Edexcel A-Level Chemistry Paper 2, June 2024: Question 1

✅ Correct Answer

D: 2-bromo-2-methylpropane

💡 Key Knowledge

• Tertiary halogenoalkanes hydrolyse the fastest via an S_N1 mechanism.
• Carbon-halogen bond strength dictates that C-Br undergoes substitution faster than C-Cl.
• Silver nitrate reacts with released Br⁻ ions to form a cream precipitate of AgBr.

❌ Common Errors

Choosing primary isomers (1-bromobutane or 1-bromo-2-methylpropane) because students confuse substitution rates with steric hindrance trends or mistake structural stability for reaction speed.

✅ Correct Answer

C: CH₃COCl and CH₃NH₂

💡 Key Knowledge

Forming an N-substituted amide requires an acyl chloride (providing an acyl group) reacting with a primary amine (e.g., methylamine, CH₃NH₂ ). Ammonia ( NH₃ ) yields a non-substituted primary amide instead.

🧠 Exam Technique

Break down reactant names: "N-substituted" means the nitrogen atom of the amide group must be attached to an alkyl chain, which can only be supplied by a primary amine.

✅ Correct Answer Points

• One molecule of ammonia acts as a nucleophile.
• It attacks the carbon atom with a partial positive charge ( Cδ⁺ ).
• A second molecule of ammonia acts as a base.
• It removes a proton ( H⁺ ion) from the intermediate.

🧠 Mechanism Breakdown (Examiner Guidance)

If drawing the mechanism instead of writing text:

• Left side: Curly arrow from the lone pair on the nitrogen of NH₃ to the electron-deficient C , and a curly arrow breaking the C-Br bond to form Br⁻ .
• Right side: Show the intermediate species with a positive charge on nitrogen, and a curly arrow from an NH₃ lone pair to one of the H atoms, plus a bond-breaking arrow towards the N atom.

❌ Common Errors

Stating that the bromide ion ( Br⁻ ) acts as the base removing the proton. The mark scheme explicitly penalises referencing Br⁻ as the base in this context; it must be a second molecule of ammonia.

✅ Correct Answer

A: heat in a sealed tube, ethanol

💡 Key Knowledge

Ammonia gas would escape if heated under standard reflux because of its low boiling point. Therefore, a sealed tube (or pressure vessel) is required to keep ammonia in contact with the halogenoalkane under heating. Ethanol is used as a mutual solvent because halogenoalkanes are insoluble in pure water.

📐 Step-by-Step Calculation

Equation: CH₃CH₂CH₂CH₂OH + NaBr + H₂SO₄ → CH₃CH₂CH₂CH₂Br + NaHSO₄ + H₂O

Step 1: Find relative molecular masses (Mᵣ) of reactants or products

• Mᵣ of CH₃CH₂CH₂CH₂OH (butan-1-ol) = (4 × 12.0) + (10 × 1.0) + 16.0 = 74.0
• Mᵣ of NaBr = 23.0 + 79.9 = 102.9
• Mᵣ of H₂SO₄ = (2 × 1.0) + 32.1 + (4 × 16.0) = 98.1
• Total reactants mass = 74.0 + 102.9 + 98.1 = 275.0
• Alternatively, sum of products: 136.9 ( C₄H₉Br ) + 120.1 ( NaHSO₄ ) + 18.0 ( H₂O ) = 275.0

Step 2: Apply the Atom Economy formula

Atom Economy = (Mᵣ of desired product / Total Mᵣ of all reactants) × 100
Atom Economy = (136.9 / 275.0) × 100 = 49.782%

Step 3: Format to required significant figures / decimal places

Rounding to one decimal place gives: 49.8%

❌ Common Calculation Traps

• Incorrect rounding: Rounding 49.782% to 49.7% instead of 49.8% loses the final accuracy mark.
• Omitting stoichiometry: Check balancing numbers carefully—here all stoichiometric coefficients are 1, but always multiply Mᵣ values by their balancing numbers if greater than 1.