AQA A-Level Chemistry AS Paper 2, June 2025: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify the correct IUPAC name for a branched fluoroalkene including E/Z stereochemistry.

Practise this question

Question

Question 12 asks 'What is the name of this compound according to IUPAC rules?' followed by a skeletal formula of an alkene. The structure features a C=C double bond where the left carbon is bonded to a methyl group and hydrogen, and the right carbon is bonded to a fluorine atom pointing downwards and an isopropyl group pointing upwards. Four multiple-choice options are given: A: E-3-fluoro-2-methylpent-3-ene, B: Z-3-fluoro-2-methylpent-4-ene, C: E-3-fluoro-4-methylpent-2-ene, D: Z-3-fluoro-4-methylpent-2-ene.
Question text

12 What is the name of this compound according to IUPAC rules?

[1 mark]

A E–3-fluoro-2-methylpent-3-ene

B Z–3-fluoro-2-methylpent-4-ene

C E–3-fluoro-4-methylpent-2-ene

D Z–3-fluoro-4-methylpent-2-ene

Mark scheme

Show the mark scheme Mark scheme for question 12 shows the correct answer as option D, with 1 mark awarded under assessment objective AO3, naming the compound Z-3-fluoro-4-methylpent-2-ene.

12 D 1 (AO3) Z–3-fluoro-4-methylpent-2-ene

How to answer it

AQA A-Level Chemistry • Paper 2 • Organic Chemistry

IUPAC Nomenclature & Cahn-Ingold-Prelog (E/Z) Isomerism

What this question tests

This multiple-choice question assesses your ability to systematically apply IUPAC nomenclature rules and stereochemical priority rules to a substituted haloalkene shown as a skeletal formula:

  • Identifying the longest continuous carbon chain that contains the principal functional group (alkene, C=C).
  • Correctly numbering the carbon skeleton to give the double bond the lowest possible locant.
  • Alphabetical ordering of substituents (prefix naming).
  • Assigning E / Z stereodescriptors using the Cahn–Ingold–Prelog (CIP) priority rules based on atomic number.

Question 12 Breakdown

Multiple Choice (1 Mark)

✅ Correct Answer

D: Z–3-fluoro-4-methylpent-2-ene

Mark Scheme: Award 1 mark for selecting option D (AO3).

💡 Cahn–Ingold–Prelog (CIP) Rules

  • Compare atoms directly bonded to each alkene carbon.
  • Higher atomic number = higher priority.
  • C2 atom: Bonded to Carbon ( -CH₃ , Z = 6) vs Hydrogen ( -H , Z = 1).
    → -CH₃ is higher priority.
  • C3 atom: Bonded to Fluorine ( -F , Z = 9) vs Carbon ( -CH(CH₃)₂ , Z = 6).
    → -F is higher priority.

📐 Step-by-Step Derivation of the Name

  1. Find the main chain: The longest carbon chain containing the C=C double bond has 5 carbons → base chain is pentene.
  2. Number the chain:
    • Numbering from left-to-right gives the double bond position 2 ( pent-2-ene ).
    • Numbering from right-to-left would give the double bond position 3 ( pent-3-ene ).
    • Functional group priority requires the lowest locant for the alkene → pent-2-ene. This eliminates options A and B immediately!
  3. Identify and order substituents:
    • Position 3 has a fluoro group ( 3-fluoro ).
    • Position 4 has a methyl branch ( 4-methyl ).
    • Alphabetical arrangement: 'f' comes before 'm' → 3-fluoro-4-methylpent-2-ene .
  4. Determine stereochemistry (E vs Z):
    • Draw the plane/axis of the C=C double bond.
    • At carbon 2, the higher priority methyl group ( -CH₃ ) points downwards (the implicit H points upwards).
    • At carbon 3, the higher priority fluorine atom ( -F ) points downwards.
    • Both high-priority groups are on the same side (both pointing down) → Z (from German zusammen, "together").

🧠 Exam Technique & Shortcuts

  • Eliminate in pairs: Check the chain numbering first. Alkene at C2 vs C3? Position 2 wins, so strike out A and B instantly.
  • Remember the mnemonic: Z = "on ze Zame Zide". If both priority groups point to the same side of the double bond axis, it is Z.
  • Draw out implicit hydrogens: In skeletal formulas, alkene carbons with one drawn substituent always have an implicit hydrogen pointing into the open corner of the trigonal planar 120° arrangement.

❌ Common Traps & Errors

  • Group size misconception: Thinking the large isopropyl group ( -CH(CH₃)₂ ) has higher priority than fluorine. CIP rules look only at atomic number of the immediately attached atom (F = 9 > C = 6).
  • Numbering from the substituent end: Numbering from the right gives substituents lower locants (2-methyl, 3-fluoro) but makes the alkene position 3. The principal functional group (C=C) must take priority for the lowest number.
  • Confusing cis/trans with E/Z: cis/trans cannot be used unambiguously here because the double bond has three different attached groups. Always use E/Z with CIP rules.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.4 Alkenes

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