Edexcel A-Level Chemistry Paper 3, June 2017: Question 2

21 marks · Hard difficulty · Practical Techniques and Data Analysis

Investigate the preparation, purification, spectroscopic analysis, and oxidation calculations of 3-methylbutan-2-one from 3-methylbutan-2-ol.

Practise this question

Question

Question text

2 This question is about the preparation of a sample of the ketone, 3-methylbutan-2-one.

A student’s research suggested that 3-methylbutan-2-one may be prepared by

oxidising 3-methylbutan-2-ol with acidified potassium dichromate(VI) solution.

The student sets up the apparatus as shown in the diagram. You may assume that all

the equipment is suitably clamped.

The student adds dilute sulfuric acid to the pear-shaped flask. A mixture of

potassium dichromate(VI) and 3-methylbutan-2-ol is then added slowly to the dilute

sulfuric acid in the flask.

water

from tap

pear-shaped

flask

reaction mixture

anti-bumping

granules conical flask

water to

sink

ice–water mixture

(a) Identify the two changes that must be made to the apparatus before heating the

pear-shaped flask, giving a reason for each change.

(4)

… *P48954A0436*

(b) Draw the skeletal formulae for 3-methylbutan-2-ol and 3-methylbutan-2-one.

(2)

3-methylbutan-2-ol

3-methylbutan-2-one

heated. The distillate formed is collected in the conical flask.

On testing the distillate with pH paper, it is found that its pH is 2.

The student suggests that this pH is due to the formation of 3-methylbutanoic acid.

(i) Give a reason why 3-methylbutanoic acid cannot be formed in the reaction.

(1)

… *P48954A0536*

(ii) Deduce the formula of the compound that could cause the distillate to have a

pH value of 2.

(1)

(iii) Solid sodium carbonate is added to the distillate. The sodium carbonate

disappears and fizzing occurs.

Write an equation, including state symbols, for the reaction that occurs

between sodium carbonate and the compound you have identified in (c)(ii).

(2)

(d) The organic mixture was separated from the aqueous layer and dried.

The infrared spectrum of the organic mixture is shown.

6 4000 3000 2000 1500 1000 500

*P48954A0636*wavenumber / cm–1

(i) By reference to any relevant peak(s), deduce how the infrared spectrum shows

that the mixture contains 3-methylbutan-2-one.

(2)

(ii) From the infrared spectrum, the student concludes that the mixture contains

another organic compound.

The mixture is redistilled and the fraction that boils in the range 93–95°C is collected.

The boiling temperature of 3-methylbutan-2-one is 94°C.

Predict any change(s) you would see in the infrared spectrum after redistillation,

justifying your answer.

(2)

(e) The mass spectrum of pure 3-methylbutan-2-one is shown.

100 *P48954A0736*

10 20 30 40 50 60 70 80 90

m/z

(i) State how you would find the molar mass of 3-methylbutan-2-one from the

mass spectrum.

(1)

(ii) The mass spectrum shows a peak at m/z = 43.

Draw the displayed formulae of two fragment ions that might be responsible

for this peak.

(2)

(f ) The sample of purified 3-methylbutan-2-one is found to have a mass of 2.15 g.

This mass of 3-methylbutan-2-one represents a yield of 62.5% by mass.

(i) Write an equation, using molecular formulae, for the oxidation of

3-methylbutan-2-ol to 3-methylbutan-2-one.

Use [O] to represent the oxidising agent.

(2)

(ii) Calculate the mass of 3-methylbutan-2-ol that the student uses at the start of

8 the preparation.

*P48954A0836* (2)

(Total for Question 2 = 21 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answers Additional Guidance Mark

Number

1(b)(ii) (2)

[Cu(H O) ]2+ + 4NH → [Cu(NH ) (H O) ]2+ + 4H O Ignore state symbols even if incorrect

26 3 3 4 2 2 2

Ignore balanced sulfate ions

LHS of equation correct (1) Do not award just Cu2+ on LHS

Allow

[Cu(OH)2(H2O)4] + 4NH3 →

RHS of equation correct (1) [Cu(NH ) (H O) ]2+ + 2H O +2OH-

34 2 2 2

Do not award for [Cu(NH ) ]2+ /

[Cu(NH ) ]2+ on RHS

(Total for Question 1 = 11 marks)

Question

Acceptable Answers Additional Guidance Mark

Number

2(a) An answer that makes reference to the following points: First and second change can be in either (4)

order

Ignore prior refluxing

Ignore water bath

(First change) Adjust so that the flow of water goes in Allow just “water should enter the

at the bottom of the condenser and out at the top of condenser at the bottom”

the condenser OR

(1) Just “water should leave at the top”

Just “swap the tubes around”

(Reason) Keeps condenser full of water / water

removes (any) air in the condenser / allows more

efficient / better cooling / prevents ‘air-lock’ (1)

(Second change) (Replace funnel and) seal with a Allow replacing the funnel with a tap /

thermometer or a stopper (1) dropping funnel

(Reason) Prevents vapour / gas / product / reactants Ignore thermometers used to measure

escaping (1) boiling temperatures

Question

Acceptable Answers Additional Guidance Mark

Number

2(b) (2)

