The carboxyl group
The carboxyl group (−COOH) is a carbonyl (C=O) and a hydroxyl (O−H) on the same carbon.
- The carboxyl carbon is sp²; the group is flat with angles near 120°.
- The O−H oxygen shares its lone pair with C=O by resonance. So the C=O here is less reactive to nucleophiles than in aldehydes and ketones. That is why carboxylic acids do not give Tollens or 2,4-DNP tests.
- Names: replace the 'e' of the alkane with -oic acid: methanoic acid (HCOOH, formic), ethanoic acid (CH₃COOH, acetic), benzoic acid (C₆H₅COOH).
Physical properties
- Very high boiling points: two acid molecules join by two hydrogen bonds to form a dimer, even in the vapour.
- Small acids (up to 4 carbons) mix with water (H-bonds). Longer ones hardly dissolve. Benzoic acid is almost insoluble in cold water.
Preparation of carboxylic acids
- Oxidation of primary alcohols and aldehydes: KMnO₄ (neutral, acidic or alkaline) or K₂Cr₂O₇/H₂SO₄. RCH₂OH → RCHO → RCOOH.
- Oxidation of alkylbenzenes: hot alkaline KMnO₄ turns the whole side chain (any length, if the ring-carbon has an H) into −COOH: toluene → benzoic acid.
- Hydrolysis of nitriles and amides: R−C≡N + H₂O (H⁺ or OH⁻) → R−CONH₂ → R−COOH.
- From Grignard reagents: RMgX + CO₂ (dry ice) → RCOOMgX → (H₃O⁺) → RCOOH. The acid has one carbon more than the alkyl halide.
- Hydrolysis of acyl halides, anhydrides and esters: with water (or base then acid) → acid.
Acidity of carboxylic acids
RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. The acid dissociation constant Ka measures how much H⁺ it gives. We usually use pKa = −log Ka: smaller pKa = stronger acid.
Why more acidic than phenol and alcohol?
- In the carboxylate ion the negative charge is shared equally by two oxygen atoms (two equal resonance forms, both C−O bonds become the same length).
- In phenoxide the charge also spreads into the ring, but onto carbon atoms, which hold negative charge less well. Alcohols cannot spread it at all.
- Typical pKa: HCl ≈ −7, CH₃COOH 4.76, C₆H₅COOH 4.19, HCOOH 3.75, phenol 10, ethanol ≈ 16.
Effect of substituents
- Electron-withdrawing groups (−NO₂, −CN, −F, −Cl, −Br, C₆H₅) spread the negative charge more → stronger acid. More such groups → stronger (Cl₃CCOOH is much stronger than CH₃COOH).
- Stronger pull → stronger acid: FCH₂COOH > ClCH₂COOH > BrCH₂COOH.
- The effect fades with distance: 2-chlorobutanoic > 3-chloro > 4-chloro.
- Electron-donating groups (alkyl) make the ion less stable → weaker acid: HCOOH is stronger than CH₃COOH.
Reactions of carboxylic acids
Breaking the O−H bond
- With metals (Na) → salt + H₂; with NaOH → salt + water.
- With NaHCO₃ → salt + CO₂ (brisk fizz). Phenols do not do this, so it is a test that tells them apart.
Breaking the C−OH bond
- Esterification: RCOOH + R′OH ⇌ RCOOR′ + H₂O (conc. H₂SO₄ or dry HCl). The OH comes from the acid.
- Acid chloride: with PCl₅, PCl₃ or SOCl₂ (SOCl₂ is best: SO₂ and HCl are gases).
- Anhydride: heat with P₂O₅ (loses water between two molecules).
- Amide: with NH₃ → ammonium salt → heat → RCONH₂.
Reactions of the −COOH group
- Reduction: LiAlH₄ (or B₂H₆) → primary alcohol. NaBH₄ does not reduce COOH.
- Decarboxylation: sodium salt + soda-lime (NaOH + CaO), heat → hydrocarbon with one carbon less + Na₂CO₃. Kolbe electrolysis of the salt also removes CO₂.
Substitution in the chain and ring
- Hell–Volhard–Zelinsky (HVZ): acids with α-H + Cl₂ or Br₂ and red phosphorus → α-halo acid.
- Benzoic acid: −COOH is deactivating and meta-directing; it does not undergo Friedel–Crafts reactions.
Uses of carboxylic acids
- Methanoic acid: rubber, textile, dyeing and leather industries.
- Ethanoic acid: vinegar in food; solvent; making esters, cellulose acetate and dyes.
- Hexanedioic (adipic) acid: making nylon-6,6.
- Benzoic acid and sodium benzoate: food preservatives. Esters of benzoic acid are used in perfumes.
- Higher fatty acids (palmitic, stearic): soaps and detergents.
Try it: the fizz test at home
Put half a teaspoon of baking soda (NaHCO₃) in a cup. Predict first: will vinegar make it fizz? Add a spoon of vinegar. The bubbles are CO₂: acetic acid is strong enough to push H⁺ onto HCO₃⁻. In the 3D free play, set Cl = 3 and predict whether trichloroacetic acid would fizz even faster (yes: pKa 0.65 is far smaller than 4.76).
