Edexcel A-Level Chemistry Paper 3, November 2021: Question 8

18 marks · Hard difficulty · Extended Writing

Describe chemical tests, mechanisms, stereochemistry, NMR spectra, and redox reactions for carbonyl compounds including pentan-2-one and pentan-3-one.

Practise this question

Question

Question text

8 This question is about some carbonyl compounds with the molecular formula C5H10O.

(a) Describe a chemical test, and its result, to distinguish between

pentan-2-one, CH3CH2CH2COCH3, and pentan-3-one, CH3CH2COCH2CH3.

(2)

(b) Pentan-2-one reacts with hydrogen cyanide in the presence of cyanide ions to

form 2-hydroxy-2-methylpentanenitrile.

H H H OH H

H C C C C C H

H H H CN H

2-hydroxy-2-methylpentanenitrile

(i) Draw the mechanism for the reaction between pentan-2-one and

hydrogen cyanide in the presence of cyanide ions.

Include curly arrows and any relevant lone pairs.

(4)

(ii) The product of this reaction, 2-hydroxy-2-methylpentanenitrile, has a

chiral centre.

Explain why a racemic mixture of 2-hydroxy-2-methylpentanenitrile is formed

in this reaction.

(2)

5 10

three peaks.

The high resolution 1H NMR spectrum of this aldehyde has two peaks and neither

of them is split. *P67806A02636*

Deduce the displayed formula of this aldehyde.

Justify your answer by referring to both NMR spectra.

(4)

*(d) Describe the oxidation and reduction reactions of pentanal and pentan-3-one. 27

Include the reagents and the structures of the organic products formed in any*P67806A02736*

reactions that take place.

(6)

… *P67806A02836*

(Total for Question 8 = 18 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

8 (a) A description that makes reference to the following (2)

points:

• (add a solution of) iodine and alkali / sodium hydroxide / (1) Allow names or formulae but if both are given, both

potassium hydroxide / hydroxide ions (and warm) must be correct

(add a solution of) potassium iodide in sodium

chlorate(I) (and warm)

• (only) pentan-2-one give a (pale) yellow precipitate / ppt(e) Stand alone mark

/ solid (1) Allow antiseptic smell

Ignore observation for pentan-3-one unless also

stated that it gives a yellow precipitate

Number

8(b)(i) An answer that makes reference to the Example of mechanism: (4)

following points:

• curly arrow from lone pair on C of CN− (1)

to C of ketone group

• curly arrow from C=O to, or just beyond, (1)

• intermediate (1)

• curly arrow from lone pair on O− to H

and curly arrow from H-CN bond to

anywhere on CN (1)

Allow C3H7 and CH3 for propyl and methyl groups

Allow CN bond displayed

Ignore correct dipoles, penalise an incorrect dipole once only

Do not award M3 if C+ is shown on intermediate

For M4, allow curly arrow from lone pair on O− to H+ ion / H O

molecule

Penalise incorrect ketone once only in M3 intermediate

Penalise curly arrow from -ve charge instead of lone pair once only

Number

8(b)(ii) An explanation that makes reference to the following (2)

points:

• pentan-2-one / ketone is planar about the carbonyl (1) Allow bonds about C=O are (trigonal) planar

carbon or

the carbonyl carbon is (trigonal) planar

Do not award planar molecule / reference to planar

intermediate / ion

• so the CN−/ nucleophile attacks (equally) from above Do not award multiple directions

and below / either side (of the plane) (1)

Number

8(c) An answer that makes reference to the following Example of displayed formula: (4)

points:

• displayed formula of aldehyde (1)

Allow CH3 groups but aldehyde group must be displayed

• three different carbon environments indicated (1) Example of three carbon environments:

• two different proton environments indicated

(1)

Example of two proton environments:

• no splitting as there are no hydrogens on the Stand alone mark

adjacent carbon atom(s) (1)

Question

Acceptable Answers Additional Guidance Mark

Number

8(d)* This question assesses a student’s ability to show a coherent and Guidance on how the mark scheme should be applied: (6)

logically structured answer with linkages and fully-sustained The mark for indicative content should be added to the

reasoning. mark for lines of reasoning. For example, an answer with

five indicative marking points that is partially structured

Marks are awarded for indicative content and for how the answer with some linkages and lines of reasoning scores 4 marks

is structured and shows lines of reasoning. (3 marks for indicative content and 1 mark for partial

structure and some linkages and lines of reasoning).

