Reaction types, mechanisms, nucleophiles and electrophilesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student summarises five reactions of ethene and related compounds. Reaction 1: ethene reacts with hydrogen bromide to form bromoethane. Reaction 2: bromoethane is warmed with aqueous sodium hydroxide to form ethanol and sodium bromide. Reaction 3: bromoethane is warmed with ethanolic sodium hydroxide to form ethene, sodium bromide and water. Reaction 4: ethanol is warmed with acidified potassium dichromate(VI) to form ethanal. Reaction 5: many ethene molecules join under high pressure to form poly(ethene).

   (a)

   Which row correctly classifies Reaction 1 and Reaction 2?

   [1 mark]

   • AReaction 1 is substitution and Reaction 2 is addition
   • BReaction 1 is addition and Reaction 2 is substitution
   • CReaction 1 is addition and Reaction 2 is elimination
   • DReaction 1 is elimination and Reaction 2 is addition

   (b)

   Which reaction is an oxidation?

   [1 mark]

   • AReaction 4
   • BReaction 1
   • CReaction 3
   • DReaction 5

   (c)

   Name the type of reaction in Reaction 3 and in Reaction 5, and state what happens to the organic molecules in each.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Bromoethane reacts with aqueous hydroxide ions to form ethanol: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The reaction can be explained by a mechanism in which curly arrows show the movement of pairs of electrons.

   (a)

   Which statement defines a nucleophile?

   [1 mark]

   • AA species that accepts a pair of electrons
   • BA species that donates a single electron
   • CA species that donates a pair of electrons to form a covalent bond
   • DA species that is attracted to a negatively charged atom

   (b)

   Why does the hydroxide ion attack the carbon atom in the C–Br bond?

   [1 mark]

   • AThe carbon is δ− because carbon is more electronegative than bromine
   • BThe carbon is δ+ because the C–Br bond is non-polar
   • CThe carbon is δ+ because bromine is a larger atom
   • DThe carbon is δ+ because bromine is more electronegative than carbon

   (c)

   Describe, in terms of the movement of electron pairs, how bromoethane forms ethanol in this reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethene reacts with bromine at room temperature to form 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. The reaction occurs readily even though ethene and bromine are both non-polar molecules.

   (a)

   Explain why ethene is attacked by electrophiles, and why bromine behaves as an electrophile in this reaction.

   [3 marks]

   (b)

   Explain, in terms of bond polarity and electron density, why bromoethane reacts with nucleophiles whereas ethene reacts with electrophiles.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies three reactions. Reaction A: propene reacts with hydrogen bromide to form 2-bromopropane. Reaction B: 1-bromobutane is warmed with aqueous potassium hydroxide to form butan-1-ol. Reaction C: 2-bromobutane is warmed with ethanolic potassium hydroxide to form an alkene.

   (a)

   Classify Reactions A, B and C by reaction type. For Reactions A and B, name the mechanism and explain what makes the organic molecule react with the attacking species.

   [6 marks]

   (b)

   Explain what is meant by heterolytic bond breaking. Classify the hydroxide ion, a water molecule and the hydrogen atom in hydrogen bromide as nucleophiles or electrophiles, and explain how bond polarity decides whether a compound reacts with nucleophiles or with electrophiles.

   [6 marks]

   Total for question 4: 12 marks

End of questions