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Carboxylic acids: Class 12 Chemistry
Structure and naming of carboxylic acids, preparation, physical properties, acidity and the effect of substituents, reactions of the carboxyl group, derivatives such as esters, acid chlorides and amides, and uses.
Key points
Carboxylic acids contain the carboxyl group -COOH, which is a carbonyl group joined to a hydroxyl group. Their general formula is R-COOH, or CₙH₂ₙO₂ for saturated monocarboxylic acids. The carboxyl carbon is sp² hybridised and the group is planar. Common examples are methanoic acid (HCOOH, in ant stings), ethanoic acid (CH₃COOH, in vinegar), benzoic acid and citric acid.
IUPAC names replace the final e of the alkane by oic acid. The carboxyl carbon is always carbon 1. CH₃CH₂COOH is propanoic acid, and CH₃CH(CH₃)COOH is 2-methylpropanoic acid. Common names are formic, acetic, propionic and butyric acids. Dicarboxylic acids such as ethanedioic (oxalic) acid have two -COOH groups.
Preparation of carboxylic acids:
- Oxidation of primary alcohols or aldehydes with acidified K₂Cr₂O₇ or KMnO₄.
- Hydrolysis of nitriles: R-CN + 2H₂O + H⁺ → R-COOH + NH₄⁺. This lengthens the chain by one carbon.
- Reaction of Grignard reagents with CO₂ followed by dilute acid: R-MgX + CO₂ → R-COOMgX, then R-COOH.
- Hydrolysis of esters, acid chlorides and amides.
- Oxidation of alkylbenzenes with hot KMnO₄ gives benzoic acid.
Carboxylic acids are polar and form strong hydrogen bonds. Two molecules join as a dimer through two hydrogen bonds. Their boiling points are therefore higher than those of alcohols of similar mass: ethanoic acid boils at 118 °C and propan-1-ol at 97 °C. The first four members dissolve in water in any proportion, and solubility falls with chain length. Lower acids have sharp smells. Benzoic acid is a white solid, only slightly soluble in cold water.
Acidity: in water a carboxylic acid ionises partly: RCOOH ⇌ RCOO⁻ + H⁺. The carboxylate ion is stabilised by resonance, with the negative charge shared equally between the two oxygens. This makes carboxylic acids stronger than alcohols and phenols, but they are weak acids compared with mineral acids. Typical Kₐ for ethanoic acid is 1.8 × 10⁻⁵ mol/dm³.
Substituents change acid strength. Electron-withdrawing groups (Cl, F, NO₂) pull electrons away, stabilise the anion and increase acidity. Trichloroethanoic acid is much stronger than ethanoic acid. The closer the group is to the COOH, the larger the effect. Electron-donating alkyl groups reduce acidity, so methanoic acid is stronger than ethanoic acid. Strength order: Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH.
Reactions as an acid:
- With metals: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.
- With alkalis: CH₃COOH + NaOH → CH₃COONa + H₂O.
- With carbonates and hydrogencarbonates: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. This effervescence is the test for carboxylic acids, because phenols and alcohols do not respond.
Reactions of the carboxyl group:
- Esterification with an alcohol and conc. H₂SO₄ gives an ester and water. The reaction is reversible, so excess alcohol or removal of water raises the yield.
- With PCl₅, PCl₃ or SOCl₂ the OH is replaced by Cl, giving an acid chloride R-COCl.
- With NH₃ it first forms an ammonium salt, which on heating gives an amide R-CONH₂.
- Reduction with LiAlH₄ gives a primary alcohol.
- Heating the sodium salt with soda lime removes CO₂ and gives an alkane (decarboxylation).
Acid derivatives are ranked by reactivity toward nucleophiles: acid chloride > acid anhydride > ester > amide. Acid chlorides react quickly with water, alcohols and ammonia, and are used to make esters and amides. Esters have fruity smells and are used in perfumes and flavours, such as ethyl ethanoate. Hydrolysis of an ester with alkali (saponification) gives the carboxylate salt and an alcohol, and is the basis of soap making from fats.
Uses: ethanoic acid as vinegar (4 to 8% solution), in making cellulose acetate and as a solvent. Methanoic acid is used in leather and textile processing. Benzoic acid and its sodium salt are food preservatives. Long-chain acids (fatty acids) are used in soaps, and ethanedioic acid removes rust and ink stains.
Study tip: The acid strength order and its reason (inductive effect and resonance of the anion) is asked almost every year, so write it with a short explanation. Make a conversion chart linking alcohol, aldehyde, acid, acid chloride, ester and amide, with the reagent for each arrow. In titration numericals always write the balanced equation first to find the mole ratio.
Definitions and flashcards
Say the meaning first, then open the card to check.
Carboxyl group
The -COOH group, a carbonyl and a hydroxyl joined on one carbon.
Dimer
Two molecules held together, as carboxylic acids are by two hydrogen bonds.
Carboxylate ion
The anion RCOO⁻ formed when a carboxylic acid loses H⁺.
Esterification
Reaction of an acid with an alcohol to give an ester and water.
Acid chloride
A derivative R-COCl in which OH is replaced by Cl.
Amide
A derivative R-CONH₂ in which OH is replaced by NH₂.
Saponification
Alkaline hydrolysis of an ester or fat to give a carboxylate salt and alcohol.
Decarboxylation
Loss of CO₂ from a carboxylic acid or its salt.
