Edexcel A-Level Chemistry AS Paper 2, June 2016: Question 1

✅ Correct Answer

B (2,3-dimethylhexane)

💡 Key Knowledge

• Find the longest continuous carbon chain (6 carbons = hexane).
• Number the chain from the end that gives substituents the lowest possible locants (from right to left gives methyl groups at positions 2 and 3 rather than 4 and 5).
• Combine identical substituents using the prefix 'di-'.

❌ Common Errors

Choosing a wrong numbering direction or miscounting the longest carbon chain, leading to incorrect locants like 4,5-dimethylhexane.

✅ Correct Answer

A (3)

💡 Key Knowledge

Pentane has 3 structural isomers: pentane (straight chain), 2-methylbutane (branched), and 2,2-dimethylpropane (double branched).

🧠 Exam Technique

Systematically draw straight chains first, then shorten the main chain by one carbon to use as a methyl branch, checking for duplicates.

✅ Correct Answer

Skeletal formula of heptane (7-carbon zig-zag chain) reacting to form the skeletal formula of cycloheptane (7-membered ring) plus H₂ .

💡 Key Knowledge

Reforming involves converting straight-chain alkanes into cyclic alkanes and hydrogen gas. You must use clear skeletal formulae for the organic structures as requested.

❌ Common Errors

Using molecular, structural, or displayed formulae instead of skeletal drawings for the organic molecules. Max 1 mark can be awarded if extra reactants/products (like excess hydrogen) are included incorrectly.

✅ Correct Answer

D ( σ bond broken, homolytic fission)

💡 Key Knowledge

Single covalent bonds are σ (sigma) bonds. Homolytic fission occurs when the bonded pair of electrons is split equally between the two atoms, forming two radicals.

❌ Common Errors

Confusing homolytic fission (radical formation, equal split) with heterolytic fission (unequal split forming ions).

✅ Correct Answer

Step 1: C₂H₆ + Cl• → C₂H₅• + HCl
Step 2: C₂H₅• + Cl₂ → C₂H₅Cl + Cl•

💡 Key Knowledge

Propagation steps always feature a molecule and a radical reacting to form a new molecule and a new radical. Notice how the chlorine radical ( Cl• ) is regenerated in step 2.

🧠 Exam Technique

Ensure radical dots ( • ) are clearly positioned on the correct species. Equations can be written in either order, but both are required for full marks.

❌ Common Errors

Forgetting to include radical dots or mixing up propagation steps with initiation or termination steps. Missing radical dots are usually penalised once across the whole script.

✅ Correct Answer

Two ethyl radicals react together: C₂H₅• + C₂H₅• → C₄H₁₀ (or described in words as two ethyl radicals combining).

💡 Key Knowledge

Trace amounts of longer-chain alkanes like butane form during free radical substitution via a termination step where two alkyl radicals collide and bond.

❌ Common Errors

Attempting to write initiation, propagation, or ionic equations instead of focusing on the radical combination step. Do not use molecules or ions as reactants here.