AQA A-Level Chemistry Paper 3, 2023: Question 33

1 mark · Easy difficulty · Multiple Choice

Distinguishing Two Carbonyl Compounds This question requires identifying a method to distinguish between two compounds with different carbonyl functional groups. Options include using spectroscopic techniques (infrared or mass spectrometry) or chemical reactions such as oxidation with potassium dichromate(VI) or reaction with Tollens’ reagent, which is specific to aldehydes. Understanding the reactivity and spectral differences of aldehydes and ketones is key.

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Question

AQA A-Level Chemistry Paper 3, 2023: Question 33
Question text

33 The skeletal formulas of two compounds are shown.

Which method would distinguish between samples of these compounds?

[1 mark]

A comparing fingerprint regions of their infrared spectra

B obtaining molecular masses from their high resolution mass spectra

C warming with acidified potassium dichromate(VI) solution

D warming with Tollens’ reagent

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 33

33 A 1 (AO1) comparing fingerprint regions of their infrared spectra

How to answer it

Distinguishing Between Compounds

Question

Which method would distinguish between samples of these compounds?

  • A: Comparing fingerprint regions of their infrared spectra
  • B: Obtaining molecular masses from their high resolution mass spectra
  • C: Warming with acidified potassium dichromate(VI) solution
  • D: Warming with Tollens’ reagent

Correct Answer: A

The correct method is to compare the fingerprint regions of their infrared spectra. Each compound has a unique fingerprint region in its IR spectrum, which allows for precise identification.

Why this works:

  • The fingerprint region (below 1500 cm-1) is highly specific to each compound.
  • Even isomers have distinct patterns in this region, making it an effective method for distinguishing between them.

Why Other Options Are Incorrect

  • B: High resolution mass spectra provide molecular masses but cannot distinguish between compounds with the same molecular formula.
  • C: Acidified potassium dichromate(VI) solution is used to oxidise primary and secondary alcohols but does not apply to ketones or aldehydes in this case.
  • D: Tollens’ reagent is used to test for aldehydes but cannot distinguish ketones from other compounds.

Final Answer

A: Comparing fingerprint regions of their infrared spectra

Topics

Organic Chemistry · 3.3.6 Organic Analysis

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.