Carboxylic Acids & Esters
1. Carboxylic Acids
Definition: organic compounds containing the functional group –C(=O)–O–H (–COOH). Naming: the –COOH carbon is always C1; suffix -oic acid; an OH elsewhere is written as the prefix hydroxy- (e.g. 2-hydroxypropanoic acid = lactic acid).
Physical properties
- Higher m.p. and b.p. than alcohols/alkanes of similar size — hydrogen bonding (between –C=O and –O–H) plus permanent dipoles; acids often exist as hydrogen-bonded dimers.
- Small carboxylic acids are soluble in water (hydrogen bonding with water); solubility falls as the chain gets longer. They are only weak acids — partially dissociated, unlike HCl or H₂SO₄.
Formation
| Method | Conditions | Where |
|---|---|---|
| Oxidation of a primary alcohol or an aldehyde | acidified K₂Cr₂O₇, reflux (alcohol) / [O] (aldehyde) | done in Alcohols & Carbonyls |
| Hydrolysis of a nitrile (R–CN → R–COOH) | dil. H₂SO₄ or HCl, heat under reflux (alkali then dil. HCl also works) | done in Halogenoalkanes (board says “Alcohols”) |
| Hydrolysis of an ester | dil. acid or alkali, then acid | section 3 below |
2. Reactions of Carboxylic Acids
① All the “acid-like” reactions — salt formation
| Reacts with | Gives | Example | Observation |
|---|---|---|---|
| With a metal (Na, Mg…) | salt + H₂ | CH₃COOH + Na → CH₃COONa + ½H₂ | effervescence (H₂) |
| With a base / metal hydroxide | salt + H₂O | CH₃CH₂COOH + KOH → CH₃CH₂COOK + H₂O | neutralisation (no visible change) |
| With a metal oxide | salt + H₂O | 2CH₃CH₂COOH + CaO → (CH₃CH₂COO)₂Ca + H₂O | oxide dissolves |
| With a carbonate / hydrogencarbonate | salt + H₂O + CO₂ | 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂ | effervescence (CO₂) — test for –COOH |
② Esterification
Already discussed in Alcohols: carboxylic acid + alcohol ⇌ ester + water, conc. H₂SO₄ + heat.
③ Reduction
- LiAlH₄ only (in dry ether). NaBH₄ cannot be used — it is not strong enough to reduce a carboxylic acid (nor an ester).
- The reaction always gives the primary alcohol — it cannot be stopped at the aldehyde.
- Sir: sometimes the question doesn’t ask for reagent/solvent — then just say “reduction”.
④ Chlorination — acyl chloride
PCl₅ is also the test for the –OH group (steamy fumes of HCl). Product: an acyl chloride (R–COCl). (PCl₃ and SOCl₂ do the same — but PCl₅ is the one usually asked at AS.)
3. Esters
Definition: organic compounds containing the group –C(=O)–O– (R–COO–R′). Uses: perfumes, glues, artificial food flavourings (fruity smells — e.g. pineapple, banana).
Hydrolysis of esters
Hydrolysis = splitting by water — the reverse of esterification. Two ways:
| Condition | Reaction | Notes |
|---|---|---|
| ① Acidic conditions | ester + H₂O ⇌ alcohol + carboxylic acid; catalyst H⁺ (dil. H₂SO₄), heat under reflux | reversible — the equilibrium mixture contains all four, hard to separate |
| ② Alkaline conditions | ester + NaOH → alcohol + sodium carboxylate; heat under reflux | goes to completion (irreversible); alcohol is easily distilled off; then add dil. HCl to convert the salt to the carboxylic acid, and distil that off |
4. Hydroxy-carboxylic Acids
Definition: organic compounds containing both –OH and –COOH groups. Heated with conc. H₂SO₄ they form esters with themselves — cyclic esters — in two ways:
| Type | What happens | Board examples |
|---|---|---|
| ① Self-esterification | the –OH and the –COOH of the same molecule react (H₂O lost; ring closes) | HOCH₂CH₂COOH → cyclic ester (ring: –CH₂–CH₂–O–C(=O)–); CH₃CH(OH)COOH → cyclic ester (board draws a 3-membered ring) |
| ② With another unit | two molecules each use their OH on the other’s COOH ⇒ a cyclic diester (2 ester links) | HOCH₂CH₂COOH (×2) → 8-membered ring; CH₃CH(OH)COOH (×2) → lactide (6-membered ring) |
5. Past-Paper MCQs (board)
