AQA A-Level Chemistry Paper 3, June 2025: Question 18

1 mark · Easy difficulty · Multiple Choice

Identify the correct structural formula for 4-chloro-2-methylpent-2-enoic acid from four multiple choice options.

Practise this question

Question

Question 18 asks: 'What is the correct structural formula for 4-chloro-2-methylpent-2-enoic acid?' with 1 mark indicated. Four options with selection lozenges are provided: A: CH3CCl=CHCH(CH3)COOH, B: (CH3)2C=CHCHClCOOH, C: CH3CHClCH=C(CH3)COOH, D: (CH3)2CHCH=CClCOOH.
Question text

18 What is the correct structural formula for 4-chloro-2-methylpent-2-enoic acid?

[1 mark]

A CH3CCl=CHCH(CH3)COOH

B (CH3)2C=CHCHClCOOH

C CH3CHClCH=C(CH3)COOH

D (CH3)2CHCH=CClCOOH

Mark scheme

Show the mark scheme Mark scheme table for question 18 showing the correct answer as 'C', with 1 mark (AO1), and confirming the formula CH3CHClCH=C(CH3)COOH.

18 C 1 (AO1) CH3CHClCH=C(CH3)COOH

How to answer it

IUPAC Naming & Structural Formulae of Multifunctional Alkenoic Acids

📋 What this question tests

This question evaluates your core understanding of organic nomenclature rules (AO1):

  • Identifying the principal functional group to define numbering priority (carboxylic acid C=1).
  • Locating unsaturation (alkene C=C double bond position).
  • Correctly assigning branch prefixes ( -CH₃ ) and halogeno substituents ( -Cl ) along a 5-carbon parent chain.
  • Translating between systematic IUPAC names and condensed structural formulae.

Question 18 Analysis

4-chloro-2-methylpent-2-enoic acid [1 Mark]

✅ Correct Answer

Correct Option: C

CH₃CHClCH=C(CH₃)COOH

Mark Scheme: C (AO1) — 1 mark for the single correct selection.

💡 Key Knowledge

  • Priority Rule: The carboxylic acid carbon atom is automatically designated as C1.
  • Parent Chain: "pent" means a 5-carbon continuous chain containing the principal functional group.
  • Suffix / Infix: "-2-en-" indicates the C=C double bond starts at carbon 2 and connects to carbon 3.
  • Substituents:
    • "2-methyl" → a -CH₃ group attached at C2.
    • "4-chloro" → a -Cl group attached at C4.

📐 Step-by-Step Chain Construction

Break the systematic IUPAC name down from the principal functional group (C1) through to the end of the chain (C5):

Step 1: Identify C1
Carboxylic acid carbon: -COOH (Carbon 1).
Step 2: Add C2 with its methyl substituent and double bond
C2 has a methyl branch and a double bond: =C(CH₃)- . Attached to C1, this forms: =C(CH₃)COOH .
Step 3: Complete the double bond at C3
The double bond is between C2 and C3: -CH= . Joined to C2: -CH=C(CH₃)COOH .
Step 4: Attach C4 with the chlorine substituent
C4 is bonded to a hydrogen and a chlorine: -CHCl- . Joined: -CHClCH=C(CH₃)COOH .
Step 5: Cap the 5-carbon chain at C5
Terminal methyl group: CH₃- .
Full condensed formula written from left-to-right (C5 → C1):
CH₃–CHCl–CH=C(CH₃)–COOH, which matches Option C.

🧠 Exam Technique & Option Elimination

  • Option A: CH₃CCl=CHCH(CH₃)COOH
    Numbering from COOH gives the double bond at C3 and chlorine at C4. This is 4-chloro-2-methylpent-3-enoic acid (wrong alkene position).
  • Option B: (CH₃)₂C=CHCHClCOOH
    Chlorine is at C2 and two methyls are at C4. This is 2-chloro-4-methylpent-3-enoic acid.
  • Option D: (CH₃)₂CHCH=CClCOOH
    Chlorine is at C2 and methyl branch is at C4. This is 2-chloro-4-methylpent-2-enoic acid (substituents inverted).

❌ Common Errors & Traps

  • Numbering from the wrong end: Starting numbering from the terminal alkyl group instead of the -COOH carbon leads directly to choosing Option A.
  • Confusing the positions of substituents: Selecting D because it has the double bond at position 2, but failing to check that chlorine is at C2 instead of C4.
  • Missing the C-count in the main chain: Mistaking condensed brackets such as (CH₃)₂C= in B and D as a simple straight chain rather than a branched terminus.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.