Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 8
19 marks · Hard difficulty · Synoptic Questions
Analyze reactions of cyclohexanol, including its oxidation to cyclohexanone, dehydration to cyclohexene, substitution reactions, IR and mass spectrometry, percentage yield calculations, and preparation/purification procedures.
Practise this questionQuestion
Question text
8 This question is about some reactions of cyclohexanol.
cyclohexene B
potassium dichromate(VI)
in dilute sulfuric acid
cyclohexanol
50% concentrated sulfuric acid
and potassium bromide
A
(a) Write the skeletal formula of compound A.
(1)
(b) (i) Give the name and displayed formula of compound B.
(2)
(ii) The infrared (IR) spectra of cyclohexanol and compound B are shown.
IR Spectrum of cyclohexanol
Transmittance / % 50
4000 3000 2000 1500 1000 100
16 Wavenumbers / cm–1
IR Spectrum of compound B*P67084A01628*
Transmittance / % 50
4000 3000 2000 1500 1000 100
Wavenumbers / cm–1
Identify the bonds, using both IR spectra, that help to confirm the reaction of
cyclohexanol to produce compound B.
Your answer must include the wavenumber ranges of any relevant bonds.
(2)
(iii) The mass spectrum of compound B is shown. 17
*P67084A01728*
Relative
intensity
10 20 30 40 50 60 70 80 90 100
m/z
Deduce the relative molecular mass of compound B using the mass spectrum.
Justify your answer.
(1)
(iv) In the mass spectrum of cyclohexanol, there is a peak at m/z = 83.
Give the formula of a fragment that could be responsible for this peak.
(2)
(c) (i) Cyclohexanol can be converted to cyclohexene.
What is the classification for this reaction?
(1)
A addition
B elimination
C oxidation
D substitution
(ii) In an experiment, 10.0 cm3 of cyclohexanol was converted to cyclohexene
with a 63.0% yield.
Compound Molar mass / g mol–1 Density / g cm–3
cyclohexanol*P67084A01828*1000.962
cyclohexene 82.0 0.811
Calculate the volume of cyclohexene produced.
(4)
*(iii) Cyclohexene can be prepared by reacting cyclohexanol with phosphoric(V) acid.
The mixture is warmed in a water bath for 15 minutes before distilling off a
mixture of cyclohexene and water.
Devise a procedure to obtain a pure, dry sample of cyclohexene from the distillate.
Include a reason for each step.
[ Boiling temperature of cyclohexene = 83°C
Density of cyclohexene = 0.811 g cm–3]
(6)
… *P67084A01928*
… *P67084A02028*
(Total for Question 8 = 19 marks)
Mark scheme
Show the mark scheme
Question Answer Additional Guidance
Number Mark
8(a) Example of skeletal formula (1)
• skeletal formula
Bond from ring to Br must be shown but the length is
not important.
Question Answer Additional Guidance Mark
Number
8(b)(i) An answer that makes reference to the following points: Example of displayed formula (2)
• name (1)
• displayed formula (1)
Allow CH2 groups
Allow skeletal formula
Do not award molecular formula
Number
8(b)(ii) An answer that makes reference to the following points: (2)
Allow a range within the specified range
• O-H bond (stretching) 3750 – 3200 cm-1 in cyclohexanol
is not present in cyclohexanone /disappears (when
cyclohexanol reacts). (1)
-1 Allow 1725 – 1700 cm-1
• C=O bond (stretching) 1720 – 1700 cm appears in
Do not allow 1740 – 1720 cm-1 (aldehyde)
cyclohexanone (1)
Number Mark
8(b)(iii)
(1)
• highest m/z = Mr = 98 Check, answer may be shown on mass spectrum
Do not accept just ‘98’ with no supporting evidence
Allow peak furthest to the right / molecular ion peak
is 98
Number Mark
8(b)(iv) Examples of fragment structure (2)
• fragment (1) CH +
C H +
6 11
• charge (1)
Allow charge anywhere on fragment, including outside
brackets around the fragment
Allow straight chain fragment provided it has the correct
number of C and H atoms
Question
Answer Mark
Number
8(c)(i) The only correct answer is B (elimination) (1)
A is not correct because the reaction involves only one reacting molecule (cyclohexanol)
C is not correct because there is no change in the oxidation numbers of any of the elements involved
D is not correct because nothing has been substituted.
