Aldehydes and ketonesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A chemist compares three compounds of similar relative molecular mass: butane (Mr 58, boiling temperature −1 °C), propanal (Mr 58, boiling temperature 49 °C) and propan-1-ol (Mr 60, boiling temperature 97 °C).

   (a)

   Which statement about the intermolecular forces between propanal molecules is correct?

   [1 mark]

   • AThere are permanent dipole–permanent dipole attractions and London forces, but no hydrogen bonds
   • BThere are hydrogen bonds between the C=O oxygen of one molecule and the C–H hydrogen of another
   • CThere are only London forces because the C=O bond is non-polar
   • DThere are ionic attractions between the carbon and oxygen atoms of different molecules

   (b)

   What is the main reason for the boiling temperature of propan-1-ol being higher than that of propanal?

   [1 mark]

   • APropan-1-ol has much weaker London forces than propanal
   • BPropanal has a larger relative molecular mass than propan-1-ol
   • CPropan-1-ol molecules form hydrogen bonds with each other but propanal molecules cannot
   • DThe C=O bond in propanal is non-polar

   (c)

   Explain why propanal has a higher boiling temperature than butane, even though their relative molecular masses are the same.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician has two unlabelled liquids, one is propanal and the other is propanone. She tests a sample of each with Tollens' reagent, with Fehling's solution and with acidified potassium dichromate(VI) solution, warming gently where necessary.

   (a)

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

   [1 mark]

   • AA brick-red precipitate forms
   • BThe solution turns from orange to green
   • CA pale yellow precipitate forms
   • DA silver mirror forms on the inside of the test tube

   (b)

   Which species is reduced when Tollens' reagent reacts with propanal?

   [1 mark]

   • APropanal
   • B[Ag(NH₃)₂]⁺
   • CNH₃
   • DOH⁻

   (c)

   Write an equation, using [O] to represent the oxidising agent, for the reaction of propanal with acidified potassium dichromate(VI), and state the colour change seen for propanal but not for propanone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Compound X has the molecular formula C₄H₈O. It gives an orange precipitate with 2,4-dinitrophenylhydrazine solution. It gives no reaction when warmed with Tollens' reagent. It gives a pale yellow precipitate when warmed with iodine and sodium hydroxide solution.

   (a)

   Deduce the structure and name of compound X, explaining each observation.

   [3 marks]

   (b)

   Describe how the orange precipitate can be used to confirm the identity of X, and explain why the precipitate must be purified first.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research chemist studies two carbonyl compounds, ethanal and butanone. In one experiment ethanal is reacted with hydrogen cyanide in the presence of a small amount of potassium cyanide. In a second experiment ethanal and butanone are each reduced separately with lithium tetrahydridoaluminate(III) in dry ether.

   (a)

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

   [6 marks]

   (b)

   Compare the reduction of ethanal with that of butanone using lithium tetrahydridoaluminate(III) in dry ether. Your answer should include the products, equations using [H], why dry ether is used and the optical activity of each product mixture.

   [6 marks]

   Total for question 4: 12 marks

End of questions