AQA A-Level Chemistry Paper 3, June 2018: Question 28

1 mark · Medium difficulty · Multiple Choice

Determine how many structural isomers with the molecular formula C5H10O react with Tollens' reagent.

Practise this question

Question

Multiple choice question 28: 'How many structural isomers with the molecular formula C5H10O react with Tollens’ reagent? [1 mark]' with options: A 3, B 4, C 5, D 6, each with a selection oval next to it.
Question text

28 How many structural isomers with the molecular formula C5H10O react with Tollens’

reagent?

[1 mark]

A 3

B 4

C 5

D 6

Mark scheme

Show the mark scheme Mark scheme table row showing question number 28 with the correct answer B.

28 B

How to answer it

Isomerism & Tollens' Reagent: Deducing Aldehydes of C₅H₁₀O

📌 What this question tests

This question assesses your ability to combine organic test knowledge with structural isomerism. Specifically, identifying which functional group reacts with Tollens' reagent (aldehydes vs ketones), and systematically deducing all possible chain isomers for an aldehyde containing five carbon atoms without missing branched variations or duplicating structures.

Question 28 Breakdown

Determining the number of structural isomers of C₅H₁₀O reacting with Tollens' reagent

✅ Correct Answer

B: 4

There are exactly four structural aldehyde isomers with the molecular formula C₅H₁₀O.

Award 1 mark for option B.

💡 Key Knowledge

  • Tollens' Reagent: Contains the diamminesilver(I) complex ion, [Ag(NH₃)₂]⁺.
  • Aldehydes are oxidised by Tollens' reagent to carboxylate ions, reducing Ag⁺ to form a metallic silver mirror.
  • Ketones do not react because they cannot be readily oxidised without breaking C–C bonds.
  • Formula C₅H₁₀O has the general formula CₙH₂ₙO (one degree of unsaturation), which corresponds to aliphatic carbonyl compounds (aldehydes and ketones). Therefore, only aldehydes qualify.

📐 Step-by-Step Deduction: Systematic Derivation of Isomers

Because the compound must be an aldehyde, it must terminate in a -CHO group (carbon 1). We systematically vary the remaining 4-carbon framework:

# IUPAC Name Structural Formula Chain Skeleton
1 Pentanal CH₃CH₂CH₂CH₂CHO Straight 5-carbon unbranched chain
2 2-Methylbutanal CH₃CH₂CH(CH₃)CHO 4-carbon chain, methyl branch at C-2
3 3-Methylbutanal (CH₃)₂CHCH₂CHO 4-carbon chain, methyl branch at C-3
4 2,2-Dimethylpropanal (CH₃)₃CCHO 3-carbon chain, two methyl branches at C-2

Total unique structural isomers = 4.

🧠 Exam Technique

  • Decode the reagent immediately: Write "Must be an aldehyde (-CHO)" right next to the question text.
  • Vary chain length systematically:
    1. Longest chain (5 carbons): 1 option.
    2. 4 carbons + 1 methyl branch: test positions C-2 and C-3 (2 options).
    3. 3 carbons + 2 methyl branches: both must be on C-2 (1 option).
  • Sum up the totals: 1 + 2 + 1 = 4. This ensures none are missed and no duplicates are counted.

❌ Common Errors & Traps

  • Counting ketones: Students who forget that Tollens' reagent only oxidises aldehydes also count ketones (e.g. pentan-2-one, pentan-3-one, 3-methylbutan-2-one), choosing option D (6) or higher.
  • Including stereoisomers: 2-Methylbutanal possesses a chiral carbon (C-2) and exhibits optical isomerism. However, the question strictly asks for structural isomers, so enantiomers must not be counted separately.
  • Overlooking 2,2-dimethylpropanal: Missing the most branched isomer leads to selecting A (3).

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.6 Organic Analysis · 3.3.8 Aldehydes and Ketones

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.