1. Carboxylic Acids: Structure & Acidity
Carboxylic acids contain the carboxyl functional group (-COOH). They are weak acids that dissociate partially in aqueous solution:
RCOOH(aq) ⇌ RCOO⁻(aq) + H⁺(aq)
Carboxylic acids are much stronger acids than alcohols because the resulting carboxylate ion (RCOO⁻) is stabilised by delocalisation of negative charge across both oxygen atoms.
Characteristic Reaction with Carbonates
Carboxylic acids react with sodium carbonate or sodium hydrogencarbonate with vigorous effervescence (bubbles of CO₂ gas):
2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)
This rapid effervescence provides the standard chemical test to distinguish carboxylic acids from phenols and alcohols (which are not acidic enough to liberate CO₂ from carbonates).
Carboxylic acids are weak acids: only a small fraction of the molecules dissociate. They are still strong enough to liberate CO₂ from carbonates. This is the test for a carboxylic acid in Required Practical 5 (RP5).
2. Esterification & Ester Hydrolysis
Esterification (Fischer Esterification)
Carboxylic acids react reversibly with alcohols in the presence of a concentrated sulfuric acid (H₂SO₄) catalyst under reflux:
RCOOH + R'OH ⇌ RCOOR' + H₂O
Esters are named with the alkyl group from the alcohol first, followed by the alkanoate stem from the carboxylic acid (e.g. ethanoic acid + methanol → methyl ethanoate).
Uses of Esters
- Solvents: for example ethyl ethanoate in glues and nail-varnish remover.
- Plasticisers: added to polymers to make them more flexible.
- Perfumes: many esters have pleasant smells.
- Food flavourings: many esters have fruity tastes and smells.
Ester Hydrolysis: Acid vs Base
| Hydrolysis Type | Reagents & Conditions | Chemical Equation | Yield & Reversibility |
|---|---|---|---|
| Acid Hydrolysis | Dilute HCl or H₂SO₄; reflux | RCOOR' + H₂O ⇌ RCOOH + R'OH | Reversible equilibrium (ester + water ⇌ acid + alcohol); poor yield (equilibrium mixture of reactants and products). |
| Base Hydrolysis (Saponification) | Aqueous NaOH; reflux | RCOOR' + OH⁻ → RCOO⁻ + R'OH | Goes to completion (ester + NaOH → carboxylate salt + alcohol); high yield. (Carboxylate salt is acidified to recover acid). |
3. Vegetable Oils, Fats, Soaps & Biodiesel
Animal fats and vegetable oils are triesters of glycerol (propane-1,2,3-triol) with long-chain fatty acids (triglycerides).
- Saponification (Soap Making): Alkaline hydrolysis of triglycerides with hot aqueous NaOH produces glycerol (propane-1,2,3-triol) and sodium salts of long-chain fatty acids, which act as soap.
- Biodiesel Production: Triglycerides react with methanol in the presence of a sodium hydroxide / potassium hydroxide catalyst in a transesterification process to yield glycerol and a mixture of methyl esters (biodiesel).
The equations below use a general triglyceride. R is a long hydrocarbon chain. The three R groups can be different.
- Soap is the sodium salt of a long-chain carboxylic acid, RCOO⁻Na⁺.
- Biodiesel is a mixture of methyl esters of long-chain carboxylic acids, RCOOCH₃.
4. Acyl Chlorides & Acid Anhydrides
Acyl chlorides (RCOCl) and acid anhydrides ((RCO)₂O) are reactive acid derivatives that undergo nucleophilic addition-elimination reactions at room temperature without requiring an acid catalyst. The amides they make are shown here too:
| Nucleophile | Reaction with Ethanoyl Chloride (CH₃COCl) | Reaction with Ethanoic Anhydride ((CH₃CO)₂O) | Organic Product Class |
|---|---|---|---|
| Water (H₂O) | CH₃COCl + H₂O → CH₃COOH + HCl(g) | (CH₃CO)₂O + H₂O → 2CH₃COOH | Carboxylic acid |
| Alcohol (CH₃OH) | CH₃COCl + CH₃OH → CH₃COOCH₃ + HCl(g) | (CH₃CO)₂O + CH₃OH → CH₃COOCH₃ + CH₃COOH | Ester |
| Ammonia (NH₃) | CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl | (CH₃CO)₂O + 2NH₃ → CH₃CONH₂ + CH₃COONH₄ | Primary amide |
| Primary Amine (CH₃NH₂) | CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃⁺Cl⁻ | (CH₃CO)₂O + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃COONH₃CH₃ | N-substituted amide |
With a primary amine, the equation is often written with one mole of amine: CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl. The HCl then reacts with a second molecule of amine to form the salt CH₃NH₃⁺Cl⁻, as in the table. The same applies to ammonia.
5. Nucleophilic Addition-Elimination Mechanism Diagram
All four nucleophiles (water, alcohols, ammonia and primary amines) react by the same three steps:
- Addition: a lone pair on the nucleophile attacks the δ+ carbonyl carbon. The C=O π electrons move onto the oxygen.
