OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 4

1 mark · Medium difficulty · Multiple Choice

Identify which compound can be refluxed with acidified potassium dichromate(VI) to form an organic product with molecular formula C5H8O2.

Practise this question

Question

Multiple choice question asking to identify a compound that reacts with acidified potassium dichromate(VI) under reflux to form C5H8O2. Four structures A, B, C, and D are shown, each containing both an alcohol group and either a carbonyl or aldehyde group.
Question text

4 Which compound can be refluxed with acidified potassium dichromate(VI) to form an organic

product with molecular formula C5H8O2?

A

HO O

B HO O

O

HO

C

O

D HO

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme showing the correct answer is C with a mark value of 1.

4 C 1 2.3

How to answer it

Oxidation of Alcohols and Aldehydes

What this question tests

This question assesses your knowledge of the chemical reactions involved in the oxidation of organic functional groups using acidified potassium dichromate(VI) under reflux. Specifically, it tests your understanding of how primary alcohols, secondary alcohols, and aldehydes behave upon oxidation, and your ability to track molecular formulas and structural changes.

Question Analysis & Answer

Multiple Choice Part (4)

Identifying the correct structure after refluxing with acidified K₂Cr₂O₇

✅ Correct Answer: C

Compound C contains a secondary alcohol group ( -CH(OH)- ) and a ketone group ( -C=O ). Under reflux with acidified potassium dichromate(VI):

  • The secondary alcohol is oxidized to a ketone.
  • The existing ketone is unreactive toward further oxidation.
  • The molecular formula of C is C₅H₁₀O₂ . Losing 2H atoms during the oxidation of the secondary alcohol yields the final product formula C₅H₈O₂ .

💡 Key Knowledge

  • Primary alcohols oxidise to aldehydes (on distillation) and further to carboxylic acids (under reflux).
  • Secondary alcohols oxidise to ketones (both under distillation and reflux).
  • Aldehydes readily oxidise to carboxylic acids under reflux conditions.
  • Ketones cannot be oxidised further under normal laboratory conditions.

🧠 Exam Technique

  • Count the total number of carbon atoms in the starting molecules to ensure they match the target formula ( C₅ ). All options have 5 carbons, so look at functional groups next.
  • Since reflux is specified, any aldehyde present would be oxidised to a carboxylic acid, and any primary alcohol would also reach the carboxylic acid stage.
  • Check hydrogen loss: Going from C₅H₁₀O₂ to C₅H₈O₂ requires a loss of 2 hydrogen atoms (an oxidation/dehydrogenation step), which fits exactly with converting -CH(OH)- to -C=O .

❌ Common Errors

  • Failing to account for reflux: Assuming aldehydes formed from primary alcohols stop at the aldehyde stage under reflux. Reflux drives the reaction fully to the carboxylic acid.
  • Misidentifying functional groups: Confusing tertiary alcohols (which do not oxidise) with secondary alcohols.
Mark Scheme Allocation: 1 mark awarded for selecting C. (Module: 2.3 Organic Chemistry)

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.2 Alcohols, haloalkanes and analysis · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.