AQA A Level Chemistry 3.3.9 Carboxylic Acids and Derivatives Exam Questions

About AQA A Level Chemistry 3.3.9 Carboxylic Acids and Derivatives

This A Level topic on the AQA course covers carboxylic acids and the compounds made from them. You need the reactions of carboxylic acids with carbonates and alcohols, the uses of esters, acid and base hydrolysis of esters, the formation of soap and biodiesel from vegetable oils and fats, and acylation using acyl chlorides and acid anhydrides with water, alcohols, ammonia and primary amines. The nucleophilic addition-elimination mechanism is essential. Required practical 10 includes preparing a pure organic solid, such as aspirin, and testing its purity. Examiners test naming of esters and amides, mechanisms and reasons for using ethanoic anhydride in industry.

What you need to know

• Carboxylic acids Weak acids that react with carbonates to release CO2: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2. With alcohols and a concentrated sulfuric acid catalyst they form esters and water.
• Esters Named from the alcohol part then the acid part, e.g. ethanol and ethanoic acid give ethyl ethanoate. Esters are used as solvents, plasticisers, perfumes and food flavourings.
• Hydrolysis Acid hydrolysis is reversible and gives the carboxylic acid and alcohol. Base hydrolysis with NaOH goes to completion and gives the carboxylate salt and alcohol.
• Fats, soap and biodiesel Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol). Base hydrolysis gives soap (sodium salts of long-chain acids) and glycerol. Reaction with methanol and a catalyst gives biodiesel, a mixture of methyl esters.
• Acylation Acyl chlorides and acid anhydrides react with water to give carboxylic acids, with alcohols to give esters, with ammonia to give amides and with primary amines to give N-substituted amides. Acyl chlorides react vigorously and release HCl.
• Addition-elimination mechanism The nucleophile's lone pair attacks the δ+ carbonyl carbon and the C=O π electrons move to oxygen. The C=O reforms, Cl- is eliminated and H+ is removed from the nucleophile.
• Ethanoic anhydride and aspirin Ethanoic anhydride is used to make aspirin industrially because it is cheaper, less corrosive, reacts less vigorously, is less easily hydrolysed and does not produce HCl gas. Purity of the solid product is checked by melting point.

Common mistakes in 3.3.9 Carboxylic Acids and Derivatives

• Naming esters the wrong way round. The alkyl group from the alcohol comes first, then the -oate from the acid.
• Showing the product of base hydrolysis as the carboxylic acid. In excess alkali it is the carboxylate salt.
• Leaving out the reformation of C=O and the loss of H+ in the addition-elimination mechanism.
• Saying a sharp melting point far below the data value means the product is pure. Impurities lower the melting point and widen the range.
• Forgetting the second molecule of amine or ammonia that reacts with HCl to form an ammonium salt.

Try three 3.3.9 Carboxylic Acids and Derivatives exam-style questions