OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 26

5 marks · Medium difficulty · Extended Response

Compare the oxidation of two structural isomers of C3H8O including conditions, functional groups of products, and balanced equations.

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Question

Question 26 asks to compare the oxidation of two alcohols that are structural isomers of C3H8O. It requires stating reaction conditions, functional groups of organic products, and balanced equations using [O] for the oxidizing agent and structures for organic compounds.
Question text

26 This question is about the oxidation of two alcohols that are structural isomers of C3H8O.

Compare the oxidation of these two structural isomers using different reaction conditions.

For each reaction include:

• the reaction conditions

• the functional group of any organic product

• a balanced equation.

In your equations, use [O] to represent the oxidising agent and show any organic compounds as

structures.

… [5]

Mark scheme

Show the mark scheme Mark scheme for question 26 awarding 5 marks: 1 mark for correct structural isomers (propan-1-ol and propan-2-ol), 1 mark for reaction conditions (distillation for aldehyde, reflux for carboxylic acid or ketone), 2 marks for functional groups of organic products, and 1 mark for at least one correct balanced equation.

AO

Question Answer Marks Guidance

element

26 ANNOTATE WITH TICKS AND CROSSES

Correct structural isomers of C3H8O 1 mark Throughout,

CH3CH2CH2OH AND CH3CHOHCH3 ✓ ALLOW any combination of skeletal OR structural

OR displayed formula as long as unambiguous

Reaction conditions 1 mark

Distillation for aldehyde

AND reflux for carboxylic acid OR ketone ✓

Functional group of organic product 2 marks IF functional group is NOT given,

CH3CH2CH2OH → aldehyde ALLOW propanal / RCHO

OR → carboxylic acid ✓ ALLOW propanoic acid / RCOOH

CH3CHOHCH3 → ketone ✓ ALLOW propanone / ….. RCOR

IGNORE small slips in formulae (assessed in

equation)

One correct equation 1 mark

CH3CH2CH2OH + [O] → CH3CH2CHO + H2O

OR

CH3CHOHCH3 + [O] → CH3COCH3 + H2O

OR

CH3CH2CH2OH + 2[O] → CH3CH2COOH + H2O ✓

How to answer it

Oxidation of Alcohols (C₃H₈O Isomers)

📌 What this question tests

This question assesses your ability to identify structural isomers of alcohols, recall and contrast the chemical reaction conditions required for oxidising primary versus secondary alcohols, name organic functional groups, and write balanced chemical equations using [O] to represent the oxidising agent.

Question Breakdown & Mark Scheme Guide

Total Marks: 5

💡 Key Knowledge: Isomers of C₃H₈O

  • The formula C₃H₈O represents two structural alcohol isomers: propan-1-ol (primary) and propan-2-ol (secondary).
  • Primary alcohols can be oxidised twice (to aldehydes, then carboxylic acids).
  • Secondary alcohols can only be oxidised once (to ketones).

✅ Correct Answers (Mark 1)

Identification of the two structural isomers:

CH₃CH₂CH₂OH (propan-1-ol)
AND
CH₃CHOHCH₃ (propan-2-ol)

🧠 Exam Technique & Conditions (Mark 2)

  • Distillation: Used with primary alcohols to isolate the volatile aldehyde before it oxidises further.
  • Reflux: Used to ensure complete oxidation of primary alcohols to carboxylic acids, or for oxidising secondary alcohols to ketones.

❌ Common Errors

  • Confusing the setup for distillation and reflux.
  • Failing to specify that primary alcohols can yield either an aldehyde (via distillation) or a carboxylic acid (via reflux).
  • Omitting structural formulas in equations, using molecular formulas instead.

✅ Functional Groups & Equations Breakdown (Marks 3, 4 & 5)

To secure the remaining marks, your functional group references and balanced equations must align:

  • Propan-1-ol to Aldehyde: CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O (Product functional group: aldehyde / propanal)
  • Propan-1-ol to Carboxylic Acid: CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O (Product functional group: carboxylic acid / propanoic acid)
  • Propan-2-ol to Ketone: CH₃CHOHCH₃ + [O] → CH₃COCH₃ + H₂O (Product functional group: ketone / propanone)
Examiner Note: You only need to provide one correct balanced equation matching your stated conditions to gain the equation mark, but stating functional groups for both isomers is required for full marks.

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.