| Q | Question | Working / answer |
|---|---|---|
| 2 | reagent to distinguish MCPA (–COOH) from 2,4-D (–CH₂CH₂OH) | Na₂CO₃(aq): effervescence only with the acid ⇒ D ✓ (Na reacts with both; AgNO₃ — both have aryl Cl; Fehling’s — no aldehyde) |
| 3 | lardolure RCH(CH₃)O–C(=O)–H hydrolysed with acid | a methanoate ester: HCOO–R′ ⇒ methanoic acid + alcohol RCH(CH₃)OH ⇒ D ✓ |
| 4 | banana ester CH₃CO₂CH₂CH(CH₃)CH₂CH₃ — which pair produces it? | acyl part = ethanoic acid, alkyl part = 2-methylbutan-1-ol ⇒ C ✓ (A, B, D have the wrong acid/alcohol) |
| 7 | malic acid: substance reacting with all three –OH groups | Na reacts with both –COOH and the alcoholic OH ⇒ C ✓ (KOH and Na₂CO₃ react only with COOH; ethanol/H₂SO₄ esterifies only COOH) |
| 25 | strobilurin ester warmed with dil. H₂SO₄, then H₂/Pd | the ester is hydrolysed by aqueous acid ⇒ –COOH (+ methanol); H₂/Pd then saturates the C=C bonds ⇒ saturated acid ⇒ A ✓ (the acid is not reduced to an alcohol — no LiAlH₄) |
| Q | Question | Working / answer |
|---|---|---|
| 39 | santonin: warm dil. H₂SO₄ then cold acidified KMnO₄ ⇒ X; H atoms displaced by Na? | hydrolysis of the lactone ⇒ COOH + OH; cold KMnO₄ turns each of the 2 C=C into a diol (4 OH) ⇒ COOH (1) + OH (1) + 4 OH = 6 ⇒ D ✓ |
| 46 | ethanedioic acid + ethanol (conc. H₂SO₄) ⇒ diester C₆H₁₀O₄ | both –COOH esterify ⇒ CH₃CH₂O–C(=O)–C(=O)–OCH₂CH₃ ⇒ B ✓ |
| 70 | caprolactone (cyclic ester) from which compound by one reaction | ring closes from a hydroxy-acid: HO(CH₂)₅COOH (6-hydroxyhexanoic acid) ⇒ C ✓ |
| 82 | lactide formed on heating with acid catalyst from which compound? | 2-hydroxypropanoic acid (lactic acid) — two molecules form the cyclic diester ⇒ C ✓ |
| 86 | chlorogenic acid + excess Na₂CO₃(aq) at room temperature | only the –COOH reacts (Na₂CO₃ is too weak for alcohols; room temperature doesn’t hydrolyse the ester) ⇒ A ✓ (the sodium carboxylate) |
6. Long CIE-style Questions (board)
V gives a brick-red precipitate with Fehling’s ⇒ aldehyde ⇒ HCHO (methanal, 1 C). W: fruity-smelling liquid ⇒ ester ⇒ HCOOCH₃ (2 C). X (3 C, C atoms bonded together, effervescence with Na₂CO₃ ⇒ –COOH; oxidation with reflux gives Y which does not react with 2,4-DNPH ⇒ not a carbonyl ⇒ primary alcohol + carboxylic acid): HOCH₂CH₂COOH.
(c)(ii) Y = HOOC–CH₂–COOH (propanedioic acid). (d) X + conc. H₂SO₄, warm ⇒ cyclic diester Z, C₆H₈O₄ (two X molecules; –H₂O × 2) — draw the 8-membered ring with two ester links. (ii) esterification.
(b) four ester isomers: W HCOOCH₂CH₂CH₃ · X CH₃COOCH₂CH₃ · Y CH₃CH₂COOCH₃ · Z HCOOCH(CH₃)₂. Q is hydrolysed with aq. H₂SO₄, and the mixture is then refluxed with acidified dichromate to give a single product R. Tests on R: no reaction with Tollens’ ⇒ not an aldehyde; none with 2,4-DNPH ⇒ not a carbonyl; effervescence with Na₂CO₃ ⇒ –COOH. (d) R = CH₃COOH (so Q gives the same acid from both parts: ethanol → ethanoic acid, and ethanoic acid) ⇒ Q = X, ethyl ethanoate. (e) none of the esters is chiral — no carbon with 4 different groups.
- R–COOH: –COOH carbon is C1, suffix -oic acid. High m.p./b.p. (H-bonding); small ones soluble; weak acids.
- Acid reactions: + metal → salt + H₂; + base/oxide → salt + H₂O; + carbonate → salt + H₂O + CO₂ (effervescence = test for –COOH).
- LiAlH₄/dry ether reduces acid → 1° alcohol (not NaBH₄; can’t stop at aldehyde). PCl₅ → acyl chloride (+ HCl + POCl₃); steamy fumes = –OH test.
- Ester R–COO–R′: alkyl (alcohol) + alkanoate (acid); made by esterification (conc. H₂SO₄, heat, reversible). Uses: perfumes, glues, flavourings.
- Hydrolysis: acid (H⁺/H₂O, reflux) ⇌ alcohol + acid; alkali (NaOH, reflux) → alcohol + carboxylate (irreversible), then dil. HCl → acid.
- Hydroxy-acids (–OH + –COOH) with conc. H₂SO₄: cyclic ester (self) or cyclic diester with another molecule (lactide from lactic acid).