Identify the most and the least acidic compound in each of the following sets. Leave the remaining answer in each set blank. a) difluoroacetic acid: fluoroacetic acid: trifluoroacetic acid: b) benzoic acid: formic acid: propanoic acid: c) p-cyanobenzoic acid: benzoic acid: p-aminobenzoic acid:

(a) Trifluoro- > fluoroacetic acid

(b) Methanoic > propanoic acid

(c) 4-Cyanobenzoic > 4-aminobenzoic acid

Explanation:

Electron-withdrawing groups (EWGs) increase acidity by inductive or resonance removal of electrons from the carboxyl group.

Electron-donating groups (EDGs) decrease acidity by inductive or resonance donation of electrons to the carboxyl group.

Most carboxylic acids have pKₐ values around 4 - 5. Here are my explanations for the differences,. with pKₐ values for confirmation.

(a) Difluoroacetic, fluoroacetic, and trifluoroacetic acids

F is the most electronegative element in the Periodic Table. It has the greatest effect on acidity.

 CF₃-COOH — 3 EWGs — most acidic — pKₐ = 0.2

F₂CH-COOH — 2 EWGs —                         pKₐ = 1.3

FCH₂-COOH —  1 EWG   — least acidic — pKₐ = 2.6

CF₃-COOH > FCH₂-COOH

(b) Benzoic, methanoic, and propanoic acids  

The ethyl group is a weak EDG by the inductive effect (or by hyperconjugation) .

The phenyl group is an EDG by resonance, but an EWG by the inductive effect. The two effects almost cancel, but the resonance effect is stronger (It's an unfair battle. Resonance usually wins over inductive effects).

CH₃CH₂-COOH — Stronger EDG — weakest — pKₐ =4.88

    C₆H₅-COOH — EDG —                                    pKₐ = 4.20

          H-COOH — Reference —     strongest — pKₐ = 3.75

H-COOH > CH₃CH₂-COOH

(c) 4-Cyanobenzoic, benzoic, and 4-aminobenzoic acids

N≡C-C₆H₄COOH — 1 EWG —       strongest — pKₐ = 3.55

    H-C₆H₄COOH — Reference —                     pKₐ = 4.20

H₂N-C₆H₄COOH — 1 EDG —        weakest —  pKₐ = 4.77

N≡C-C₆H₄COOH > H₂N-C₆H₄COOH