The questions are on the halogenoalkanes test. Take it under real conditions before reading the mark scheme.
Quick answer: Alkanes react with halogens in UV light by free radical substitution, a chain reaction of initiation, propagation and termination. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on halogenoalkanes, from radical substitution to ozone layer, with the equivalent answers to accept.
Compare your work with these answers and the marking points. Credit is given for correct steps, so look for the specific mark you missed and revise the related mechanism.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.4
Question 1 Radical substitution (3 marks)
- Cl₂ → 2Cl• (UV light).
- Cl• + CH₄ → •CH₃ + HCl and •CH₃ + Cl₂ → CH₃Cl + Cl•.
Marking: 1 initiation, 1 each propagation step.
Question 2 Mechanism (3 marks)
- Nucleophilic substitution (SN2).
- The lone pair on OH⁻ attacking the δ+ carbon, forming a new bond.
- It is inverted.
Marking: 1 mark each.
Question 3 Classify (1 mark)
- Tertiary: the carbon bonded to Br is attached to three other carbons.
Marking: 1 mark.
Question 4 Comparing rates (3 marks)
- 1-iodobutane > 1-bromobutane > 1-chlorobutane.
- The C–I bond (228 kJ mol⁻¹) is weaker than C–Br (290) and C–Cl (346). Less energy is needed to break it, so it is hydrolysed faster.
Marking: 1 for order, 1 for bond enthalpy trend, 1 for link to rate.
Question 5 Reagents (2 marks)
- KCN (potassium cyanide) in ethanol and heat under reflux.
Marking: 1 for KCN in ethanol, 1 for reflux.
Question 6 Percentage yield (3 marks)
- n = 6.00 ÷ 92.5 = 0.06486 mol; theoretical mass = 0.06486 × 74.0 = 4.80 g; yield = 3.10 ÷ 4.80 × 100 = 64.6 %.
Marking: 1 for amount, 1 for theoretical mass, 1 for yield.
Question 7 SN1 or SN2 (3 marks)
- rate = k[(CH₃)₃CBr].
- The slow step is the loss of Br⁻ to form the carbocation, which involves only the halogenoalkane. The nucleophile reacts in a fast step afterwards.
Marking: 1 for equation, 1 for slow step, 1 for nucleophile in the fast step.
Question 8 Ozone layer (2 marks)
- Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂.
Marking: 1 each step.
