OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 18

12 marks · Hard difficulty · Structured Questions

Identify carbonyl compounds and their structures using chemical tests, mass spectrometry, and 1H NMR spectroscopy.

Practise this question

Question

A three-part structured chemistry question about carbonyl compounds. Part (a) asks to describe a chemical test for a carbonyl group and how to identify it, and a test to distinguish aldehydes. Part (b) provides percentage composition and mass spectra for two unknown compounds F and G to identify their structures. Part (c) provides relative peak areas and an 1H NMR spectrum for organic compound H to deduce its structure.
Question text

18 This question is about carbonyl compounds.

(a) (i) Describe a chemical test to confirm the presence of a carbonyl group.

How could the product of this test be used to identify the carbonyl compound?

Your answer should not include spectroscopy.

… [3]

(ii) Describe a chemical test that would identify whether a carbonyl compound is an

aldehyde.

Your answer should include the reagent and observations.

… [1]

(b) A student is provided with two unknown carbonyl compounds, F and G.

The compounds are analysed and found to have identical percentage compositions by

mass:

C: 62.07%; H: 10.34%; O: 27.59%

The mass spectra of the two compounds are shown below.

Mass spectrum of F

Relative

intensity

10 15 20 25 30 35 40 45 50 55 60 65 70 75

m/z

Mass spectrum of G

Relative

intensity

10 15 20 25 30 35 40 45 50 55 60

m/z

Use the results to identify the structures of the two compounds.

Include relevant peaks present in the mass spectrum of each compound.

F G

[4]

(c) The organic compound H contains carbon, hydrogen and oxygen only and has an Mr of

114.0.

Compound H has two carbonyl groups and no other functional groups.

The 1H NMR spectrum of organic compound H is shown below.

Expansion of multiplet

centred at δ = 2.9ppm

43 2 1 0

chemical shift, δ/ppm

The numbers by the peaks are the relative peak areas.

Analyse the spectrum to suggest a possible structure for compound H.

Show all your reasoning.

Compound H

[4]

Mark scheme

Show the mark scheme Mark scheme showing accepted answers for the carbonyl identification questions, including 2,4-DNP and Tollens' tests, mass spectrometry calculations and fragment identifications for compounds F and G, and 1H NMR peak assignments and structure for compound H.

AO

Question Answer Marks Guidance

element

18 (a) (i) (Add) 2,4-dinitrophenylhydrazine AND orange/yellow/red 3 AO1.2 ALLOW errors in spelling

precipitate × 3 ALLOW 2,4(-)DNP OR 2,4(-)DNPH

ALLOW Brady’s reagent or Brady’s Test

ALLOW solid OR crystals OR ppt as alternatives

for precipitate

Take melting point (of crystals) Mark second and third points independently of

response for first marking point

Compare to known values/database

DO NOT ALLOW 2nd and 3rd marks for taking and

comparing boiling points OR chromatograms

18 (a) (ii) Tollens’ (reagent) 1 AO1.2 ALLOW ammoniacal silver nitrate OR Ag+/NH

AND

Silver (mirror/precipitate/ppt/solid) ALLOW black ppt OR grey ppt

ALLOW Cr O 2–/H+

AND

Turns green

IGNORE reference to conditions,

e.g. Heat or reflux

-------------------------------------------------------------

IF other reagents are seen e.g. Fehling’s or

Benedict’s, contact your Team Leader

AO

element

18 (b) Initial ratios 4 CHECK spectra for annotations that may be

62.07 10.34 27.59 worthy of credit

C, 12.0 : H, 1.0 : O, 16.0

OR

C, 5.1725 : H, 10.34 : O, 1.724

OR

C, 3 : H, 6 : O, 1 AO1.2 Mark can be awarded from a correct molecular

formula

AO2.5

(Molecular formula =) C3H6O

AND

Evidence of 58 in working or from labelled peak in one of

the spectra AO3.2

For F

evidence for fragment ion m/z=29 linked to CH CH (+) AO3.2 IGNORE m/z=15 (as this is not unique)

OR CHO(+) IGNORE m/z=43

IGNORE incorrect fragments

IGNORE charges on fragment ions

ALLOW any combination of skeletal OR structural

F = OR displayed formula as long as unambiguous

AND

G =

AO

element

18 (c) 1H NMR 4 AO3.1 ALLOW any combination of skeletal OR

×3 structural OR displayed formula as long as

δ = 1.1 ppm/doublet linked to 2 x CH3 unambiguous

CHECK spectra for annotations that may be

worthy of credit

ALLOW δ values ± 0.2 ppm, as a range or a value

within the range

δ = 2.2 ppm/singlet linked to CH3–C=O AO3.2 IGNORE HC–C=O linked to δ = 2.2 ppm

OR ×1

δ = 2.9 ppm/multiplet linked to CH(CH3)2 OR HC–C=O IGNORE additional chemical environments (taken

from the data sheet) that align with the given

chemical shifts

Structure

Any structure with molecular formula C6H10O2 and has 2

carbonyl groups

Total 12

How to answer it

Carbonyl Compounds Analysis & Structure Elucidation

What this question tests

This question assesses your practical knowledge of chemical tests for carbonyl groups and aldehydes (2,4-DNP and Tollens' reagent), your ability to calculate empirical and molecular formulae from percentage composition and mass spectra fragments, and your competence in interpreting ¹H NMR spectra (chemical shifts, integration, and splitting patterns) to deduce complex organic structures.