OH Do not penalise ‘connectivity’ to OH unless

O-H-C

Allow O-H for OH

Penalise non-skeletal formulae once only

(1)

Question

Acceptable Answers Additional Guidance Mark

Number

2(c)(i) 3-methylbutan-2-ol / secondary alcohols cannot be Allow (1)

oxidised to a carboxylic acid only primary alcohols can be oxidised to

OR carboxylic acids

3-methylbutanone / the product / ketones cannot be

(further) oxidised

Question

Acceptable Answers Additional Guidance Mark

Number

2(c)(ii) H2SO4 Ignore ‘sulfuric acid’ (1)

Question

Acceptable Answers Additional Guidance Mark

Number

2(c)(iii) Example of equation: (2)

all formulae correct (1)

Na2CO3(s) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + CO2(g)

all state symbols correct (1) OR

Na CO (s) + 2H+(aq) → 2Na+(aq) + H O(l) + CO (g)

23 2 2

Na2CO3(s) + 2H2SO4(aq) → 2NaHSO4(aq) + H2O(l) +

CO2(g)

Use of NaCO3 or H2CO3 scores zero

Allow any acid.

M2 consequential on M1 being awarded, or a ‘near-miss’

Question

Acceptable Answers Additional Guidance Mark

Number

2(d)(i) An explanation that makes reference to the following (2)

points:

peak at 1720 (cm−1) (1) Allow any absorbance between

1720 to 1700 (cm−1)

shows presence of a C=O bond / carbonyl (1)

Marks cannot be awarded if ANY incorrect other

peaks are identified e.g. peak due to C=C /

peak due to O-H

Ignore references to alkane C-H bonds /

fingerprint region

Do not award just ‘ketone’ for MP2

Question

Acceptable Answers Additional Guidance Mark

Number

2(d)(ii) An answer that makes reference to the following points: (2)

peak between 3750 and 3200 (cm−1) will disappear / Allow

will be absent from the spectrum any absorbance between

OR 3750 to 3200 (cm−1)

Peak(s) above 3000(cm−1) will disappear / will be

absent from the spectrum (1) Ignore references to fingerprint region

(because) 3-methylbutan-2-ol / the alcohol / O-H has

now been removed (1)

Question

Acceptable Answers Additional Guidance Mark

Number

2(e)(i) (identify the peak at the) highest/largest m/z value Allow (1)

Peak (furthest) to the right/last peak on the

spectrum

Do not award the mark for

“largest peak” / “highest peak”

Ignore

“parent ion” / molecular ion peak / References

to m/z = 86

Question

Acceptable Answers Additional Guidance Mark

Number

2(e)(ii) H H H H O (2)

Allow positive charge anywhere on structure

H C C C H H C C +

H H H

Ignore open bonds

Penalise non-displayed formulae once only

(1) (1)

Ignore brackets around the structure

Penalise missing charge once only

Question

Acceptable Answers Additional Guidance Mark

Number

2(f)(i) C5H12O + [O] → C5H10O + H2O Molecular formulae must be used throughout (2)

left-hand side of equation correct Allow [O] above the arrow

(1)

right-hand side of equation correct Do not award for C5H11OH as the alcohol

(1)

Ignore state symbols if incorrect or conditions

mentioned

Question

Acceptable Answers Additional Guidance Mark

Number

2(f)(ii) Example of calculation (2)

calculation of moles of both of C5H10O and C5H12O (1) Moles C5H10O = 2.15 = 0.025(0) (mol)

86.0

and

moles C5H12O = 0.025(0) x 100 = 0.04(00)

62.5

calculation of mass of C5H12O (1) (So) mass of C5H12O = 0.04(00) x 88 = 3.52 g

calculation of theoretical mass of C5H10O and moles of Theoretical mass C5H10O = 2.15 x 100 = 3.44 g

C5H10O (1) 62.5

and

moles C5H10O = 3.44 = 0.04(00) = mol C5H12O

86.0

calculation of mass of C5H12O (1) (So) mass of C5H12O = 0.04(00) x 88 = 3.52 g

Correct answer with no working scores (2)

Allow TE from MP1

Award 1 mark for 3.36 g, 1.375 g or 2.2 g

(Total for Question 2 = 21 marks)

How to answer it

Preparation, Spectroscopy & Calculations of 3-methylbutan-2-one

What this question tests

This question assesses practical organic chemistry techniques (distillation setup corrections), structural representation (skeletal and displayed formulas), organic reaction mechanisms and limitations (oxidation of secondary alcohols), qualitative analysis of functional groups via Infrared (IR) spectroscopy, interpretation of Mass Spectrometry data (molecular ion and fragment ions), and quantitative stoichiometry involving percentage yields.

Part (a): Practical Apparatus Correction

Identifying apparatus errors and functional reasons in distillation

✅ Correct Answers (4 Marks)

Change 1: Adjust water flow so it enters at the bottom of the condenser jacket and leaves at the top.
Reason 1: Ensures the condenser fills completely with water, maintaining efficient cooling and preventing air locks.
Change 2: Replace the open funnel with a thermometer or a solid stopper/bung.
Reason 2: Prevents volatile organic vapours, reactants, or products from escaping into the laboratory.