The following table shows how the marks should be awarded for If there are no linkages between points, the same five

indicative content. indicative marking points would yield an overall score of

3 marks (3 marks for indicative content and no marks for

Number of Number of marks linkages).

indicative awarded for

marking points indicative

seen in answer marking points

5–4 3

3–2 2

The following table shows how the marks should be awarded for

structure and lines of reasoning.

Number of marks In general it would be expected that 5 or 6 indicative

awarded for points would get 2 reasoning marks, and 3 or 4 indicative

structure of answer points would get 1 mark for reasoning, and 0, 1 or 2

and sustained line of indicative points would score zero marks for reasoning.

reasoning

Answer shows a coherent and logical 2

structure with linkages and fully

sustained lines of reasoning

demonstrated throughout.

Answer is partially structured with 1

some linkages and lines of reasoning.

Answer has no linkages between 0

points and is unstructured.

Comment:

Look for the indicative marking points first, then consider the

mark for structure of answer and sustained line of reasoning

Indicative content Reagents - Allow names or formulae but if both are given,

both must be correct

Products - Allow any combination of displayed and

structural formulae / skeletal formulae

Allow C4H9 / C3H7 for the alkyl groups

• IP1 Reagents and conditions for oxidation –

acidified potassium dichromate((VI)) / K Cr O and H+/H SO Allow acidified dichromate((VI)) ions / Cr O 2−

22 7 2 4 2 7

or acidified sodium dichromate((VI)) / Na Cr O and and H+/H SO

22 7 2 4

H+/H SO Allow acidified manganate((VII)) ions / MnO −

24 4

or Fehling’s solution / Benedict’s solution and H+/H SO

or Tollens’ reagent / ammoniacal silver nitrate / Ag(NH )2+ Ignore reference to heat

Do not award just Cu2+ for Fehling’s / Benedict’s

• IP2 Oxidation of aldehyde – structure of CH3CH2CH2CH2COOH

pentanoic acid

• IP3 Oxidation of ketone–pentan-3-one / ketone is not (easily)

oxidised

• IP4 Reagents and conditions for reduction –

lithium tetrahydridoaluminate((III)) / lithium aluminium Allow lithal

hydride and dry ether /ethoxyethane (followed by a dilute Ignore hydrogen and platinum (catalyst)

acid) Ignore reference to heat

or sodium tetrahydridoborate((III)) / sodium borohydride and

aqueous / methanol solution (followed by a dilute acid)

• IP5 Reduction of aldehyde – structure of CH3CH2CH2CH2CH2OH

pentan-1-ol

• IP6 Reduction of ketone – structure of pentan-3-ol CH3CH2 CH(OH)CH2CH3

(Total for Question 8 = 18 marks)

How to answer it

Reactions, Mechanisms & Spectroscopy of Carbonyl Compounds

What this question tests

This comprehensive question assesses your knowledge of carbonyl chemistry (aldehydes and ketones with molecular formula C₅H₁₀O). It tests chemical identification tests (triiodomethane), nucleophilic addition mechanisms with curly arrows, stereochemistry (chiral centers and racemic mixtures), structure elucidation using combined ¹³C and high-resolution ¹H NMR spectroscopy, and comparative redox reactions (oxidation and reduction) with full structural identification and prose structuring.

Part (a): Chemical Identification Test

Distinguishing pentan-2-one and pentan-3-one

✅ Correct Answer

Reagent: Add alkaline iodine solution (I₂ / NaOH or warm aqueous iodine with sodium hydroxide / sodium chlorate(I)).

Result:
• pentan-2-one gives a pale yellow precipitate (or solid) with an antiseptic smell.
• pentan-3-one shows no reaction / no yellow precipitate forms.

💡 Key Knowledge

Pentan-2-one is a methyl ketone ( CH₃CO- group), which reacts positively in the triiodomethane (iodoform) test. Pentan-3-one is a diethyl ketone and lacks this specific structural feature.

❌ Common Errors

Students often lose marks by omitting the state of the precipitate (must state solid/precipitate, not just "yellow solution") or confusing methyl ketones with general carbonyl compounds.

Mark breakdown: (2 marks) 1 mark for correct reagents, 1 mark for the correct observation of pentan-2-one (mentioning pale yellow precipitate).

Part (b)(i): Nucleophilic Addition Mechanism

Reaction of pentan-2-one with hydrogen cyanide

✅ Correct Answer

The mechanism proceeds via nucleophilic addition:

Curly arrow from lone pair on the cyanide ion ( :CN⁻ ) to the carbon atom of the C=O carbonyl group.
Curly arrow breaking the C=O double bond, with both electrons moving to the oxygen atom.
Formation of a negatively charged intermediate species containing an alkoxide ion ( -O⁻ ).
Curly arrow from the lone pair on O⁻ to the hydrogen atom of an H-CN molecule, accompanied by a curly arrow breaking the H-CN bond back to form :CN⁻ .

