Edexcel A-Level Chemistry Paper 3, June 2024: Question 7

22 marks · Hard difficulty · Open Response

Synthesise 1-ethylcyclohexan-1-ol from cyclohexene involving mechanisms, reaction schemes, reflux apparatus, and carbonyl derivatives.

Practise this question

Question

An organic reaction scheme showing the synthesis of 1-ethylcyclohexan-1-ol from cyclohexene via bromocyclohexane, compound A, and cyclohexanone, followed by sub-questions about mechanisms, reflux apparatus setup with a drying tube, and 2,4-DNPH derivatisation.

Question text

7 The alcohol 1-ethylcyclohexan-1-ol can be synthesised from cyclohexene using the

reaction scheme shown.

Step 1 Br

HBr Step 2

Compound A

cyclohexene bromocyclohexane

Step 3

OH Step 4 O

CH3CH2MgBr

1-ethylcyclohexan-1-ol cyclohexanone

(a) Draw the mechanism for Step 1.

Include curly arrows, and relevant lone pairs and dipoles.

(4)

(b) Devise a reaction scheme for Step 2 and Step 3 of the synthesis.

Include in your answer all reagents and conditions, the type of reaction in each

step, and the structure of A.

(5)

and magnesium to be warmed under reflux with a suitable solvent. The apparatus

used for this reaction is shown.

Tube X anhydrous calcium chloride

glass wool

water out

*P74455A01432*

water in

flask with solvent

and reactants

gentle heat

(i) Name the solvent used.

(1)

(ii) Explain why the anhydrous calcium chloride in Tube X is necessary.

(2)

… *P74455A01532*

… *P74455A01632*

(iii) Give a reason why it is best practice for the water in the condenser to flow in

the direction shown in the diagram.

(1)

(d) Carbonyl compounds such as cyclohexanone can be identified by the reaction

with 2,4-dinitrophenylhydrazine.

(i) Give the colour of the precipitate formed in this reaction.

(1)

(ii) Complete the simplified mechanism for the first two steps of this reaction by

adding four curly arrows.

(4)

R R′

R′

.. +

H2N N

NH H NH

NO2 H NO2

Step 1

NO2 NO2

*P74455A01732*

H+ O OH

R′ R′

R N R N

H NH H NH

H NO2 NO2

.. +

H O Step 2 H O

NO2 NO2

(iii) Describe how you would obtain pure dry crystals from the precipitate.

(4)

… *P74455A01832*

(Total for Question 7 = 22 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

7(a) An answer that makes reference to the following points: (4)

• curly arrow from C=C to ∂+ H in HBr Penalise use of half arrows once only in points 1, 3 and 6

• dipole on HBr

• curly arrow from H-Br bond to, or just beyond, Br∂–

• structure of carbocation Ignore drawing of hydrogen atoms on carbocation unless

the number of hydrogen atoms is too many

• lone pair on Br−

• curly arrow (from lone pair on) Br– to C+

6 points scores (4)

5 points scores (3)

3 or 4 points scores (2)

2 points scores (1)

1 point scores (0)

Question

Answer Additional Guidance Mark

Number

7(b) An answer that makes reference to the following points: (5)

Each mark is standalone

• (reaction of) bromocyclohexane with KOH(aq) /NaOH(aq) (1) Accept aqueous ethanolic for (aq) or aqueous

Ignore reference to reflux / heating

Do not award just ethanolic KOH/ NaOH

• (nucleophilic) substitution (1) Accept hydrolysis

• structure of A (cyclohexanol) (1)

Accept displayed formula

Ignore name of A, even if incorrect

• reflux / heat with sulfuric acid and sodium dichromate ((VI)) (1) Allow acidified dichromate ((VI)) / H+ and Cr O 2–

Accept K2Cr2O7 for Na2Cr2O7

Ignore concentration

Allow distillation/distil for reflux/heat

Do not award HCl for sulfuric acid

Allow reference to oxidising agent/oxidised

Ignore redox

• oxidation (1)

Questio

n Answer Additional Guidance Mark

Number

7(c)(i) (1)

• (dry) ether / ethoxyethane Accept diethyl ether

Ignore any formulae, even if incorrect

Do not award if given with any additional substance

Question

Answer Additional Guidance Mark

Number

7(c)(ii) An explanation that makes reference to the following points: (2)

• to prevent moist air entering (1) Allow to keep the reactants dry

Allow reference to absorbing water

Ignore just drying agent

Do not award to remove water from the solution or

coming out from the solution

• as Grignard reagent will not form in presence of water (1) Allow ‘to ensure Grignard Reagent doesn’t break

down (to an alkane)’ / Grignard reagents react with

water / avoid hydrolysis of Grignard reagents

Allow reference to ethane/alkane forming

Ignore reference to just reacting with the reactants

Question

Answer Additional Guidance Mark

Number

7(c)(iii) An answer that makes reference to the following point: (1)

• to ensure efficient cooling / prevent air bubbles forming in condenser Allow to fill the condenser with water

Question

Answer Additional Guidance Mark

Number

7(d)(i) An answer that makes reference to the following point: (1)

• orange / yellow Allow red

Allow any combination of these 3 colours

Ignore any shades

Question

Answer Additional Guidance Mark

Number

7(d)(ii) An answer that makes reference to the following points: (4)

• arrow from C=O bond to O (1)

• arrow from lone pair on O− to H+ (1)

• arrow from lone pair in O in H2O to H in NH2 (1) Accept arrow to either of the two hydrogen atoms

• arrow from N−H bond to N+ (1)

