AQA A-Level Chemistry AS Paper 2, June 2025: Question 8

1 mark · Easy difficulty · Multiple Choice

Identify the IUPAC name of a branched aldehyde from its skeletal structure.

Practise this question

Question

Question 08 asks: 'What is the name of this compound according to IUPAC rules?' followed by the skeletal structure of a four-carbon aldehyde chain with a methyl branch on carbon-3 (an aldehyde carbonyl at one terminus). Four multiple-choice options are provided: A (2-methylbutanal), B (3-methylbutanal), C (2-methylbutan-1-one), and D (3-methylbutan-1-one).
Question text

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

[1 mark]

A 2-methylbutanal

B 3-methylbutanal

C 2-methylbutan-1-one

D 3-methylbutan-1-one

Mark scheme

Show the mark scheme Mark scheme table for question 08 indicating that the correct answer is B (3-methylbutanal), worth 1 mark (AO2).

08 B 1 (AO2) 3-methylbutanal

How to answer it

IUPAC Nomenclature of Branched Aldehydes

What this question tests

This question assesses your ability to apply standard IUPAC nomenclature rules to organic skeletal structures. Specifically, it tests:

  • Identifying the principal carbonyl functional group (aldehyde vs. ketone) from a skeletal formula.
  • Determining the correct numbering of the longest continuous carbon chain, prioritising the aldehyde carbon as C1.
  • Correctly identifying and assigning locants to branched alkyl side chains (methyl substituent).
Question 08 • Multiple Choice • 1 Mark (AO2)

Identifying 3-methylbutanal

✅ Correct Answer

B: 3-methylbutanal

The structure represents an aldehyde with a 4-carbon parent chain and a methyl branch at carbon-3.

📐 Step-by-Step Systematic Naming

  1. Identify functional group: A terminal carbonyl group ( C=O at the end of the chain) is an aldehyde. The suffix is -al.
  2. Number the main chain: The carbonyl carbon must be C1. Counting from the right:
    C1 (CHO) → C2 (CH₂) → C3 (CH) → C4 (CH₃)
  3. Identify parent chain length: 4 carbons = butanal. (No need for "butan-1-al" as aldehydes are always at position 1).
  4. Locate substituents: A single methyl ( -CH₃ ) group branches off carbon-3.
  5. Combine name: 3-methylbutanal.

💡 Key Knowledge

  • Aldehyde vs. Ketone: Aldehydes have the carbonyl carbon bonded to at least one hydrogen atom (at the end of a chain). Ketones have the carbonyl carbon bonded to two other carbon atoms (within a chain).
  • C1 Priority: In aldehydes, the carbonyl carbon always takes number 1 by definition. Therefore, position 1 is omitted from the name (e.g., butanal, not butan-1-al).
  • Ketones cannot be "1-one": An organic compound named butan-1-one cannot exist under IUPAC rules because a carbonyl at carbon-1 is an aldehyde, not a ketone.

🧠 Exam Technique: Elimination Strategy

  • Step 1: Check the suffix. Look at the terminal C=O . It is an aldehyde ( -al ). Immediately eliminate options C and D.
  • Step 2: Check numbering direction. Aldehydes mandate numbering starting at the carbonyl carbon.
    Numbering from the left gives the methyl group position 2, but makes the carbonyl carbon 4 (incorrect).
    Numbering from the right gives the carbonyl carbon 1, making the methyl group position 3 (correct).
  • Eliminate option A → Select B in under 15 seconds!

❌ Common Errors

  • Numbering from the wrong end: Selecting 2-methylbutanal (Option A) by mistakenly numbering from the side closest to the branch rather than giving the principal functional group ( CHO ) priority.
  • Confusing aldehydes and ketones: Choosing Option C or D. Remember: ketones must be situated between two carbon atoms (locants must be 2 or higher, e.g., butan-2-one).
  • Miscounting skeletal carbons: Forgetting that the carbonyl carbon itself is part of the parent carbon chain count.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.8 Aldehydes and Ketones

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.