Carboxylic acids and esters

Carboxylic acids are organic compounds containing the carboxyl group –COOH. They are very common in nature and everyday life: ethanoic acid in the kitchen (vinegar is a 3–9 % solution of it), citric and malic acids in fruit, methanoic acid in ants and nettles, stearic and oleic acids in fats. Although carboxylic acids are weak, they show all the general properties of inorganic acids and, in addition, form esters with alcohols. This topic is the last link in the oxidation chain of alcohols and aldehydes and the start of the topic of esters and fats.

§1. Structure and classification

The carboxyl group combines a carbonyl group (C=O) and a hydroxyl group (–OH). The carbonyl group draws electron density from the oxygen, the O–H bond becomes strongly polarised and the hydrogen easily leaves as a proton — so, unlike alcohols, carboxylic acids are acidic.

• Saturated monocarboxylic acids: methanoic (formic) HCOOH, ethanoic (acetic) CH₃COOH, propanoic C₂H₅COOH, butanoic C₃H₇COOH.
• Higher fatty acids: palmitic C₁₅H₃₁COOH, stearic C₁₇H₃₅COOH, oleic C₁₇H₃₃COOH (one double bond), linoleic C₁₇H₃₁COOH (two).
• Dicarboxylic: ethanedioic (oxalic) acid HOOC–COOH; aromatic: benzoic acid C₆H₅COOH; unsaturated: propenoic (acrylic) acid CH₂=CH–COOH.

Functional-group isomers are esters: C₃H₆O₂ fits propanoic acid, methyl ethanoate and ethyl methanoate.

Physical properties: the lower acids are pungent liquids that mix well with water; the higher fatty acids are solids insoluble in water. Carboxyl groups form hydrogen-bonded dimers, so acids boil even higher than alcohols of the same mass. Pure ethanoic acid freezes at 17 °C into an ice-like solid ("glacial acetic acid"). The lower acids burn the skin, so they must be handled with care.

§2. Acidic properties

Carboxylic acids are weak electrolytes: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Acid strength depends on the group attached: alkyl groups release electrons and weaken the acid, while halogen atoms withdraw electrons and strengthen it:

They undergo all the reactions typical of inorganic acids:

The last reaction shows that carboxylic acids are stronger than carbonic acid — a way to tell them from phenol and alcohols, which do not release CO₂ from carbonates. In turn, strong inorganic acids displace them from their salts: CH₃COONa + HCl → CH₃COOH + NaCl.

Salt names: salts of methanoic acid are methanoates (formates, HCOONa), of ethanoic acid ethanoates (acetates, CH₃COONa), of propanoic acid propanoates, and of fatty acids stearates and palmitates (soaps). These salts come from a weak acid and a strong base, so they undergo anion hydrolysis and their solutions are alkaline: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. The dicarboxylic ethanedioic acid gives two series of salts (acid and normal); its calcium salt is insoluble, which is used to detect Ca²⁺. Ethanedioic acid is also a reducing agent: in acid it decolourises KMnO₄ solution and is oxidised to CO₂.

Worked example

Neutralising 3.7 g of a saturated monocarboxylic acid used 0.05 mol of NaOH. Identify the acid.

1. n(acid) = n(NaOH) = 0.05 mol → M = 3.7/0.05 = 74 g/mol.
2. CₙH₂ₙO₂: 14n + 32 = 74 → n = 3.
3. C₂H₅COOH — propanoic acid.

§3. Reactions of the carboxyl group and the hydrocarbon chain

• Esterification (conc. H₂SO₄, reversible): CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The –OH of the water comes from the acid and the H from the alcohol.
• Halogenation of the chain (at the α-carbon next to the carboxyl group, red phosphorus catalyst): CH₃COOH + Cl₂ → ClCH₂COOH + HCl. Chloroethanoic acid with ammonia gives aminoethanoic acid (glycine), so this reaction opens the way to amino acid synthesis.
• Methanoic acid also contains an aldehyde group (H–CO–OH), so it is a reducing agent: it gives a silver mirror and decolourises KMnO₄: HCOOH + Ag₂O → CO₂ + H₂O + 2Ag↓. Heated with conc. H₂SO₄ it decomposes: HCOOH → CO + H₂O (a laboratory source of CO).
• Unsaturated acids (oleic, propenoic) undergo addition at the double bond and decolourise bromine water; oleic acid is hydrogenated to stearic acid.

Worked example

9.2 g of methanoic acid reacted with excess ammoniacal silver oxide solution. What mass of silver is deposited?

1. n(HCOOH) = 9.2/46 = 0.2 mol.
2. HCOOH → 2Ag: n(Ag) = 0.4 mol.
3. m = 0.4·108 = 43.2 g.

§4. Preparation and uses

In the food industry ethanoic acid is a preservative and condiment, and benzoic acid and its salts are preservatives. Ethanoic acid is used to make esters (solvents), acetate fibre and medicines (aspirin); methanoic acid is used to tan leather and disinfect; fatty acids are used to make soap. Citric acid is an acidity regulator in food, and ascorbic acid (vitamin C) is essential for the body.

§5. An official test item

44 g of a mixture of two isomers C₄H₈O₂ gave 22 g of salt with NaHCO₃. The isomer that did not react was treated with NaOH, giving ethanol, 20 % of which evaporated. Find the mass of alcohol remaining.

Worked example

1. The mixture is 44/88 = 0.5 mol. Only the acid reacts with NaHCO₃; the salt C₃H₇COONa (M = 110): 22/110 = 0.2 mol of acid.
2. The other isomer is 0.5 − 0.2 = 0.3 mol and gives ethanol with NaOH → it is ethyl ethanoate CH₃COOC₂H₅.
3. Ethanol 0.3 mol = 0.3·46 = 13.8 g.
4. 80 % remains: 13.8·0.8 = 11.04 g.

Check yourself

1. What mass of 10 % NaOH solution neutralises 12 g of ethanoic acid?

2. Which carboxylic acid gives the silver mirror test?

3. How many acids and esters fit the formula C₃H₆O₂?

4. Which acid is the strongest: CH₃COOH, ClCH₂COOH, Cl₃CCOOH?

5. What volume of CO₂ (STP) is released when 6 g of ethanoic acid reacts with excess Na₂CO₃?

6. How can oleic acid be distinguished from stearic acid?

7. What volume of H₂ (STP) is released when 4.8 g of magnesium reacts with excess ethanoic acid?

8. What is the IUPAC name of acetic acid?

Common mistakes

• Thinking ethanoic acid is a strong acid: it is a weak electrolyte.
• Forgetting the reducing (aldehyde) behaviour of methanoic acid.
• Writing esterification as irreversible.
• Taking the amount of CO₂ equal to the amount of acid in the reaction with a carbonate: the ratio is 2 : 1.
• Thinking a halogen weakens the acid: it strengthens it.