Concept 06 / 07Aldehydes, Ketones and Carboxylic Acids
Acidity of Carboxylic Acids and Effect of Substituents
Why carboxylic acids are acidic (resonance-stabilised carboxylate with equal C-O bonds), comparison with phenols and alcohols, reactions with metals, alkalis and bicarbonates, and effects of electron-withdrawing and electron-donating substituents, their number and position, on acid strength in aliphatic and aromatic acids.
Acidity. Carboxylic acids ionise in water: , with typically to (ethanoic acid ). They are much weaker than mineral acids but stronger than phenols () and alcohols (about ).
The carboxylate ion is stabilised by resonance between two equivalent structures, with the negative charge shared equally by two electronegative oxygens; both C-O bonds are identical (). In phenoxide, the charge is delocalised onto less electronegative ring carbons, and the structures are not equivalent, so stabilisation is weaker. Hence acids > phenols in acidity.
Evidence: carboxylic acids liberate from sodium hydrogencarbonate (brisk effervescence), a test that distinguishes them from phenols: . They also react with active metals () and with alkalis.
Effect of substituents (see figure). Any group that stabilises the carboxylate ion increases acidity; any that destabilises it decreases acidity.
Electron-withdrawing groups ( or : halogens, , CN, ) increase acidity; the effect grows with the number of such groups ( , , ), with their electronegativity ( > > ), and falls off with distance from COOH (2-chlorobutanoic > 3-chloro > 4-chloro).
Electron-donating groups (alkyl, ) decrease acidity: () > () > ().
Attaching a phenyl or vinyl group (on carbon, more electronegative) increases acidity relative to aliphatic analogues: benzoic acid () is stronger than ethanoic acid.
Overall order: .
Substituted benzoic acids: electron-withdrawing groups () increase acidity (4-nitrobenzoic acid ), electron-donating groups (, ) decrease it (4-methoxybenzoic acid ). Ortho-substituted benzoic acids are almost always stronger than benzoic acid, whatever the substituent (ortho effect: steric twisting of COOH out of the ring plane).
Key Takeaways:
- Carboxylate: two equivalent resonance structures, charge on two O atoms.
- Acidity: RCOOH > ArOH > ROH; only acids liberate from .
- groups increase acidity (more, closer, more electronegative = stronger); groups decrease it.
- Benzoic acid () is stronger than acetic acid ().
- Pitfall: formic acid is stronger than acetic acid because it lacks the electron-releasing methyl group, not because of any special resonance.
Brain-Teaser 1: Arrange in increasing acidity: , , , , .
Solution: .
Brain-Teaser 2: Why is 4-nitrobenzoic acid stronger than benzoic acid, while 4-methoxybenzoic acid is weaker?
Solution: The nitro group withdraws electrons (, ), dispersing the negative charge of the carboxylate and stabilising it. The methoxy group donates electrons to the ring by resonance (), increasing electron density near the carboxylate and destabilising it.
Brain-Teaser 3: How would you distinguish phenol from benzoic acid with a single reagent?
Solution: Add sodium hydrogencarbonate solution: benzoic acid dissolves with brisk effervescence of ; phenol, a much weaker acid, does not react. (Neutral also works: violet with phenol, buff precipitate with benzoate.)
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