Exam corner
Expect: acidity order questions with reasons, 'why is RCOOH more acidic than phenol', distinguish phenol and benzoic acid (NaHCO₃ test), preparations with Grignard reagents, name reactions (HVZ, decarboxylation, Kolbe), and pKa/Ka calculations of 1–3 marks.
Key formulas and definitions
- pKa = −log₁₀ Ka and Ka = 10^(−pKa)
- For a weak acid HA of concentration c: [H⁺] ≈ √(Ka·c), degree of dissociation α ≈ √(Ka/c)
- pH = −log₁₀ [H⁺]
- RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂↑
- RCOOH + R′OH ⇌ RCOOR′ + H₂O (H⁺)
- RMgX + CO₂ → RCOOMgX → RCOOH (H₃O⁺)
- RCOONa + NaOH (CaO, heat) → RH + Na₂CO₃
Worked examples
1. The Ka of ethanoic acid is 1.8 × 10⁻⁵. Find its pKa.
Step 1: pKa = −log Ka = −log(1.8 × 10⁻⁵). Step 2: log 1.8 = 0.255, so log Ka = 0.255 − 5 = −4.745. Step 3: pKa = 4.745 ≈ 4.75. Answer: pKa ≈ 4.75.
2. Chloroethanoic acid has pKa 2.86. Find Ka and say how many times stronger it is than ethanoic acid (pKa 4.76).
Step 1: Ka = 10^(−2.86) = 1.38 × 10⁻³. Step 2: Ratio of Ka = 10^(4.76 − 2.86) = 10^1.9 ≈ 79. Answer: Ka ≈ 1.4 × 10⁻³; about 80 times stronger.
3. Find the pH of 0.1 M ethanoic acid (Ka = 1.8 × 10⁻⁵).
Step 1: [H⁺] ≈ √(Ka × c) = √(1.8 × 10⁻⁵ × 0.1) = √(1.8 × 10⁻⁶). Step 2: √1.8 = 1.34, so [H⁺] = 1.34 × 10⁻³ M. Step 3: pH = −log(1.34 × 10⁻³) = 3 − 0.127 = 2.87. Answer: pH ≈ 2.87.
4. What percentage of 0.01 M ethanoic acid is ionised? (Ka = 1.8 × 10⁻⁵)
Step 1: α ≈ √(Ka / c) = √(1.8 × 10⁻⁵ / 0.01) = √(1.8 × 10⁻³). Step 2: √(18 × 10⁻⁴) = 4.24 × 10⁻². Step 3: % = 0.0424 × 100 = 4.2%. Answer: about 4.2% (so most molecules stay as CH₃COOH).
5. How many grams of CO₂ are given off when 6.0 g of ethanoic acid reacts fully with NaHCO₃? (C = 12, H = 1, O = 16)
Step 1: Molar mass of CH₃COOH = 60 g/mol, so moles = 6.0 / 60 = 0.10 mol. Step 2: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂: 1 mol acid gives 1 mol CO₂. Step 3: Mass of CO₂ = 0.10 × 44 = 4.4 g. Answer: 4.4 g CO₂.
6. 12 g of ethanoic acid is heated with excess ethanol; 11 g of ethyl ethanoate forms. Find the % yield. (CH₃COOC₂H₅ = 88 g/mol)
Step 1: Moles of acid = 12 / 60 = 0.20 mol. Step 2: 1 mol acid → 1 mol ester, so the expected ester = 0.20 × 88 = 17.6 g. Step 3: % yield = 11 / 17.6 × 100 = 62.5%. Answer: 62.5% (esterification is reversible, so the yield is never 100%).
7. Arrange in increasing acidity: CH₃COOH, FCH₂COOH, ClCH₂COOH, HCOOH.
Step 1: CH₃ pushes electrons → weakest (pKa 4.76). HCOOH has no alkyl → stronger (3.75). Step 2: Halogens pull electrons: Cl (2.86), F pulls more (2.59). Answer: CH₃COOH < HCOOH < ClCH₂COOH < FCH₂COOH.
8. Which Grignard reagent and what else do you need to make propanoic acid? Show the steps.
Step 1: Propanoic acid CH₃CH₂COOH has 3 C; the COOH carbon comes from CO₂. Step 2: So the Grignard must carry 2 C: CH₃CH₂MgBr. Step 3: CH₃CH₂MgBr + CO₂ → CH₃CH₂COOMgBr; then H₃O⁺ → CH₃CH₂COOH. Answer: ethylmagnesium bromide + dry ice, then dilute acid.
Common mistakes
- Saying a larger pKa means a stronger acid. It is the opposite: smaller pKa = larger Ka = stronger acid.
- Thinking NaBH₄ reduces −COOH. Only strong LiAlH₄ (or diborane) does.
- Taking the OH from the alcohol in esterification. The OH leaves from the acid and the H from the alcohol.
- Expecting carboxylic acids to give 2,4-DNP or Tollens tests like aldehydes. Resonance makes their C=O unreactive to these.