Oxidation of alcoholsAQA A-Level Chemistry: Subtopic test

1. 1
   A school technician is given three unlabelled liquids, each with the molecular formula C₄H₁₀O: butan-1-ol, butan-2-ol and 2-methylpropan-2-ol. She warms a sample of each with acidified potassium dichromate(VI) solution.

   (a)

   Which of the three liquids is a tertiary alcohol?

   [1 mark]

   • AButan-1-ol
   • B2-Methylpropan-2-ol
   • CButan-2-ol
   • DAll three are tertiary alcohols

   (b)

   What is the organic product when butan-2-ol is oxidised by acidified potassium dichromate(VI)?

   [1 mark]

   • AButanal
   • BButanoic acid
   • C2-Methylpropanal
   • DButanone

   (c)

   State the colour of the mixture after warming for butan-2-ol and for 2-methylpropan-2-ol, and explain each result.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Propan-1-ol can be converted into either propanal or propanoic acid using acidified potassium dichromate(VI), depending on how the apparatus is set up. In one experiment the alcohol is heated in a flask fitted with a distillation head and the product is collected as it forms. In a second experiment the mixture is heated under reflux with excess oxidising agent for a long time.

   (a)

   Which compound is formed when propan-1-ol is heated under reflux with excess acidified potassium dichromate(VI)?

   [1 mark]

   • APropanoic acid
   • BPropanal
   • CPropanone
   • DPropene

   (b)

   Why does distilling the product as it forms give propanal rather than propanoic acid?

   [1 mark]

   • APropanal has a higher boiling point than propan-1-ol so it stays in the flask
   • BDistillation supplies extra oxygen to the mixture
   • CPropanal has no hydrogen bonding, so it has a lower boiling point and is removed before it can be oxidised further
   • DDistillation neutralises the acid that would otherwise form

   (c)

   Write two equations, using [O] to represent the oxidising agent, for the oxidation of propan-1-ol to propanal and then propanal to propanoic acid.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A forensic chemist has two colourless liquids, pentanal and pentan-2-one, which both have the molecular formula C₅H₁₀O. She tests a sample of each with warm Fehling's solution and with warm Tollens' reagent.

   (a)

   Describe the observations the chemist would make when each liquid is tested with Fehling's solution and with Tollens' reagent.

   [3 marks]

   (b)

   Explain why only one of the liquids reacts, and write an equation, using [O] for the oxidising agent, for the reaction of pentanal.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical company wants to make ethanal and ethanoic acid from ethanol using acidified potassium dichromate(VI). A quality-control chemist must also be able to tell the products apart from similar compounds using simple test-tube reactions.

   (a)

   Explain how the apparatus and conditions are chosen to obtain ethanal from ethanol, and how ethanoic acid is obtained instead. Include equations using [O] and the colour change of the oxidising agent.

   [6 marks]

   (b)

   The chemist has four unlabelled liquids: propan-1-ol, propan-2-ol, propanal and propanone. Describe a sequence of test-tube reactions that would identify each liquid, giving the observation for each.

   [6 marks]

   Total for question 4: 12 marks

End of questions