Formula sheet
| Name | Formula | What it means |
|---|---|---|
| Saturated monocarboxylic acids | CₙH₂ₙ₊₁COOH | General formula |
| Acid dissociation constant | Kₐ = [H⁺][RCOO⁻] / [RCOOH] | A larger Kₐ means a stronger acid |
| Concentration from titration | M₁V₁ / n₁ = M₂V₂ / n₂ | For acid and base with coefficients n₁ and n₂ |
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why do carboxylic acids have higher boiling points than alcohols of similar mass? (2 marks)
Carboxylic acid molecules form a dimer joined by two hydrogen bonds, while alcohols form one hydrogen bond per pair. The larger intermolecular attraction needs more energy to overcome.
Why is ethanoic acid a stronger acid than ethanol? (2 marks)
The ethanoate ion is stabilised by resonance, with the charge shared by two oxygen atoms. The ethoxide ion has the charge on one oxygen only, and is also destabilised by the electron-pushing ethyl group. So ethanoic acid loses H⁺ more easily.
How can you distinguish a carboxylic acid from phenol? (2 marks)
Add sodium hydrogencarbonate solution. A carboxylic acid gives brisk effervescence of CO₂ and phenol does not, because phenol is a weaker acid than carbonic acid. Phenol gives a violet colour with neutral FeCl₃, which carboxylic acids do not.
Explain the effect of chlorine atoms on the acidity of ethanoic acid. (3 marks)
Chlorine is electronegative and pulls electron density away through the sigma bonds (the inductive effect). This disperses the negative charge of the carboxylate ion and stabilises it. More chlorine atoms give a larger effect, so trichloroethanoic acid is much stronger than ethanoic acid.
Describe the preparation of ethyl ethanoate in the laboratory. (3 marks)
Ethanoic acid is heated with excess ethanol and a few drops of conc. H₂SO₄: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The acid is the catalyst and also removes water to shift the equilibrium. The mixture is distilled and the ester, which has a fruity smell, is washed with sodium carbonate solution to remove acid and then dried.
Compare the reactivity of acid chlorides, anhydrides, esters and amides, and state why acid chlorides are the most reactive. (5 marks)
The order is acid chloride > acid anhydride > ester > amide. In an acid chloride the carbonyl carbon is attached to the very electronegative Cl, so it is strongly positive, and chloride is an excellent leaving group. In amides the nitrogen lone pair is shared with the carbonyl group by resonance, which lowers the positive charge on carbon and makes NH₂⁻ a poor leaving group. Acid chlorides therefore react with water, alcohols and ammonia in the cold, while amides need heating with acid or alkali.
Numericals with worked steps
6 g of ethanoic acid is neutralised by sodium hydroxide. Find the mass of NaOH needed and the mass of sodium ethanoate formed. (C = 12, H = 1, O = 16, Na = 23)
- CH₃COOH + NaOH → CH₃COONa + H₂O.
- Molar mass of CH₃COOH = 24 + 4 + 32 = 60 g/mol; moles = 6 / 60 = 0.1 mol.
- Moles of NaOH = 0.1 mol; molar mass 40 g/mol; mass = 4 g.
- Molar mass of CH₃COONa = 82 g/mol; mass = 0.1 × 82 = 8.2 g.
Answer: 4 g of NaOH and 8.2 g of sodium ethanoate
0.2 mol of ethanoic acid reacts with excess ethanol to form ethyl ethanoate with 60% yield. Find the mass of ester. (C = 12, H = 1, O = 16)
- CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.
- Moles of ester (theoretical) = 0.2 mol.
- Molar mass of CH₃COOC₂H₅ = 48 + 8 + 32 = 88 g/mol.
- Theoretical mass = 0.2 × 88 = 17.6 g.
- Actual mass = 17.6 × 60 / 100 = 10.56 g.
Answer: 10.56 g of ethyl ethanoate
25 cm³ of vinegar needs 20 cm³ of 0.1 mol/dm³ NaOH for neutralisation. Find the concentration of ethanoic acid in mol/dm³ and g/dm³. (Molar mass = 60 g/mol)
- Moles of NaOH = 0.1 × 20 / 1000 = 0.002 mol.
- Ratio 1 : 1, so moles of acid = 0.002 mol in 25 cm³.
- Molarity = 0.002 × 1000 / 25 = 0.08 mol/dm³.
- Mass concentration = 0.08 × 60 = 4.8 g/dm³.
Answer: 0.08 mol/dm³, which is 4.8 g/dm³
6 g of ethanoic acid is added to excess sodium hydrogencarbonate. Find the volume of CO₂ at STP. (1 mol gas = 22.4 dm³)
- CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂.
- Moles of acid = 6 / 60 = 0.1 mol.
- Moles of CO₂ = 0.1 mol.
- Volume = 0.1 × 22.4 = 2.24 dm³.
Answer: 2.24 dm³ of CO₂
Common mistakes
- Saying that phenol releases CO₂ from carbonates. Only carboxylic acids do this.
- Numbering the chain from the wrong end. The carboxyl carbon is always C-1.
- Forgetting that esterification is reversible, and writing a single arrow without an acid catalyst.
- Thinking that electron-donating groups make an acid stronger. They make it weaker.
- Writing the formula of ethanoic acid as CH₃COH. The correct form is CH₃COOH.