OxfordAQA International A Level Chemistry 3.3.9 Carboxylic Acids and Derivatives Exam Questions

About OxfordAQA International A Level Chemistry 3.3.9 Carboxylic Acids and Derivatives

Carboxylic acids and derivatives is an A2 organic topic examined in Unit 4. You need to describe carboxylic acids as weak acids that release carbon dioxide from carbonates, explain esterification and the uses of esters, and describe fats and oils as esters of glycerol. You also need acid and alkaline hydrolysis of esters, the formation of soap and the production of biodiesel. The second part covers acyl chlorides, acid anhydrides and amides, and the nucleophilic addition-elimination reactions of acyl chlorides with water, alcohols, ammonia and primary amines. The advantages of ethanoic anhydride in making aspirin are required, and Required practical 10 prepares and tests the purity of an organic solid.

What you need to know

• Carboxylic acids Carboxylic acids are weak acids. They react with carbonates to release carbon dioxide, which is a test for the COOH group.
• Esters Carboxylic acid + alcohol ⇌ ester + water, using a concentrated sulfuric acid catalyst. Esters are used as solvents, plasticisers, perfumes and food flavourings.
• Hydrolysis, soap and biodiesel Acid hydrolysis of an ester is reversible; alkaline hydrolysis is not and gives a carboxylate salt. Fats and oils are esters of glycerol; alkaline hydrolysis gives soap and glycerol. Biodiesel is a mixture of methyl esters made by reacting vegetable oils with methanol and a catalyst.
• Acyl chlorides Acyl chlorides react by nucleophilic addition-elimination with water (carboxylic acid), alcohols (ester), ammonia (primary amide) and primary amines (N-substituted amide), releasing HCl.
• Mechanism The nucleophile's lone pair attacks the delta positive carbonyl carbon and the C=O electrons move to oxygen. The C=O then reforms, Cl- is eliminated, and H+ is lost.
• Aspirin and Required practical 10 Ethanoic anhydride is used instead of ethanoyl chloride for aspirin because it is cheaper, less corrosive, reacts less violently and does not produce HCl. Required practical 10 involves preparing a solid, purifying it by recrystallisation and testing purity by melting point.

Common mistakes in 3.3.9 Carboxylic Acids and Derivatives

• Calling the acyl chloride reaction nucleophilic substitution. The mechanism is nucleophilic addition-elimination.
• Saying alkaline hydrolysis gives a carboxylic acid. It gives the carboxylate salt, which must be acidified to give the acid.
• Forgetting the catalyst or reversibility in esterification. Concentrated sulfuric acid is the catalyst and the reaction is reversible.
• Saying a pure solid melts over a range. A pure solid has a sharp melting point; impurities lower it and broaden the range.

Try three 3.3.9 Carboxylic Acids and Derivatives exam-style questions