Allow M2 to M4 to be drawn on the product of Step 1 if

not drawn in Step 2

Ignore any dipoles added, even if incorrect

Penalise any additional incorrect curly arrows over four

Exemplar diagram

Question

Answer Additional Guidance Mark

Number

7(d)(iii) An answer that makes reference to the following points: (4)

• solid dissolved in minimum amount of hot solvent (1) Allow any named solvents

• (hot filtration then) solution allowed to cool and M2 dependent on M1 or near miss

(re)crystallise/precipitate (1) Allow solid for crystal/ppt

• solid filtered under reduced pressure (1) Allow ‘Buchner filtration’ / suction filtration

Cooling and recrystalising follows M1 so M2 dependent

M3 and M4 must apply to crystals/solid being present

however derived

Filtering and drying of dissolved solid is nonsense so M3

and M4 would both be lost

• rinsed with (cold) solvent Allow desiccator / warm oven as alternative for (sheets of)

and filter paper

dried between (sheets of) filter paper (1) Allow drying with paper towels

(Total for Question 7 = 22 marks)

How to answer it

Organic Synthesis, Reaction Mechanisms & Purification

What this question tests

This multi-step synoptic organic chemistry question evaluates your understanding of electrophilic addition mechanisms, nucleophilic substitution, oxidation pathways, Grignard reagent preparation conditions (reflux techniques and anhydrous environments), addition-elimination mechanisms with 24-DNPH, and laboratory purification techniques including filtration under reduced pressure and recrystallisation.

Part (a)

Electrophilic Addition Mechanism

✅ Correct Answer Requirements (4 Marks)

Curly arrow starting from the C=C double bond pointing to the delta-positive H atom on HBr.
Partial charges (delta-plus on H, delta-minus on Br) clearly indicated on the HBr molecule.
Curly arrow from the H-Br bond breaking completely towards the Br atom, forming a bromide ion.
Correct intermediate carbocation structure (bromocyclohexane carbocation) and a bromide ion ( Br⁻ ) with a lone pair, showing a curly arrow pointing from the lone pair to the positively charged carbon.

❌ Common Errors

Drawing curly arrows starting in "empty space" rather than precisely from bonds or lone pairs.
Omitting partial charges ( δ+ and δ- ) on the attacking electrophile.
Using half-headed (fishhook) radicals arrows instead of full-headed curly arrows for heterolytic fission.

Mark allocation: 6 points available mapped to a 4-mark sliding scale (6 points = 4 marks; 5 points = 3 marks; 3–4 points = 2 marks; 2 points = 1 mark).

Part (b)

Multi-Step Synthesis Route

💡 Reagents, Conditions & Identification

Step 2 Reagent/Conditions: KOH(aq) or NaOH(aq) with heating/reflux.
Step 2 Reaction Type: Nucleophilic substitution (or hydrolysis).
Compound A Structure: Cyclohexanol ( C₆H₁₁OH - a cyclohexane ring with an -OH group attached).
Step 3 Reagent/Conditions: Acidified potassium dichromate( VI ) ( H⁺/K₂Cr₂O₇ or H⁺/Na₂Cr₂O₇ ) under reflux or distillation.
Step 3 Reaction Type: Oxidation.

🧠 Exam Technique

Always state both the specific reagent (including relevant states like (aq) ) and the required energy condition (heat/reflux) to secure reagent marks. Vague answers like "sodium hydroxide" without state symbols often lose marks.

Mark allocation: 5 marks total (1 mark per point: Step 2 reagent/conditions, Step 2 type, Structure of A, Step 3 reagent/conditions, Step 3 type).

Parts (c)(i) - (c)(iii)

Grignard Reagents & Reflux Apparatus

✅ Correct Answers

(i) Solvent: Dry ether (or ethoxyethane). Other ethers are accepted, but must include "dry".
(ii) Calcium Chloride Purpose: To prevent moist air from entering the apparatus (1 mark) because water destroys Grignard reagents by protonating them back to alkanes (1 mark).
(iii) Water Direction in Condenser: Enters at the bottom and leaves at the top to ensure the water jacket remains completely full, giving efficient cooling and preventing air bubbles.

❌ Common Errors

Failing to specify that the ether solvent must be dry in (c)(i).
Reversing the water flow direction in the condenser (top-to-bottom), which leaves upper sections dry and reduces condensation efficiency.

Mark allocation: (i) 1 mark, (ii) 2 marks, (iii) 1 mark. Total = 4 marks.

Parts (d)(i) - (d)(iii)

Carbonyl Identification & Purification

💡 Core Knowledge & Mechanism

(i) 2,4-DNPH Test: Forms an orange or yellow precipitate.
(ii) Mechanism Curly Arrows (4 marks):
1. Arrow from C=O pi bond to oxygen atom.
2. Arrow from lone pair on -O⁻ back down or handling proton transfer to H⁺ .
3. Arrow from lone pair on water molecule oxygen to hydrogen attached to nitrogen ( NH₂ ).
4. Arrow from N-H bond to the positive nitrogen atom ( N⁺ ).
(iii) Purification Steps: Dissolve impure solid in a minimum volume of hot solvent (1), hot filtration followed by cooling to allow recrystallisation (2), filter crystals under reduced pressure using a Buchner funnel (3), rinse with cold solvent and dry between filter papers or in a desiccator (4).

🧠 Recrystallisation Checklist

Never skip key terms when describing purification: always mention a minimum volume of hot solvent, hot filtration for insoluble impurities, cooling for crystal growth, and reduced pressure filtration.

Mark allocation: (i) 1 mark, (ii) 4 marks, (iii) 4 marks. Total = 9 marks.

Topics

Organic Chemistry · Core Practicals · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III · Core Practical 7: Identify unknown organic liquids and inorganic solids · Core Practical 15: Analyse organic and inorganic unknowns

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.