Reactions of aldehydes and ketonesAQA A-Level Chemistry: Subtopic test

1. 1
   A technician has two unlabelled colourless liquids, one of which is propanal and the other propanone. She warms a sample of each separately with Tollens' reagent and with Fehling's solution in a hot water bath.

   (a)

   What is observed when propanal is warmed with Tollens' reagent?

   [1 mark]

   • AA red precipitate forms
   • BA silver mirror forms on the inside of the tube
   • CThe solution turns from orange to green
   • DThe solution stays colourless with no change

   (b)

   What is observed when propanal is warmed with Fehling's solution?

   [1 mark]

   • AThe blue solution turns to a silver mirror
   • BThe blue solution turns green
   • CThe blue solution stays blue
   • DThe blue solution forms a red precipitate

   (c)

   State what is observed when propanone is tested with each reagent, and explain why propanal reacts differently.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist reduces a sample of butanal and a sample of butanone separately, using NaBH₄ in aqueous solution at room temperature.

   (a)

   What is the organic product when butanone is reduced with NaBH₄?

   [1 mark]

   • AButan-2-ol
   • BButan-1-ol
   • CButanal
   • DButanoic acid

   (b)

   Which species acts as the nucleophile in the reduction with NaBH₄?

   [1 mark]

   • AH⁺
   • BOH⁻
   • CH⁻
   • DNa⁺

   (c)

   Write an overall equation for the reduction of butanal, using [H] to represent the reductant, and name the type of mechanism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two isomeric carbonyl compounds, pentanal and pentan-2-one, are each treated with NaBH₄ in aqueous solution. The organic products are then separated and identified.

   (a)

   Identify the organic product formed from each carbonyl compound, and state the class of alcohol in each case.

   [3 marks]

   (b)

   Describe the mechanism for the reduction of pentan-2-one by NaBH₄ in aqueous solution, including the movement of electron pairs and the intermediate formed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist has an unlabelled bottle that contains either pentanal or pentan-3-one. After identifying the compound as pentan-3-one, she wants to convert all of it into pentan-3-ol.

   (a)

   Describe how Tollens' reagent could be used to show that the bottle contains pentan-3-one and not pentanal. Include the observations, and explain the difference in terms of oxidation.

   [6 marks]

   (b)

   Describe how pentan-3-one can be reduced to pentan-3-ol. Your answer should include the reagent, an overall equation using [H], and the mechanism, explaining why the reaction is nucleophilic addition.

   [6 marks]

   Total for question 4: 12 marks

End of questions