Aldehydes, Ketones and Carboxylic Acids · Lesson 11 of 15
Carboxylic Acid Strength and Substituent Effects
“Use conjugate-base stability, Ka and pKa to explain and compare acid strength.”
• Write acid dissociation and neutralisation equations. • Relate Ka and pKa to relative acid strength. • Explain carboxylate stabilisation compared with phenoxide and alkoxide. • Predict inductive and resonance substituent effects with distance and position.
Acidity is an equilibrium, not the number of H atoms
A carboxylic acid donates its O–H proton to water: RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. The C–H hydrogens of its hydrocarbon group do not dissociate in the same way. A stronger acid has a greater equilibrium tendency to transfer that proton in a specified solvent and temperature. Concentration, extent of dissociation and intrinsic acid strength are related but distinct: a concentrated weak acid is not thereby a strong acid.
The negative sign reverses the order: pKa 3 corresponds to Ka 10⁻³, greater than 10⁻⁵ for pKa 5. A difference of one pKa unit means a tenfold difference in Ka, not a tenfold difference in acid concentration. pKa is also not pH: pH describes hydrogen-ion activity in a particular solution, which depends on concentration and other species. Numerical values should be compared under compatible conditions.
A stable conjugate base makes proton loss easier
An alkoxide holds most of its negative charge on one oxygen. Phenoxide delocalises charge into an aromatic ring, but its carbon-charged contributors are not equivalent to the oxygen-charged contributor. A carboxylate has two equivalent principal contributors with negative charge on either oxygen. Delocalisation over two electronegative atoms is especially effective. Consequently ordinary carboxylic acids are generally more acidic than phenol and simple alcohols.
Do not count resonance drawings as votes. Phenoxide may be drawn with more contributors than carboxylate, but the energies, equivalence and charge placement of those contributors matter. Approximate aqueous reference values illustrate the distinction: ethanoic acid pKa 4.76, phenol about 10, ethanol about 16. Carboxylic acids are still usually weak acids in water compared with strongly dissociating mineral acids. There is no universal strength classification based only on a few arbitrary pKa intervals.
Electron withdrawal stabilises carboxylate
An electron-withdrawing substituent can reduce charge concentration in the conjugate base through inductive or resonance effects and increase acidity. Fluorine on the α-carbon makes fluoroethanoic acid stronger than ethanoic acid. For otherwise comparable α-halo acids, fluorine withdraws more strongly than chlorine, then bromine, then iodine. More halogens usually amplify the effect: trichloroethanoic acid is stronger than dichloroethanoic acid, which is stronger than chloroethanoic acid. Electron-donating alkyl groups tend to weaken acidity relative to the corresponding less donating structure.
Inductive effects fall with bond distance. An α-bromo acid is more strongly affected than its β-bromo isomer. Thus CH₃CH₂CHBrCOOH is stronger than CH₃CHBrCH₂COOH. For FCH₂CH₂CH₂COOH versus CH₃CHFCH₂COOH, the latter has fluorine at C3 rather than C4 and is expected to be stronger. Count distance from COOH, not from whichever end is written first.
| Acid | Approximate pKa | Interpretation |
|---|---|---|
| Trifluoroethanoic acid, CF₃COOH | 0.23 | Strong electron withdrawal; not the strongest possible carboxylic acid |
| 4-Nitrobenzoic acid | 3.41 | Nitro strengthens relative to benzoic acid |
| Benzoic acid | 4.19 | Reference for ring substitution |
| 4-Methoxybenzoic acid | 4.46 | Net para resonance donation weakens acidity |
| Ethanoic acid | 4.76 | Reference simple aliphatic acid |
On an aromatic ring, position affects the combination of resonance and induction. Para nitro withdraws and stabilises benzoate relative to benzoic acid; para methoxy donates by resonance and tends to weaken it despite its oxygen’s inductive withdrawal. Multiple nitro substituents can increase acidity further. Direct phenyl or vinyl attachment can also give greater acidity than simple alkyl attachment because the net electronic environment differs. Avoid declaring one substituent order valid at every position, solvent or distance; ortho steric and solvation effects can complicate simple trends.
Problem
Compare the acid strengths of 4-nitrobenzoic acid (3.41) and 4-methoxybenzoic acid (4.46).
- 1.Use Ka(nitro)/Ka(methoxy) = 10^(4.46 − 3.41) = 10^1.05.
- 2.The ratio is approximately 11.2. The nitro-substituted acid has about eleven times the Ka under the same conditions. This does not say that equal-molar solutions have an elevenfold ratio of hydrogen-ion concentrations.
Acid–base reactions give visible evidence
With hydroxide, RCOOH + OH⁻ → RCOO⁻ + H₂O. With carbonate, 2RCOOH + CO₃²⁻ → 2RCOO⁻ + CO₂ + H₂O. With sodium metal, 2RCOOH + 2Na → 2RCOONa + H₂. These observations have different gases: bicarbonate releases CO₂, while metal releases H₂. Formation of a water-soluble salt can also support extraction or separation of an acid from neutral material.
Problem
Order 2-bromobutanoic acid, 3-bromobutanoic acid, butanoic acid and 2-methylpropanoic acid by increasing strength.
- 1.The α-bromo substituent exerts stronger −I influence than β-bromo, so the two brominated acids are stronger than the comparable unhalogenated acids.
- 2.For this set, the qualitative order is 2-methylpropanoic acid < butanoic acid < 3-bromobutanoic acid < 2-bromobutanoic acid. The first comparison is a small difference among weak acids; the halogen-distance effect is much larger and more instructive.
Quick Quiz
Quiz
Which pKa corresponds to the strongest acid under identical conditions?
Explanation: Lower pKa means a larger acid dissociation constant.
Why is carboxylate strongly resonance-stabilised?
Explanation: Equivalent contributors place charge on electronegative oxygen atoms.
Which is normally stronger, fluoroethanoic or ethanoic acid?
Explanation: The nearby electron-withdrawing F stabilises the conjugate base.
What gas forms with NaHCO₃ and a suitable carboxylic acid?
Explanation: Bicarbonate neutralisation gives carbonic acid equivalents that release CO₂ and water.
A pKa decrease of two units corresponds to what Ka change?
Explanation: Ka varies as 10^(−pKa), so a decrease by two raises it by 10².
Practice Problems
- Order benzoic, 4-nitrobenzoic, 3,4-dinitrobenzoic and 4-methoxybenzoic acids by increasing strength.
- Compare CH₂FCOOH and CH₂ClCOOH, and explain why distance must also be controlled.
- Calculate Ka from pKa 4.76 and compare with an acid of pKa 3.76.
- Why does a larger number of phenoxide resonance contributors not prove phenol is more acidic than an acid?
- Write balanced ionic neutralisations with OH⁻ and CO₃²⁻.
- Compare 4-fluorobutanoic acid and 3-fluorobutanoic acid.
4-Methoxybenzoic acid < benzoic acid < 4-nitrobenzoic acid < 3,4-dinitrobenzoic acid. Para methoxy donation weakens the acid, while electron-withdrawing nitro groups stabilise carboxylate; adding another nitro group strengthens the net effect for this set.
Key Takeaways
• Ka describes proton-transfer equilibrium; pKa is its negative base-10 logarithm. • Lower pKa indicates a stronger acid under comparable conditions. • pKa is not the pH of a solution. • Equivalent oxygen-centred resonance strongly stabilises carboxylate. • Electron-withdrawing groups usually increase acidity; donation usually lowers it. • Inductive effects weaken with distance and resonance depends on position. • Bicarbonate gives CO₂, while reactive metals give H₂.