WJEC A-Level Chemistry AS Unit 2, June 2025: Question 1

1 mark · Easy difficulty · Short Answer

Draw the skeletal formula of pentane-2,4-diol.

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Question

Question 1 states: 'Draw the skeletal formula of pentane-2,4-diol.' The question is worth 1 mark, shown in brackets on the right.
Question text

1. Draw the skeletal formula of pentane-2,4-diol. [1]

Mark scheme

Show the mark scheme Mark scheme for Question 1 showing a five-carbon zig-zag skeletal chain with hydroxyl (OH) groups attached at carbon 2 and carbon 4. An additional note specifies: 'do not accept CH3 groups on the ends of the chain'. Total marks: 1 (AO2).

Marks available

Question Marking details

AO1 AO2 AO3 Total Maths Prac

do not accept CH3 groups on the ends of the chain

How to answer it

Skeletal Formula of Pentane-2,4-diol

📌 What This Question Tests

This question assesses fundamental organic chemistry skills from the AS Level syllabus:

  • Translating IUPAC nomenclature into chemical structure: Breaking down the root name (pentane) and functional group suffixes/locants (-2,4-diol).
  • Rules of skeletal formulae: Accurately representing carbon chains as zig-zag lines without explicitly writing carbon or hydrogen atoms directly attached to carbons.
  • Displaying heteroatoms & functional groups: Showing alcohol (-OH) groups correctly connected by single bonds to the correct positions along the backbone.

Question 1

Draw the skeletal formula of pentane-2,4-diol. [1 Mark]

Skeletal formula of pentane-2,4-diol with 5 carbons in a continuous chain and -OH on C2 and C4.

✅ Correct Answer

  • A continuous 5-carbon zig-zag chain (consisting of 4 line segments).
  • Two single bonds extending from carbon 2 and carbon 4 terminating at -OH groups.
  • No carbon or hydrogen symbols written along the carbon backbone.

📐 Structural Breakdown

  • "pentane" = 5 carbon chain:
    C1 — C2 — C3 — C4 — C5
  • "-2,4-diol" = 2 alcohol groups (-OH), positioned at:
    • Carbon 2
    • Carbon 4
  • Symmetry: Numbering from left-to-right or right-to-left yields the exact same structure due to internal symmetry.

💡 Key Knowledge

  • In skeletal formulae, each vertex (bend) and each line termination represents a carbon atom with its attached hydrogen atoms implied.
  • All heteroatoms (atoms other than C and H, e.g. O, N, Cl) and any hydrogens directly bonded to them (e.g. -OH) must be drawn explicitly.
  • The bond to the alcohol group must connect directly to the oxygen atom: C—OH , never to the hydrogen atom ( C—HO unless drawn on the left with O bonded).

❌ Common Errors to Avoid

  • Writing "CH₃" at the ends: The official mark scheme explicitly states:
    "do not accept CH₃ groups on the ends of the chain". Writing letters for terminal carbons voids skeletal representation.
  • Counting line segments as carbons: Drawing 5 lines instead of 4. Remember: n carbons in an open chain require n - 1 bonds (4 lines = 5 carbons).
  • Incorrect locants: Placing the -OH groups on C1 and C3, or C2 and C3 (producing pentane-1,3-diol or pentane-2,3-diol).

🧠 Examiner Technique & Tips

  • Count before drawing: First lightly tap out the vertices: Start pencil down (C1) → up (C2) → down (C3) → up (C4) → down (C5). Count the 5 points to confirm you haven't drawn butane (4 carbons) or hexane (6 carbons).
  • Keep angles around 120°: Use standard tetrahedral/zig-zag angles so vertices are unambiguous.
  • Clean connectivity: Ensure the bond lines to the -OH groups cleanly meet the vertex of the carbon chain and terminate directly on the "O" of OH.
Mark Scheme Note: Award [1 mark] for the correct skeletal representation (AO2). 0 marks if terminal CH₃ groups are shown, or if the chain length is incorrect.

Topics

Organic Chemistry · 2.4 Organic compounds · 2.7 Alcohols and carboxylic acids

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.