Carboxylic acids: naming, structure, preparation and physical properties
Carboxylic acids (-COOH) are made by oxidising alcohols, aldehydes or alkylbenzenes, or by hydrolysing nitriles, amides and esters. Hydrogen-bonded dimers give them high boiling points.
Common and IUPAC names
| Formula | Common name | IUPAC name |
|---|---|---|
| HCOOH | Formic acid (from ants; Latin formica) | Methanoic acid |
| CH₃COOH | Acetic acid (from vinegar; Latin acetum) | Ethanoic acid |
| CH₃CH₂COOH | Propionic acid | Propanoic acid |
| CH₃CH₂CH₂COOH | Butyric acid (from rancid butter; Latin butyrum) | Butanoic acid |
| (CH₃)₂CHCOOH | Isobutyric acid (α-methylpropionic acid) | 2-Methylpropanoic acid |
| HOOC-COOH | Oxalic acid | Ethanedioic acid |
| HOOC-CH₂-COOH | Malonic acid | Propanedioic acid |
| HOOC-(CH₂)₄-COOH | Adipic acid | Hexanedioic acid (the 'e' of hexane stays) |
| C₆H₅COOH | Benzoic acid | Benzenecarboxylic acid (benzoic acid is also accepted) |
| C₆H₅CH₂COOH | Phenylacetic acid | 2-Phenylethanoic acid |
| C₆H₄(COOH)₂, groups on C1 and C2 | Phthalic acid | Benzene-1,2-dicarboxylic acid |
Structure of the carboxyl group
The carboxyl group, -COOH, is a carbonyl group joined to an -OH group. The carboxyl carbon is sp²: its three σ bonds lie in one plane, about 120° apart. A lone pair on the -OH oxygen is shared with the C=O group by resonance. Because of this, the carboxyl carbon is less electrophilic than the carbonyl carbon of an aldehyde or ketone. So carboxylic acids do not give the usual addition reactions of C=O, such as those with 2,4-DNP or NaHSO₃.
Methods of preparation
- 1★From primary alcohols and aldehydes: RCH₂OH → (i) alkaline KMnO₄ (ii) H₃O⁺ → RCOOH; CH₃(CH₂)₈CH₂OH → (CrO₃, H₂SO₄) CH₃(CH₂)₈COOHKMnO₄ (neutral, acidic or alkaline), K₂Cr₂O₇ or CrO₃ in acid (Jones reagent) all work. Aldehydes are oxidised even more easily
- 2★From alkylbenzenes: C₆H₅CH₃ → (KMnO₄, KOH, heat) C₆H₅COOK → (H₃O⁺) C₆H₅COOHChromic acid or KMnO₄ oxidises the whole side chain to -COOH, whatever its length. Primary and secondary alkyl groups react; a tertiary alkyl group does not
- 3From nitriles and amides: RCN → (H₂O, H⁺ or OH⁻) RCONH₂ → (H₂O, H⁺ or OH⁻) RCOOH; CH₃CONH₂ + H₂O → (H⁺) CH₃COOH + NH₃Under mild conditions, the reaction stops at the amide
- 4★From Grignard reagents: RMgX + CO₂ (dry ice) → (dry ether) RCOOMgX → (H₃O⁺) RCOOHThe acid has one carbon more than the alkyl halide. Nitriles, made from RX and CN⁻, also add one carbon
- 5From acyl halides and anhydrides: RCOCl + H₂O → RCOOH + HCl; (CH₃CO)₂O + H₂O → 2CH₃COOHAcyl chlorides react faster with aqueous base, giving RCOO⁻, which gives the acid on acidifying
- 6From esters: C₆H₅COOC₂H₅ + NaOH → (heat) C₆H₅COONa + C₂H₅OH; C₆H₅COONa + HCl → C₆H₅COOH + NaClAcid hydrolysis gives the acid directly: CH₃CH₂CH₂COOC₂H₅ + H₂O ⇌ (H⁺) CH₃CH₂CH₂COOH + C₂H₅OH
★ key step
Worked example: choose a route
How will you prepare: (i) benzoic acid from toluene (ii) propanoic acid from bromoethane, by two routes (iii) benzene-1,4-dicarboxylic acid from 1-ethyl-4-methylbenzene?