Part (a): Chemical Tests for Carbonyls and Aldehydes

Part (a)(i): Confirming a carbonyl group [3 marks]

✅ Correct Answer

  • Reagent: Add 2,4-dinitrophenylhydrazine (2,4-DNP / Brady's reagent).
  • Observation: Orange, yellow, or red precipitate (solid/crystals) forms.
  • Identification: Take the melting point of the purified crystals and compare it with known database values.

🧠 Exam Technique

The first mark is for the correct reagent and observation. The second and third marks require a specific workflow to identify the exact compound without spectroscopy: separating the precipitate, measuring its melting point, and matching it against literature values.

❌ Common Errors

Students frequently lose marks by suggesting boiling point determination instead of melting point, or attempting to use chromatography to identify the derivative.

✅ Correct Answer (Part (a)(ii) - 1 mark)

Reagent & Observation: Use Tollens' reagent (ammoniacal silver nitrate) which forms a silver mirror (or grey/black precipitate). Alternatively, acidified potassium dichromate ( Cr₂O₇²⁻ / H⁺ ) turns green with aldehydes (no reaction with ketones).

💡 Key Knowledge

Aldehydes are easily oxidised to carboxylic acids, whereas ketones cannot be oxidised further without breaking carbon-carbon bonds. Tollens' exploits this mild reducing property.

Part (b): Mass Spectrometry & Isomer Identification

Elucidating structures for F and G from % composition and mass spectra [4 marks]

📐 Step-by-Step Calculation: Empirical Formula

  1. Initial Ratios: Divide percentage composition by relative atomic mass:
    C = 62.07 / 12.0 = 5.1725
    H = 10.34 / 1.0 = 10.34
    O = 27.59 / 16.0 = 1.724
  2. Simplify Ratio: Divide by the smallest value (1.724):
    C : H : O = 3 : 6 : 1
  3. Empirical Formula: C₃H₆O (Molar mass = 58.0 g mol⁻¹)
  4. Molecular Formula Link: The prominent molecular ion peak ( M⁺ ) at m/z = 58 confirms the molecular formula is also C₃H₆O .

✅ Fragment Analysis & Final Structures

  • Compound F: Contains an aldehyde group. Evidence of fragment peak at m/z = 29 corresponding to CH₃CH₂⁺ or CHO⁺ . Structure: propanal ( CH₃CH₂CHO ).
  • Compound G: Contains a ketone group. Structure: propanone ( CH₃COCH₃ ).

❌ Common Errors

Failing to link specific m/z peaks to structural fragments, or drawing incorrect structural isomers that do not fit a C₃H₆O carbonyl compound (e.g., cyclic ethers or unsaturated alcohols).

Part (c): ¹H NMR Spectroscopy Analysis

Deducing the structure of Compound H ( C₆H₁₀O₂ , 2 carbonyl groups) [4 marks]

💡 Spectrum Breakdown & Spectral Interpretation

  • Peak at δ = 1.1 ppm (Relative Area 6, Doublet): Indicates two equivalent CH₃ groups attached to a CH with 1 adjacent proton (splitting = n+1 = 2, doublet). Represents (CH₃)₂CH- group.
  • Peak at δ = 2.2 ppm (Relative Area 3, Singlet): Indicates a CH₃ group adjacent to a carbonyl group ( CH₃-C=O ) with no adjacent protons.
  • Peak at δ = 2.9 ppm (Relative Area 1, Multiplet/Septet): Indicates a single CH group flanked by multiple protons (the tertiary methine proton in an isopropyl group next to a carbonyl).

✅ Final Structure for Compound H

Combining the fragments gives a symmetrical or branched diketone structure:

Hexane-2,4-dione derivative / 2-methylpentane-3,5-dione structure:
(CH₃)₂CH-CO-CH₂-CO-CH₃ or specifically 5-methylhexane-2,4-dione / hexane-2,5-dione variant matching C₆H₁₀O₂ with two carbonyls: (CH₃)₂CH-CO-CO-CH₃ (biacetyl derivative) or similar arrangement consistent with the 1.1, 2.2, and 2.9 ppm shifts.

🧠 Exam Technique

Always check integration (relative peak areas) against the molecular formula to verify the number of hydrogen atoms. Use chemical shift data tables to rule out incorrect functional environments.

Topics

Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.