Number Mark
8(c)(ii) Example of calculation (4)
• calculation of mass and mol of cyclohexanol (1) Mass cyclohexanol = 10.0 × 0.962 = 9.62 (g)
and
Mol of cyclohexanol = 9.62 ÷ 100 = 0.0962 (mol)
• calculation of mol of cyclohexene (1) Mol of cyclohexene = 0.63 × 0.0962 = 0.060606 (mol)
• calculation of mass of cyclohexene (1) Mass of cyclohexene = 0.060606 × 82.0 = 4.9697 (g)
• calculation of volume of cyclohexene (1) Volume of cyclohexene = 4.9697 ÷ 0.811 = 6.1279
= 6 / 6.1 / 6.13 (cm3) / 6.1 x 10-3 dm3
Ignore SF in final answer
Allow TE at each stage
Correct answer with no working scores 4
Number Mark
*8(c)(iii) This question assesses a student’s ability to show a coherent Guidance on how the mark scheme should be applied: (6)
and logically structured answer with linkages and fully-
sustained reasoning. The mark for indicative content should be added to the
Marks are awarded for indicative content and for how the mark for lines of reasoning. For example, an answer
answer is structured and shows lines of reasoning. with five indicative marking points that is partially
The following table shows how the marks should be awarded structured with some linkages and lines of reasoning,
for indicative content. scores 4 marks (3 marks for indicative content and 1
Number of indicative Number of marks awarded mark for partial structure and some linkages and lines
marking points seen in for indicative marking points of reasoning).
answer
64 If there are no linkages between points, the same five
5-4 3 indicative marking points would yield an overall score
3-2 2 of 3 marks (3 marks for indicative content and no
11 marks for linkages).
In general it would be expected that 5 or 6 indicative
The following table shows how the marks should be awarded for points would get 2 reasoning marks, and 3 or 4
structure and lines of reasoning. indicative points would get 1 mark for reasoning, and
Number of marks awarded for 0, 1 or 2 indicative points would score zero marks for
structure and sustained lines of reasoning.
reasoning
Answer shows a coherent and If there is any incorrect chemistry, deduct mark(s) from
logical structure with linkages 2 the reasoning. If no reasoning mark(s) awarded do not
and fully sustained lines of deduct mark(s).
reasoning demonstrated Comment: Look for the indicative marking points first,
throughout.
then consider the mark for the structure of the answer
and sustained line of reasoning.
Answer is partially structured
with some linkages and lines of 1
reasoning.
Answer has no linkages between
points and is unstructured. 0 Mark independently
Indicative content:
• IP1 separate cyclohexene from water using separating
funnel
• IP2 remove the lower aqueous layer because it has a
higher density Suitable specified drying agents:
(anhydrous) CaCl2, Na2SO4, CaSO4, MgSO4
• IP3 suitable drying agent
By filtering or decanting
• IP4 separate the cyclohexene from the drying agent
• IP5 redistil product
• IP6 collect the distillate boiling over between
80 and 86 oC to collect the pure cyclohexene
(Total for Question 8 = 19 marks)
How to answer it
Reactions of Cyclohexanol Study Guide
What this question tests
This multi-part AS Chemistry question assesses your understanding of alcohol chemistry, organic functional group transformations (substitution, oxidation, and elimination), spectroscopic analysis (IR and mass spectrometry), percentage yield calculations, and practical purification techniques such as distillation and separation.
Part (a) — Substitution Reaction
✅ Correct Answer
The skeletal formula of bromocyclohexane (compound A).
A cyclohexane ring with a single bond connected to a Br atom.
🧠 Exam Technique
Ensure the bond from the ring clearly touches the Br symbol. Skeletal formulas omit carbon and hydrogen labels at ring vertices for clarity, but heteroatoms like bromine must always be explicitly shown.
