stronger than sulfuric acid (H₂SO₄) primarily due to the electronic effects of the trifluoromethyl group (CF₃) attached to the sulfonic acid group in triflic acid. Here’s why:

1. Electron-withdrawing Effects of CF₃ Group

The trifluoromethyl group (CF₃) in triflic acid is an extremely strong electron-withdrawing group. This means that the CF₃ group pulls electron density away from the sulfonic acid functional group (SO₃H), making the hydrogen ion (H⁺) in triflic acid much more readily dissociable compared to sulfuric acid.  In other words, the CF₃ group stabilizes the conjugate base (CF₃SO₃⁻) by spreading out the negative charge, making it easier for the acid to lose a proton. This results in a stronger acid.

2. Greater Acidic Proton (H⁺) Donation

The stronger electron-withdrawing effect of the CF₃ group also decreases the electron density around the oxygen atoms in the sulfonic acid group, making it much easier to release the proton (H⁺). In contrast, sulfuric acid has two sulfonic groups (–SO₃H), but they are less affected by electron-withdrawing groups, so the proton release is less facile.  Triflic acid has a lower pKa (~–14), which is indicative of a much stronger acid compared to sulfuric acid, which has a pKa around –3.

3. Increased Stability of the Conjugate Base

When triflic acid dissociates into its conjugate base (CF₃SO₃⁻), the negative charge is stabilized by the electron-withdrawing CF₃ group. This makes the conjugate base much more stable than the conjugate base of sulfuric acid (HSO₄⁻), which does not have a similar stabilizing group.  The higher the stability of the conjugate base, the stronger the acid, since a stable conjugate base encourages more proton dissociation.

4. Enhanced Electrophilic Properties

The trifluoromethyl group enhances the electrophilic nature of the triflic acid molecule, making it more willing to participate in reactions where proton donation is required, further increasing its acid strength.

Triflic acid is stronger than sulfuric acid primarily due to the strong electron-withdrawing effect of the trifluoromethyl group (CF₃), which makes the proton (H⁺) much easier to dissociate and stabilizes the conjugate base more effectively. This results in TfOH having a lower pKa and being a more potent acid than sulfuric acid.