Aldehydes and ketonesEdexcel A-Level Chemistry: Subtopic test

1. 1
   A forensic analyst is given two colourless liquids. One is propanal and the other is propanone, both with molecular formula C₃H₆O. She has Tollens' reagent, Fehling's solution, 2,4-dinitrophenylhydrazine solution and acidified potassium dichromate(VI) available.

   (a)

   Which reagent would give a positive result with both propanal and propanone?

   [1 mark]

   • ATollens' reagent
   • BFehling's solution
   • C2,4-dinitrophenylhydrazine solution
   • DAcidified potassium dichromate(VI)

   (b)

   The analyst warms a sample of each liquid with Fehling's solution. What is observed for propanal?

   [1 mark]

   • AThe blue solution forms a brick-red precipitate
   • BA silver mirror forms on the inside of the tube
   • CThe orange solution turns green
   • DThe blue solution stays unchanged

   (c)

   A positive result with 2,4-dinitrophenylhydrazine shows that both liquids are carbonyl compounds. Describe how the analyst could use this reagent to confirm that one of the liquids is propanone.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanal reacts with hydrogen cyanide, HCN, in the presence of a small amount of potassium cyanide, KCN, to form a hydroxynitrile. The product contains a carbon atom bonded to four different groups, but the product mixture shows no effect on plane-polarised light.

   (a)

   What is the name of the organic product of this reaction?

   [1 mark]

   • A3-hydroxypropanenitrile
   • B2-hydroxybutanenitrile
   • CPropanenitrile
   • D2-hydroxypropanenitrile

   (b)

   What is the role of the potassium cyanide?

   [1 mark]

   • AIt acts as an electrophile that attacks the carbonyl oxygen
   • BIt provides cyanide ions, which act as the nucleophile
   • CIt acts as an oxidising agent for ethanal
   • DIt acts as an acid that protonates the carbonyl group

   (c)

   Explain why the product mixture does not rotate the plane of plane-polarised light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A research chemist studies the reduction of propanal and propanone, and also investigates a compound X of molecular formula C₄H₈O. Compound X gives an orange precipitate with 2,4-dinitrophenylhydrazine, gives no change with Tollens' reagent and gives a pale yellow precipitate when warmed with iodine and aqueous sodium hydroxide.

   (a)

   Propanal and propanone are each reduced by lithium tetrahydridoaluminate, LiAlH₄, in dry ether. State the organic product from each and explain why the ether must be dry.

   [3 marks]

   (b)

   Deduce the structure of compound X and explain how the observations support your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Propane (boiling temperature −42 °C), propanone (boiling temperature 56 °C) and propan-1-ol (boiling temperature 97 °C) have similar relative molecular masses. Propanone mixes with water in all proportions, whereas propane is almost insoluble in water. Propanone is also used as a starting material in the synthesis of hydroxynitriles.

   (a)

   Explain the differences in boiling temperature between these three compounds, and why propanone mixes with water but propane does not.

   [6 marks]

   (b)

   Describe the mechanism for the reaction of propanone with hydrogen cyanide in the presence of cyanide ions, and explain why the product shows no optical activity.

   [6 marks]

   Total for question 4: 12 marks

End of questions