- Elimination: the C=O bond reforms and Cl⁻ leaves.
- Loss of H⁺: H⁺ is lost from the nucleophile's O or N. Cl⁻ often takes it, giving HCl.
The Same Mechanism with Other Nucleophiles
| Nucleophile | Atom whose lone pair attacks | Group bonded to C after Cl⁻ leaves | H⁺ lost from | Product with ethanoyl chloride |
|---|---|---|---|---|
| Water, H₂O | O | -O⁺H₂ | O | CH₃COOH + HCl |
| Alcohol, CH₃OH (drawn above) | O | -O⁺(H)CH₃ | O | CH₃COOCH₃ + HCl |
| Ammonia, NH₃ | N | -N⁺H₃ | N | CH₃CONH₂ + HCl (HCl + NH₃ → NH₄Cl) |
| Primary amine, CH₃NH₂ | N | -N⁺H₂CH₃ | N | CH₃CONHCH₃ + HCl (HCl + CH₃NH₂ → CH₃NH₃⁺Cl⁻) |
Acid anhydrides react by the same mechanism. The leaving group is CH₃COO⁻ instead of Cl⁻, so the by-product is ethanoic acid instead of HCl.
6. Industrial Synthesis of Aspirin
Aspirin (acetylsalicylic acid) is synthesised by the esterification of the phenolic -OH group on 2-hydroxybenzoic acid (salicylic acid) using ethanoic anhydride:
2-hydroxybenzoic acid + ethanoic anhydride → aspirin + ethanoic acid
HOC₆H₄COOH + (CH₃CO)₂O → CH₃COOC₆H₄COOH + CH₃COOH
Why Ethanoic Anhydride is Preferred Industrially over Ethanoyl Chloride
- Safety (No toxic HCl gas): Ethanoyl chloride produces choking, corrosive, highly toxic fumes of hydrogen chloride gas (HCl). Ethanoic anhydride produces harmless, recyclable ethanoic acid.
- Less corrosive, controlled exotherm: Ethanoic anhydride is less corrosive and reacts less violently with water than ethanoyl chloride, making thermal runaway easier to manage on an industrial tonnage scale.
- Cost & storage: Ethanoic anhydride is cheaper and does not hydrolyse as readily with atmospheric moisture during storage.
Required Practical 10 (RP10)
Making aspirin is a common way to carry out RP10: prepare a pure organic solid, purify it by recrystallisation and test its purity by melting point. See RP10: Preparation and Purification of an Organic Solid.
7. Worked Synthesis & Mechanism
(a) Ethylamine (CH₃CH₂NH₂)
(b) Water
Solution:
- (a) With ethylamine: Forms an N-substituted amide and an alkylammonium salt:
CH₃CH₂COCl + 2CH₃CH₂NH₂ → CH₃CH₂CONHCH₂CH₃ + CH₃CH₂NH₃⁺Cl⁻
Organic product: N-ethylpropanamide. - (b) With water: Rapid hydrolysis releasing steamy fumes of HCl:
CH₃CH₂COCl + H₂O → CH₃CH₂COOH + HCl(g)
Organic product: propanoic acid.
Equation:
C₃H₅(OOCC₁₇H₃₅)₃ + 3CH₃OH → C₃H₅(OH)₃ + 3C₁₇H₃₅COOCH₃
Reactants: 1 mole of triglyceride + 3 moles of methanol.
Products: 1 mole of glycerol (propane-1,2,3-triol) + 3 moles of methyl stearate (biodiesel).
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
8. Practice Questions
Try each question, then open the answer to check your mark points.
Question 1 (2 marks): Write an equation for the reaction of ethanoic acid with sodium carbonate. State what you would see.
Show Answer & Mark Points
- 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (1)
- Effervescence (bubbles of gas) (1)
Question 2 (3 marks): Outline the mechanism for the reaction of ethanoyl chloride with ammonia. Name the organic product.
Show Answer & Mark Points
- Curly arrow from the lone pair on N of NH₃ to the δ+ carbon, and an arrow from the C=O bond to the oxygen (1)
- Arrow from the O⁻ lone pair to reform C=O, and an arrow from the C-Cl bond to Cl so that Cl⁻ leaves (1)
- Arrow from an N-H bond to N⁺ so that H⁺ is lost; product ethanamide, CH₃CONH₂ (1)
Question 3 (2 marks): Write an equation for the alkaline hydrolysis of a triglyceride, C₃H₅(OOCR)₃, by sodium hydroxide. Name the two products.
Show Answer & Mark Points
- C₃H₅(OOCR)₃ + 3NaOH → C₃H₅(OH)₃ + 3RCOONa (1)
- Glycerol (propane-1,2,3-triol) and soap (sodium salts of long-chain carboxylic acids) (1)
Question 4 (3 marks): Give three reasons why ethanoic anhydride, not ethanoyl chloride, is used to make aspirin industrially.
Show Answer & Mark Points
- It is cheaper (1)
- It is less corrosive and reacts less violently with water (1)
- Its by-product is ethanoic acid, not toxic HCl gas (1)