1. 1
   A student sorts some common reagents into nucleophiles and electrophiles. The species considered are water (H₂O), ammonia (NH₃), the hydroxide ion (OH⁻), the cyanide ion (CN⁻), the hydrogen ion (H⁺), methane (CH₄), the sodium ion (Na⁺) and a bromine molecule (Br₂) approaching the double bond of an alkene.

   (a)

   Which of these species can act as a nucleophile?

   [1 mark]

   • ACH₄
   • BH⁺
   • CNH₃
   • DNa⁺

   (b)

   Which statement about electrophiles is correct?

   [1 mark]

   • AAn electrophile forms a new bond by donating a lone pair of electrons to its reaction partner.
   • BAn electrophile forms a new bond by accepting both bonding electrons from its reaction partner.
   • CAn electrophile must carry a positive charge.
   • DAn electrophile is attracted to regions of low electron density.

   (c)

   Explain why both H₂O and OH⁻ can act as nucleophiles, and suggest why OH⁻ is the more effective nucleophile.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   When hydrogen bromide gas dissolves in water, the H–Br bond breaks and the solution becomes strongly acidic. In a separate reaction, the C–Br bond in bromoethane, CH₃CH₂Br, breaks as the molecule reacts. Pauling electronegativities: H 2.2, C 2.6, Br 3.0.

   (a)

   Which equation shows how the H–Br bond breaks when hydrogen bromide dissolves in water?

   [1 mark]

   • AHBr → H• + Br•
   • BHBr → H⁻ + Br⁺
   • C2HBr → H₂ + Br₂
   • DHBr → H⁺ + Br⁻

   (b)

   Which pair of fragments is formed when the C–Br bond in bromoethane undergoes heterolytic fission?

   [1 mark]

   • ACH₃CH₂⁺ and Br⁻
   • BCH₃CH₂• and Br•
   • CCH₃CH₂⁻ and Br⁺
   • DCH₃CH₂Br⁺ and e⁻

   (c)

   Explain, with reference to the electronegativity values, why the H–Br bond breaks to form H⁺ and Br⁻ rather than H⁻ and Br⁺.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats 2.74 g of 1-bromobutane, CH₃CH₂CH₂CH₂Br, under reflux with an excess of aqueous sodium hydroxide for 20 minutes and isolates 1.11 g of pure butan-1-ol. The student then repeats the experiment under identical conditions using water in place of the sodium hydroxide solution, and finds that only about 5% of the 1-bromobutane has reacted after 20 minutes.

   (a)

   Deduce the equation for the reaction between 1-bromobutane and hydroxide ions, and describe the movement of electron pairs that takes place.

   [3 marks]

   (b)

   Calculate the percentage yield of butan-1-ol, and suggest why the reaction with water gave a much lower conversion than the reaction with hydroxide ions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrocarbon Q is a gas at room temperature and contains only carbon and hydrogen. 1.40 g of Q reacts completely with exactly 4.00 g of bromine, Br₂, dissolved in an inert solvent, and the orange colour of the bromine disappears. Only one product is formed. When Q reacts with hydrogen bromide, only one structural isomer with the formula C₄H₉Br is formed. A student concludes that Q is but-2-ene.

   (a)

   Explain why alkenes such as Q undergo electrophilic addition, and deduce equations for the reactions of but-2-ene with bromine, hydrogen bromide and water, naming each organic product.

   [6 marks]

   (b)

   Use the data to deduce the molecular formula of Q, and evaluate the student's conclusion that Q is but-2-ene.

   [6 marks]

   Total for question 4: 12 marks

End of questions