Dehydration of alcoholsAQA A-Level Chemistry: Subtopic test

1. 1
   Cyclohexanol, C₆H₁₁OH, is heated with concentrated phosphoric acid and the product is collected by distillation. Cyclohexanol boils at 161 °C and the alkene product, cyclohexene, boils at 83 °C.

   (a)

   What type of reaction converts cyclohexanol into cyclohexene?

   [1 mark]

   • AAddition
   • BSubstitution
   • CElimination
   • DOxidation

   (b)

   What is the role of the phosphoric acid in this reaction?

   [1 mark]

   • ACatalyst
   • BOxidising agent
   • CReducing agent
   • DNucleophile

   (c)

   Explain why cyclohexene can be separated from the reaction mixture by distillation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student prepares an organic liquid and wants to separate it from the reaction mixture by distillation. The mixture is placed in a round-bottomed flask with a few anti-bumping granules and heated with an electric heating mantle. The flask is joined through a still head to a condenser, and the distillate runs into a receiver flask. The student collects only the liquid that distils over between 80 °C and 85 °C.

   (a)

   Where should the thermometer bulb be placed in this apparatus?

   [1 mark]

   • ADipping into the liquid in the flask
   • BIn the receiver flask
   • CAbove the condenser outlet
   • DLevel with the side arm of the still head, in the vapour entering the condenser

   (b)

   How should cooling water flow through the condenser?

   [1 mark]

   • AIn at the top and out at the bottom
   • BIn at the bottom and out at the top
   • CIn at either end, which makes no difference
   • DThere should be no water flow, to keep the vapour hot

   (c)

   Explain why the student collects only the liquid that distils between 80 °C and 85 °C, and why an electric heating mantle is used rather than a Bunsen burner.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A company in a country with large sugar-cane plantations but no crude oil makes ethanol by fermentation of sugar. It then converts the ethanol to ethene and makes poly(ethene) from the ethene.

   (a)

   State the reagent and conditions for converting ethanol into ethene, name the type of reaction, and write the equation.

   [3 marks]

   (b)

   Explain why this route to poly(ethene) uses a renewable resource, write an equation for the formation of poly(ethene) from ethene, and suggest one disadvantage of the route.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Elimination of water from alcohols is used to make alkenes. A student studies the acid-catalysed dehydration of propan-2-ol and of cyclohexanol.

   (a)

   Outline the mechanism for the dehydration of propan-2-ol to propene, describing each step and the movement of electron pairs.

   [6 marks]

   (b)

   In an experiment, 15.0 g of cyclohexanol (Mᵣ = 100.0) is dehydrated to cyclohexene (Mᵣ = 82.0) and 7.38 g of cyclohexene is collected. Calculate the percentage yield and suggest two reasons why it is below 100%.

   [6 marks]

   Total for question 4: 12 marks

End of questions