WJEC A-Level Chemistry AS Unit 2, June 2025: Question 5

2 marks · Medium difficulty · Short Answer

Name the given alkene using IUPAC nomenclature including stereochemical descriptor.

Practise this question

Question

Question 5 shows the skeletal/structural formula of an alkene with a central C=C double bond. The left carbon of the double bond is bonded to a hydrogen atom below and a -CH(CH3)2 (isopropyl) group above. The right carbon of the double bond is bonded to an ethyl group (-CH2CH3) above and a methyl group (-CH3) below. Students are asked to name the compound for 2 marks.
Question text

5. Name the compound shown by the structure drawn below. [2]

CH3

H3C CH CH2CH3

C C

H CH3

Mark scheme

Show the mark scheme Mark scheme for question 5: Awards 2 marks for 'Z-2,4-dimethylhex-3-ene' (also accepts 'Z-3,5-dimethylhex-3-ene'). Awards 1 mark for 'E-2,4-dimethylhex-3-ene', '2,4-dimethylhex-3-ene', or '3,5-dimethylhex-3-ene'.

5 Z-2,4-dimethylhex-3-ene (2)

accept Z-3,5-dimethylhex-3-ene

award (1) for any of following

E-2,4-dimethylhex-3-ene

2,4-dimethylhex-3-ene

3,5-dimethylhex-3-ene

How to answer it

IUPAC Naming & Stereoisomerism of Branched Alkenes

📋 What this question tests
  • Identifying the longest carbon chain containing the C=C functional group.
  • Numbering the carbon skeleton to give substituent branches and the double bond the lowest possible locants.
  • Applying Cahn-Ingold-Prelog (CIP) priority rules across the double bond to assign E or Z configuration.
  • Standard IUPAC punctuation: correct use of hyphens, commas, and formatting of stereodescriptors.

Question 5

IUPAC Systematic Name for a Substituted Alkene [2 Marks]

✅ Correct Answer & Mark Allocation

Full 2 Marks:

Z-2,4-dimethylhex-3-ene

(Also accepted: Z-3,5-dimethylhex-3-ene )

Partial Credit [1 Mark] awarded for:
  • 2,4-dimethylhex-3-ene (omitted stereodescriptor)
  • 3,5-dimethylhex-3-ene (omitted stereodescriptor)
  • E-2,4-dimethylhex-3-ene (correct stem, incorrect E/Z assignment)

📐 Step-by-Step Deduction

  1. Find the longest continuous chain: Trace from left to right: (CH₃)₂CH– (2 carbons) → C=C (2 carbons) → –CH₂CH₃ (2 carbons).
    Total = 6 carbons → hex-3-ene.
  2. Number from the end giving lowest branch numbers:
    Left-to-right gives methyls at C2 and C4.
    (Right-to-left gives 3,5-dimethyl; 2,4 is preferred).
  3. Add substituent prefixes: Two methyl groups → 2,4-dimethylhex-3-ene.
  4. Assign CIP Priorities to C=C:
    Left C: –CH(CH₃)₂ (C) beats –H (H) → High priority is TOP.
    Right C: –CH₂CH₃ (C attached to C,H,H) beats –CH₃ (C attached to H,H,H) → High priority is TOP.
    Both high-priority groups are on the same side → Z.

💡 Key Knowledge: CIP Priority Rules

  • Step 1: Compare the atomic number (Z) of atoms directly attached to each C=C carbon.
    • Left carbon: Carbon (Z = 6) vs Hydrogen (Z = 1). Carbon wins!
  • Step 2: If directly attached atoms are identical, compare the atoms attached to them in order of decreasing atomic number until a point of difference is reached.
    • Right carbon: Ethyl (–CH₂CH₃) carbon is bonded to (C, H, H); Methyl (–CH₃) carbon is bonded to (H, H, H). Ethyl wins!
  • Step 3: If highest priority groups are on the same side → Z (zusammen = together). If on opposite sides → E (entgegen = opposite).

🧠 Exam Technique & Top-Scorer Tips

  • Always look for stereoisomerism: Whenever an alkene question shows a non-linear displayed/skeletal formula with specific geometry, ask yourself: "Does this alkene exhibit E/Z isomerism?" If so, include it in the name.
  • Mnemonic for Z: "Z-same side" (Zame Zide). Both top groups have higher priority, so it must be Z.
  • Punctuation counts: Numbers separated from numbers by commas ( 2,4 ); numbers separated from letters by hyphens ( -dimethyl ).

❌ Common Traps & Where Marks Were Lost

  • Forgetting the stereodescriptor entirely: Writing only 2,4-dimethylhex-3-ene limits the mark to 1/2.
  • Assigning E instead of Z: Students often mistakenly consider the bulky isopropyl group and the ethyl group as "opposite" without methodically ranking each carbon separately.
  • Misidentifying the main chain: Mistaking the isopropyl group for part of a 5-carbon chain (pentene) rather than unwrapping it to form a 6-carbon (hexane) parent chain.
  • Wrong double-bond locant: Hex-2-ene instead of hex-3-ene.

Topics

Organic Chemistry · 2.4 Organic compounds

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.