❌ Common Errors

Students often lose marks by saying water should enter from the top, which leaves pockets of air in the jacket and reduces cooling efficiency. Another common trap is suggesting a thermometer is needed to measure boiling temperature when the primary goal in a distillation setup is containment.

Part (b): Skeletal Formulae

Drawing organic structures accurately

✅ Correct Answers (2 Marks)

3-methylbutan-2-ol: A 4-carbon chain backbone with a methyl branch on carbon 3 and an -OH group on carbon 2. Drawn in skeletal format: a zig-zag chain ending with an OH branch pointing upwards/downwards.
3-methylbutan-2-one: A 4-carbon chain backbone with a methyl branch on carbon 3 and a double-bonded oxygen (=O) on carbon 2.

🧠 Exam Technique

Ensure skeletal lines clearly intersect at vertices representing carbon atoms. Do not penalise standard OH connectivity unless explicit H-C bonds are awkwardly drawn inside a skeletal representation.

Part (c): Reaction Limitations, Impurities, and Neutralisation

Explaining chemical limits, acidic impurities, and carbonate reactions

💡 Key Knowledge (c)(i) & (c)(ii)

(i) Why no carboxylic acid? 3-Methylbutan-2-ol is a secondary alcohol. Secondary alcohols oxidise to ketones, which cannot be further oxidised under normal laboratory conditions without breaking carbon-carbon bonds. (1 mark)
(ii) Impurity formula: H₂SO₄ (Unreacted sulfuric acid carried over from the acidifying mixture). (1 mark)

✅ Correct Answers for (c)(iii)

Equation for acid impurity reacting with sodium carbonate:

Na₂CO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

Mark breakdown: 1 mark for correct species and balancing; 1 mark for correct state symbols. (Accept valid ionic equations with H⁺).

Part (d): Infrared Spectroscopy Analysis

Interpreting diagnostic peaks and shifts after purification

✅ Correct Answers

(d)(i) Proving Ketone Presence (2 marks): Look for a sharp, strong peak around 1720 cm⁻¹ , which corresponds to the C=O (carbonyl) stretch. Do not award points for unrelated fingerprint region picks.
(d)(ii) Changes After Redistillation (2 marks): The broad peak between 3750 and 3200 cm⁻¹ will disappear (or be absent). Reason: Unreacted alcohol / O-H bonds have been successfully removed during purification.

❌ Common Errors

Many students incorrectly cite alkane C-H stretches or fingerprint region fluctuations. Always focus strictly on diagnostic functional group peaks requested by the stem.

Part (e): Mass Spectrometry & Fragmentation

Determining molar mass and drawing fragment ions

💡 Key Knowledge (e)(i)

The molar mass of 3-methylbutan-2-one is found by locating the highest/furthest right peak on the mass spectrum (the molecular ion peak, m/z = 86).

✅ Correct Answers for (e)(ii) (2 Marks)

For the fragment peak at m/z = 43 , draw the displayed formulae of the fragment ions:

CH₃-CH(CH₃)-⁺CH or CH₃-C⁺=O (Acetyl/acylium ion or carbocation fragment).
Must clearly display bonds, positive charge ( ⁺ ) anywhere on the structure, and brackets are optional.

Part (f): Stoichiometry and Percentage Yield Calculations

Step-by-step mass calculations using oxidation ratios

💡 Oxidation Equation (f)(i)

C₅H₁₂O + [O] → C₅H₁₀O + H₂O

1 mark for correct left-hand side; 1 mark for correct right-hand side. Use [O] as the oxidising agent.

📐 Step-by-Step Calculation (f)(ii) (2 Marks)

Find moles of product (3-methylbutan-2-one, C₅H₁₀O):
Molar mass of C₅H₁₀O = (5 × 12.0) + (10 × 1.0) + 16.0 = 86.0 g mol⁻¹
Moles = mass / molar mass = 2.15 g / 86.0 g mol⁻¹ = 0.0250 mol
Account for the percentage yield (62.5%):
Theoretical moles of C₅H₁₀O needed = (0.0250 / 62.5) × 100 = 0.0400 mol
Calculate mass of reactant (3-methylbutan-2-ol, C₅H₁₂O):
Since stoichiometry is 1 : 1, moles of reactant = 0.0400 mol
Molar mass of C₅H₁₂O = (5 × 12.0) + (12 × 1.0) + 16.0 = 88.0 g mol⁻¹
Mass = moles × molar mass = 0.0400 mol × 88.0 g mol⁻¹ = 3.52 g

Alternative methods calculating theoretical mass directly via percentage yield ratios also achieve full credit. Common alternative answers accepted: 3.52 g.

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 6: Organic Chemistry I · Topic 7: Modern Analytical Techniques I · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III · Topic 5: Formulae, Equations and Amounts of Substance · Topic 19: Modern Analytical Techniques II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.