Edexcel A-Level Chemistry AS Paper 2, June 2017: Question 4

✅ Correct Answer

• Molecular formula: C₁₀H₁₈O
• Molar mass: 154 g mol⁻¹ (units required)

💡 Key Knowledge

Each vertex in a skeletal structure represents a carbon atom. Terminals represent methyl groups (unless stated otherwise). Hydrogen atoms must be deduced by ensuring each carbon forms 4 covalent bonds.

📐 Calculation Steps

1. Count carbons: 10 carbon atoms in the main chain and branches.
2. Count hydrogens: Account for 2 double bonds (each losing 2 H compared to an alkane) and the OH group. Total = 18 hydrogens.
3. Calculate molar mass: (10 × 12.0) + (18 × 1.0) + (1 × 16.0) = 154 g mol⁻¹.

❌ Common Errors

Forgetting to include units for molar mass, or miscounting hydrogen atoms on terminal methyl groups or near double bonds.

✅ Correct Answers

(i) The furthest peak to the right (molecular ion peak / parent ion) is at m/z = 154 , which confirms the molar mass.

(ii) C₅H₅⁺ (or [C₅H₅]⁺ )

💡 Key Knowledge

The molecular ion peak corresponds to the molecular mass of the intact molecule losing one electron. Fragment peaks correspond to stable carbocations formed during fragmentation.

🧠 Exam Technique

For part (ii), ensure the positive charge is clearly indicated (e.g. superscript plus sign) on the ion formula. Brackets around the formula with an external plus sign are also fully accepted.

✅ Correct Answer

• First functional group: Alkene. Bond: C=C (Range: 1645–1669 cm⁻¹) OR C-H alkene stretch (Range: 3010–3095 cm⁻¹).
• Second functional group: Alcohol (or Hydroxyl). Bond: O-H (Range: 3200–3750 cm⁻¹).

❌ Common Errors

Writing "hydroxide" instead of "alcohol" or "hydroxyl" loses the functional group mark. Quoting wavenumber ranges outside the Data Booklet values or missing bond assignments will lose credit.

✅ Correct Answers

• Alkene Test: Add bromine water ( Br₂(aq) ). Observation: Changes from orange/brown/yellow to colourless.
• Alcohol Test: Add phosphorus pentachloride ( PCl₅ ). Observation: Misty / steamy / white fumes produced. (Alternative: Acidified potassium dichromate turns orange to green upon warming).

❌ Common Errors

Writing "clear" instead of "colourless" when describing the bromine water test is a frequent penalty. Ensure reagent names and distinct observations are explicitly paired.

✅ Correct Answer

2 isoprene molecules.

💡 Key Knowledge

Isoprene (2-methylbuta-1,3-diene) has 5 carbon atoms. Geraniol has 10 carbon atoms.

📐 Calculation & Justification

Number of carbon atoms in geraniol = 10. Number of carbon atoms in isoprene = 5. Therefore, ratio = 10 / 5 = 2.

✅ Correct Answers

When excess hydrogen bromide ( HBr ) reacts with 2-methylbuta-1,3-diene, both double bonds undergo electrophilic addition. Four distinct dibromovaler / dibromomethylbutane isomeric structures can be formed depending on Markovnikov addition across the conjugated/isolated double bond positions:

• 1,2-dibromo-2-methylbutane structure
• 2,3-dibromo-2-methylbutane structure
• 1,3-dibromo-3-methylbutane structure
• 2,4-dibromo-2-methylbutane (or equivalent positional/skeletal monobromo/dibromo combinations accepted by mark scheme)

🧠 Exam Technique

Use displayed, structural, or clear skeletal formulae. Check that all carbon atoms have 4 bonds and every added bromine atom is correctly placed. Examiners award 1 mark per correct unique isomer up to 4 marks.

❌ Common Errors

Drawing duplicate identical molecules rotated in space, or failing to add HBr across both double bonds when "excess" is specified.