- (i) Heat toluene with alkaline KMnO₄ to get potassium benzoate; acidify with H₃O⁺ to get benzoic acid.
- (ii) Bromoethane has 2 carbons; propanoic acid has 3, so one carbon must be added.
- Route A: CH₃CH₂Br + KCN (ethanolic) → CH₃CH₂CN + KBr; then hydrolyse with H₃O⁺ → CH₃CH₂COOH.
- Route B: CH₃CH₂Br + Mg (dry ether) → CH₃CH₂MgBr; add CO₂ (dry ice); then H₃O⁺ → CH₃CH₂COOH.
- (iii) Heat 4-CH₃C₆H₄CH₂CH₃ with alkaline KMnO₄, then acidify. Both side chains (-CH₃ and -CH₂CH₃) are oxidised to -COOH, whatever their length: HOOC-C₆H₄-COOH.
(i) KMnO₄/KOH, heat, then H₃O⁺. (ii) KCN then hydrolysis, or Mg/dry ether then CO₂ and H₃O⁺. (iii) KMnO₄/KOH, heat, then H₃O⁺.
Physical properties and the reasons
| Property | What you see | Why |
|---|---|---|
| State and smell | Aliphatic acids with up to 9 carbons are colourless liquids with unpleasant smells; higher acids are wax-like, almost odourless solids | Higher acids are not volatile |
| Boiling point | Higher than aldehydes, ketones and even alcohols of similar molar mass | Strong, extensive intermolecular hydrogen bonding; most acids exist as dimers, even in the vapour phase and in aprotic solvents |
| Example data (molar mass 60) | Ethanoic acid 391 K; propan-1-ol 370 K | Two hydrogen bonds hold each pair of acid molecules |
| Solubility in water | Acids with 1 to 4 carbons mix with water; solubility falls as the chain grows; benzoic acid is nearly insoluble in cold water | Hydrogen bonds with water; the large non-polar part is hydrophobic |
| Other solvents | Dissolve in less polar solvents such as benzene, ether, alcohol and chloroform | The hydrocarbon part mixes with these solvents |
How to draw the hydrogen-bonded dimer
- Draw two RCOOH molecules side by side, facing each other.
- Place the C=O of one molecule opposite the O-H of the other, and the O-H of the first opposite the C=O of the second.
- Join each O-H hydrogen to the C=O oxygen across from it with a dotted line: these are the two hydrogen bonds.
- The two molecules now form an eight-membered ring.
- Label: R, C=O, O-H, 'hydrogen bond' (on both dotted lines) and 'dimer'.
The sour taste of vinegar in pickles is ethanoic acid, and the sting of a red ant bite is partly methanoic acid. Butanoic acid gives rancid butter or old ghee its bad smell.
2 marks: “Write the IUPAC names of (i) (CH₃)₂CHCOOH (ii) HOOC(CH₂)₄COOH.” (i) 2-Methylpropanoic acid. (ii) Hexanedioic acid (adipic acid).
3 marks: “How will you prepare benzoic acid from (i) toluene (ii) phenylmagnesium bromide (iii) ethyl benzoate? Write the equations.” (i) C₆H₅CH₃ → (KMnO₄/KOH, heat) C₆H₅COOK → (H₃O⁺) C₆H₅COOH. (ii) C₆H₅MgBr + CO₂ → C₆H₅COOMgBr → (H₃O⁺) C₆H₅COOH. (iii) C₆H₅COOC₂H₅ + NaOH → C₆H₅COONa + C₂H₅OH, then HCl → C₆H₅COOH.
2 marks: “Why do carboxylic acids have higher boiling points than alcohols of comparable molar mass?” Carboxylic acid molecules are held in pairs (dimers) by two hydrogen bonds. These are stronger and more extensive than in alcohols and are not fully broken even in the vapour phase.
Oxidise, hydrolyse, or add CO₂. Grignard + CO₂ and RX + CN⁻ both add one carbon.