1. Classification, names and physical properties
Mono- (CₙH₂ₙ₊₁COOH) and dicarboxylic acids. IUPAC -oic acid: 2-methylpropanoic acid; diacids -dioic acid: ethanedioic (oxalic), propanedioic (malonic), butanedioic (succinic), hexanedioic (adipic); benzene-1,2-dicarboxylic (phthalic).
Carboxyl C sp²; H-bonded cyclic dimers → b.p. higher than alcohols of similar mass (ethanoic 391 K vs propan-1-ol 370 K). C1–C4 miscible with water.
| Common | IUPAC |
|---|---|
| Formic | Methanoic |
| Oxalic | Ethanedioic |
| Succinic | Butanedioic |
| Adipic | Hexanedioic |
Key terms
- Dimer
Two acid molecules held by two hydrogen bonds.
2. Preparation
- Oxidation of 1° alcohols and aldehydes (K₂Cr₂O₇/H⁺, KMnO₄).
- Alkylbenzenes + hot alkaline KMnO₄, then H⁺ → benzoic acid (any side chain).
- Nitrile hydrolysis: (one C more than RX).
- Grignard: , then H₃O⁺ (one C more).
| From | Product |
|---|---|
| Propylbenzene + KMnO₄ | Benzoic acid |
| C₃H₇MgBr + CO₂ | Butanoic acid |
| C₃H₇CN + H₃O⁺ | Butanoic acid |
Formulas
- Grignard carboxylation
Key terms
- Dry ice
Solid CO₂, used with Grignard reagents to make carboxylic acids.
3. Acidity
Carboxylate stabilised by two equivalent resonance structures (charge on two O; C–O both 127 pm) → pKa 4–5; stronger than phenols (10) and alcohols (16); decompose NaHCO₃.
Substituents: −I groups (F > Cl > Br; more groups; nearer –COOH) increase acidity: CCl₃COOH 0.65, CHCl₂COOH 1.26, FCH₂COOH 2.66, ClCH₂COOH 2.86; 2-, 3-, 4-chlorobutanoic 2.86, 4.05, 4.52. +I alkyl groups decrease: HCOOH 3.75 > CH₃COOH 4.76 > CH₃CH₂COOH 4.88.
Benzoic acids: 4-NO₂ 3.41, 4-Cl 3.98, H 4.19, 4-CH₃ 4.37, 4-OCH₃ 4.47; ortho-substituted acids usually stronger (ortho effect).
Key terms
- Inductive effect
Electron withdrawal (−I) or release (+I) through σ bonds; weakens with distance.
- Ortho effect
Most ortho-substituted benzoic acids are stronger than benzoic acid regardless of the substituent.
4. Acid reactions and derivatives
Na → H₂; NaOH → salt + H₂O; Na₂CO₃/NaHCO₃ → CO₂ (phenols do not).
Derivatives: PCl₅ (), PCl₃ (+ H₃PO₃), SOCl₂ (+ SO₂ + HCl) → acid chlorides; P₂O₅/heat → anhydrides; R′OH + conc. H₂SO₄ → esters (reversible); NH₃ → RCOONH₄ → (heat) RCONH₂.
| Reagent | Derivative |
|---|---|
| SOCl₂, PCl₅, PCl₃ | RCOCl |
| P₂O₅ | (RCO)₂O |
| R′OH/H⁺ | RCOOR′ |
| NH₃, heat | RCONH₂ |
Formulas
- Test with NaHCO₃
Key terms
- Acid derivative
A compound in which the –OH of –COOH is replaced: acid chloride, anhydride, ester, amide.
5. Decarboxylation, Kolbe electrolysis, HVZ and ring substitution
Soda lime (NaOH + CaO): (one C fewer). Kolbe electrolysis: at the anode (R–R). HVZ: α-H + X₂ (red P) → α-halo acid; excess Cl₂ → CCl₃COOH. Benzoic acid: –COOH deactivating, meta directing (3-nitro-, 3-bromobenzoic acid); no Friedel–Crafts.
| Salt | Soda lime | Kolbe |
|---|---|---|
| CH₃COONa | CH₄ | C₂H₆ |
| C₂H₅COONa | C₂H₆ | C₄H₁₀ |
Formulas
- Kolbe electrolysis
Key terms
- Decarboxylation
Removal of CO₂ from a carboxylic acid or its salt.
- HVZ reaction
α-Halogenation of carboxylic acids with X₂ and red phosphorus.
6. Tests and uses
All three effervesce with NaHCO₃. Formic acid: reduces Tollens’ (silver mirror), Fehling’s (red ppt), HgCl₂ (white Hg₂Cl₂ → grey). Acetic acid: neutral FeCl₃ → deep red colour (brown-red ppt on boiling); fruity ester with ethanol/H₂SO₄. Benzoic acid: neutral FeCl₃ → buff precipitate; sublimes.
Uses: formic — rubber coagulation, dyeing, tanning; acetic — vinegar, cellulose acetate, esters, solvent; benzoic — sodium benzoate preservative, medicines, dyes; adipic — nylon-6,6; terephthalic — polyester.
| Test | Formic | Acetic | Benzoic |
|---|---|---|---|
| Tollens’ | − | − | |
| Neutral FeCl₃ | − | Red | Buff ppt |
Formulas
- Formic acid with HgCl₂
Key terms
- Neutral FeCl₃ test
Distinguishes ethanoate (red colour) from benzoate (buff precipitate).
Exam tips
Common mistakes
- Ranking CH₃COOH above HCOOH in acid strength.
- Forgetting that −I effects fade with distance.
- Kolbe electrolysis of ethanoate giving methane (it gives ethane).
- Applying HVZ to benzoic acid (no α-H).
How marks are usually awarded
- Acid strength (2–3 marks): correct order plus named electronic effects.
- Preparations (1 mark each): reagent, condition and product; mention carbon count.
- Tests (1–2 marks): reagent and result for each acid.