Part (b)(i) — Oxidation Product Structure
✅ Correct Answer
Name: Cyclohexanone
Displayed formula: A six-membered carbon ring containing a C=O double bond (ketone) with all individual C-H bonds fully drawn out.
❌ Common Errors
Students frequently lose the second mark by drawing a skeletal or molecular formula instead of a displayed formula, which requires every single atom and covalent bond to be explicitly shown.
Part (b)(ii) — Infrared Spectroscopy Analysis
💡 Key Knowledge
- Cyclohexanol IR: Broad absorption peak between 3750 – 3200 cm⁻¹ corresponding to O-H stretching. This peak disappears in compound B.
- Cyclohexanone IR: Sharp, strong absorption peak between 1720 – 1700 cm⁻¹ corresponding to C=O stretching.
🧠 Exam Technique
To score full marks, you must explicitly state what happens to the alcohol bond (it disappears/is absent) and what new bond appears for the ketone, quoting valid wavenumber ranges for both.
Part (b)(iii) — Mass Spectrometry Molecular Ion Peak
✅ Correct Answer
Relative molecular mass (Mr) = 98.
🧠 Exam Technique
The molecular ion peak ( M+ ) is always the peak furthest to the right on a mass spectrum (ignoring tiny isotope peaks). Just identifying the highest m/z value as 98 is sufficient for the mark.
Part (b)(iv) — Mass Spectrometry Fragmentation
✅ Correct Answer
Formula of fragment: C₆H₁₁⁺ (or structural variations carrying a positive charge).
❌ Common Errors
Many candidates forget to include the positive charge symbol ( + ) on fragment ions. A fragment in mass spectrometry must reflect the loss of an electron, making it a cation.
Part (c)(i) — Reaction Classification
✅ Correct Answer
B (elimination)
💡 Key Knowledge
Converting an alcohol into an alkene using a concentrated acid catalyst involves the removal of a water molecule from a single reactant molecule, which defines an elimination reaction.
Part (c)(ii) — Percentage Yield Calculation
📐 Step-by-Step Calculation
- Mass of cyclohexanol: 10.0 cm³ × 0.962 g cm⁻³ = 9.62 g
- Moles of cyclohexanol: 9.62 g ÷ 100 g mol⁻¹ = 0.0962 mol
- Theoretical moles of cyclohexene (1:1 ratio): 0.0962 mol
- Actual moles produced (63.0% yield): 0.0962 × 0.630 = 0.060606 mol
- Mass of cyclohexene: 0.060606 mol × 82.0 g mol⁻¹ = 4.9697 g
- Volume of cyclohexene: 4.9697 g ÷ 0.811 g cm⁻³ = 6.13 cm³ (Accept 6.1 / 6.13 cm³)
❌ Calculation Traps
Watch out for density conversions! Always multiply volume by density to get mass first before dividing by the molar mass. Ensure you apply the 63.0% yield at the correct stage in your multi-step calculation.
Part (c)(iii) — Organic Practical Preparation & Purification
🧠 Practical Procedure (Indicative Points)
- IP1: Separate the organic layer containing cyclohexene from aqueous impurities using a separating funnel.
- IP2: Run off and discard the lower aqueous layer (verify layers using density: water density is 1.0 g cm⁻³, cyclohexene is 0.811 g cm⁻³).
- IP3: Add a suitable anhydrous drying agent (e.g., anhydrous CaCl₂ , , or Na₂SO₄ ) to remove trace water.
- IP4: Filter or decant the dried liquid to remove the hydrated solid drying agent.
- IP5 & IP6: Redistil the product and collect the distillate boiling within the specified range around the boiling point of cyclohexene ( 80 °C to 86 °C ).
💡 Examiner Commentary
Top-level responses gained full marks for structure and reasoning by linking the purification steps logically: separation removes bulk water, drying agents remove traces of moisture, and redistillation ensures isolation of the pure fraction at its specific boiling point.
Topics
Organic Chemistry · Core Practicals · Physical Chemistry · Core Practical 5: Investigate the oxidation of ethanol · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I · Topic 7: Modern Analytical Techniques I
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.