OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 2

1 mark · Medium difficulty · Multiple Choice

Identify which compound reacts with 2,4-dinitrophenylhydrazine but does not react with Tollens' reagent among four given options.

Practise this question

Question

Multiple choice question 2 asking which compound reacts with 2,4-dinitrophenylhydrazine but does not react with Tollens' reagent. The options are A: C6H5COCOOH, B: C6H5CH(OH)CHO, C: CH3COCHO, and D: CH3CH2CH(OH)CH3. A blank box is provided for the answer.
Question text

2 Which compound reacts with 2,4-dinitrophenylhydrazine but does not react with Tollens’

reagent?

A C6H5COCOOH

B C6H5CH(OH)CHO

C CH3COCHO

D CH3CH2CH(OH)CH3

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 2 is A.

2 A 1 AO1.2

How to answer it

Identifying Carbonyl Functional Groups

🔍 What this question tests

This question assesses your knowledge of the characteristic chemical reactions used to identify functional groups in organic chemistry—specifically distinguishing between aldehydes, ketones, and carboxylic acids using 2,4-dinitrophenylhydrazine (2,4-DNPH) and Tollens' reagent.

Question Analysis & Answer

Multiple Choice Question (1 Mark)

✅ Correct Answer: A

Compound C₆H₅COCOOH reacts with 2,4-DNPH (due to the carbonyl group C=O ) but does NOT react with Tollens' reagent because it contains a ketone and a carboxylic acid, not an aldehyde.

💡 Key Knowledge

  • 2,4-DNPH: Reacts with all aldehydes and ketones to form a yellow/orange precipitate.
  • Tollens' Reagent: A mild oxidizing agent that specifically reacts with aldehydes (forming a silver mirror), but does not react with ketones or carboxylic acids.

🧠 Exam Technique

Deconstruct each option by identifying its functional groups before evaluating the reagents. Eliminate options that react with both or neither.

❌ Common Errors

Confusing ketones with aldehydes. Students often assume any complex oxygen-containing organic molecule will react with Tollens' reagent, forgetting that Tollens' is specific to the -CHO aldehyde proton.

Mark Scheme Allocation: 1 mark for selecting option A (AO1.2 - Demonstrating knowledge and understanding of organic chemistry reagents and tests).

Detailed Breakdown of Options

Option A: C₆H₅COCOOH

Contains a ketone group ( C=O ) and a carboxylic acid group ( -COOH ). Result: Reacts with 2,4-DNPH (ketone), no reaction with Tollens' (not an aldehyde).

Option B: C₆H₅CH(OH)CHO

Contains an alcohol group and an aldehyde group ( -CHO ). Result: Reacts with 2,4-DNPH and does react with Tollens' reagent (silver mirror forms).

Option C: CH₃COCHO

Contains a ketone group and an aldehyde group ( -CHO ). Result: Reacts with 2,4-DNPH and does react with Tollens' reagent.

Option D: CH₃CH₂CH(OH)CH₃

Contains a secondary alcohol group ( -OH ). Result: Does not contain a carbonyl group, so it does not react with 2,4-DNPH or Tollens' reagent.

Topics

Module 6: Organic chemistry 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), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.