🧠 Exam Technique

Draw clear partial structures showing alkyl chains (e.g., C₃H₇ or fully expanded bonds). Ensure curly arrows start precisely from lone pairs or bonds, and point accurately towards atoms or bond spaces.

❌ Common Errors

Putting the dipole incorrectly on the intermediate, omitting lone pairs on attacking species, or drawing the second step's curly arrow incorrectly originating from a positive charge rather than an oxygen lone pair.

Mark breakdown: (4 marks) 1 mark for attack on C=O, 1 mark for bond breaking to O, 1 mark for intermediate structure, 1 mark for protonation step using H-CN.

Part (b)(ii): Stereochemistry & Racemic Mixtures

Why 2-hydroxy-2-methylpentanenitrile is formed as a racemic mixture

✅ Correct Answer

The carbonyl carbon in pentan-2-one is planar (trigonal planar). Because the molecule is flat around this reactive center, the incoming cyanide nucleophile ( :CN⁻ ) has an equal probability of attacking from either above or below the plane.

💡 Key Knowledge

Equal attack from both sides forms equal amounts of the two enantiomers (a 50:50 mixture), which rotates plane-polarized light equally in opposite directions, resulting in optical inactivity (a racemic mixture).

Mark breakdown: (2 marks) 1 mark for stating the carbonyl carbon is planar, 1 mark for explaining equal attack from above and below.

Part (c): Structure Elucidation via NMR Spectroscopy

Deducing the displayed formula of the aldehyde

✅ Correct Answer

Displayed Formula Structure: An aldehyde with molecular formula C₅H₁₀O that has three peaks in its ¹³C NMR and two singlet peaks (no splitting) in its high-resolution ¹H NMR.
Structure: Trimethylacetaldehyde (2,2-dimethylpropanal), where the main chain is a central carbon bonded to three methyl groups and the aldehyde group ( -CHO ).

💡 Justification by NMR:

¹³C NMR: Exactly three peaks, corresponding to the three unique carbon environments (carbonyl carbon, quaternary carbon, and equivalent methyl carbons).
¹H NMR: Two peaks corresponding to the hydrogen atoms in the aldehyde group and the twelve equivalent methyl protons.
No Splitting: Neither peak is split because there are no adjacent carbon atoms with hydrogen atoms attached (using the n+1 rule, n = 0 ).

❌ Common Errors

Failing to draw the displayed formula explicitly (showing every single bond and atom), or misunderstanding how quaternary carbons result in singlets with no neighbouring protons for splitting.

Mark breakdown: (4 marks) 1 mark for correct displayed formula, 1 mark for 3 different carbon environments, 1 mark for 2 proton environments, 1 mark for explaining no splitting due to lack of adjacent hydrogens.

Part (d): Oxidation and Reduction of Carbonyls

Comparing pentanal and pentan-3-one reactions

💡 Indicative Chemistry Content

1. Oxidation Reactions:

Reagents: Acidified potassium dichromate(VI) ( K₂Cr₂O₇ / H⁺ ) or acidified sodium/potassium manganate(VII), or Tollens' / Fehling's reagents.
Pentanal (aldehyde): Easily oxidized to pentanoic acid ( CH₃CH₂CH₂CH₂COOH ). Colour change from orange to green with dichromate.
Pentan-3-one (ketone): Resistant to oxidation under normal conditions (no reaction).

2. Reduction Reactions:

Reagents: Lithium tetrahydridoaluminate(III) ( LiAlH₄ in dry ether) or sodium tetrahydridoborate(III) ( NaBH₄ in aqueous/alcoholic solution).
Pentanal reduction: Reduced to pentan-1-ol ( CH₃CH₂CH₂CH₂CH₂OH ), a primary alcohol.
Pentan-3-one reduction: Reduced to pentan-3-ol ( CH₃CH₂CH(OH)CH₂CH₃ ), a secondary alcohol.

🧠 Structuring Top-Level Responses

This is a continuous-prose assessed question (marked out of 6 total marks: up to 4 marks for indicative chemical points, and up to 2 marks for the coherence, logical structure, and sustained line of reasoning). Use clear headings or organized paragraphs separating oxidation from reduction.

Mark breakdown: (6 marks total) Up to 4 marks for chemical accuracy (reagents, conditions, and correct organic products for both oxidation and reduction of both compounds), plus up to 2 marks for a coherent, logically structured argument with linkages.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II · Topic 19: Modern Analytical Techniques II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.