Elimination from halogenoalkanesAQA A-Level Chemistry: Subtopic test

1. 1
   A student warms 2-bromopropane, CH₃CHBrCH₃, with aqueous potassium hydroxide and obtains mainly propan-2-ol. She repeats the experiment using a solution of potassium hydroxide in ethanol and heats the mixture strongly. This time the main product is a gas that decolourises bromine water.

   (a)

   What type of reaction forms propan-2-ol from 2-bromopropane and aqueous hydroxide ions?

   [1 mark]

   • AElectrophilic addition
   • BNucleophilic substitution
   • CElimination
   • DFree-radical substitution

   (b)

   Which statement correctly describes the role of the hydroxide ion in the reaction that forms the gas?

   [1 mark]

   • AIt acts as an electrophile by accepting a pair of electrons from carbon
   • BIt acts as a nucleophile by attacking the carbon bonded to bromine
   • CIt acts as a reducing agent by donating electrons to bromine
   • DIt acts as a base by removing a hydrogen ion from a carbon atom next to the C–Br carbon

   (c)

   Write an equation for the formation of the gas in the second experiment, using structural formulae for the organic species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydroxide ions, OH⁻, can react with a halogenoalkane in two different ways. Each oxygen atom in OH⁻ has lone pairs of electrons, and the ion can also accept a proton. Both reactions start with the polar carbon–halogen bond.

   (a)

   What is a nucleophile?

   [1 mark]

   • AAn electron pair donor
   • BAn electron pair acceptor
   • CA proton donor
   • DA proton acceptor

   (b)

   What happens to the carbon–bromine bond in both the substitution and the elimination reactions of 2-bromopropane with hydroxide ions?

   [1 mark]

   • AIt breaks homolytically, with one electron going to each atom
   • BIt breaks heterolytically, with both electrons going to carbon
   • CIt breaks heterolytically, with both electrons going to bromine
   • DIt does not break, because only the C–H bond is involved

   (c)

   Explain how the role of the hydroxide ion in substitution differs from its role in elimination.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician needs to make propene from 2-bromopropane. A competing reaction, which gives propan-2-ol, takes place at the same time under some conditions, so the technician must choose the reagent and conditions with care.

   (a)

   Describe, in words, the mechanism for the elimination of HBr from 2-bromopropane by hydroxide ions, referring to the movement of electron pairs.

   [3 marks]

   (b)

   Suggest the conditions that favour elimination rather than substitution. Describe a test to show that the gas collected is unsaturated and give the result.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist compares two experiments on 2-bromopropane: one with warm aqueous potassium hydroxide and one with hot ethanolic potassium hydroxide. In each experiment both propan-2-ol and propene are formed, but in different proportions.

   (a)

   Explain how both propan-2-ol and propene can be formed from 2-bromopropane, and how the conditions affect which product predominates.

   [6 marks]

   (b)

   In a third experiment, 12.3 g of 2-bromopropane is heated with an excess of potassium hydroxide. 65% of the 2-bromopropane undergoes elimination and the rest undergoes substitution. Calculate the volume of propene formed, measured at room temperature and pressure, and the mass of propan-2-ol formed. (Molar volume of a gas at room temperature and pressure = 24.0 dm³ mol⁻¹. Ar: C = 12.0, H = 1.0, Br = 79.9, O = 16.0.)

   [6 marks]

   Total for question 4: 12 marks

End of questions