Edexcel A-Level Chemistry Paper 2, November 2020: Question 7

12 marks · Hard difficulty · Open Response

Outline the synthetic steps and reactions involved in converting but-2-en-1-ol to pentanoic acid and benzene to benzoyl chloride.

Practise this question

Question

A two-part organic synthesis question. Part (a) shows a reaction scheme with structures for but-2-en-1-ol, butan-1-ol, 1-cyanobutane, and pentanoic acid, along with four sub-questions asking about LiAlH4 reducing properties, reagents for Step 1, intermediate compounds for converting alcohols to nitriles, and reagents for nitrile hydrolysis. Part (b) asks to devise a four-step synthesis to convert benzene into benzoyl chloride using a Grignard reagent, including reagents, conditions, and intermediate structures.
Question text

7 This question is about the synthesis of organic compounds.

(a) A student suggested the following plan for the synthesis of pentanoic acid from

but-2-en-1-ol.

Step 1

OH OH

LiAlH4 in

but-2-en-1-ol dry ether butan-1-ol

Step 2 KCN

in aqueous ethanol

O

Step 3

CN

heat under reflux

OH with NaOH(aq)

pentanoic acid

(i) LiAlH is a source of hydride ions, H−.

Give a possible reason why LiAlH4 cannot be used to reduce alkenes.

(1)

(ii) Give a suitable reagent and condition for Step 1.

(2)

(iii) Step 2 is incorrect because alcohols can only be converted to nitriles via an

intermediate compound.

Identify a suitable intermediate compound by name or formula.

(1)

(iv) Step 3 involves the hydrolysis of a nitrile.

Give the additional reagent that should be added after heating under reflux

with aqueous sodium hydroxide, to produce pentanoic acid.

(1)

(b) Devise a four-step synthesis, involving the use of a Grignard reagent, to convert

benzene into benzoyl chloride.

*P62669A02232*O

C

Cl

Include the reagents and conditions for each step in the synthesis and the structures

of the intermediates.

(7)

(Total for Question 7 = 12 marks)

Mark scheme

Show the mark scheme The mark scheme provides detailed answers for both parts of Question 7. Part (a) lists acceptable points such as hydride ions being repelled by high electron density in alkenes, hydrogen with nickel/platinum catalyst for Step 1, 1-bromobutane as the intermediate, and hydrochloric acid for hydrolysing the nitrile. Part (b) details the four-step synthesis starting with bromination or chlorination of benzene, formation of the Grignard reagent with magnesium, reaction with carbon dioxide followed by acid, and finally conversion of benzoic acid to benzoyl chloride using phosphorus(V) chloride.

How to answer it

Synthesis of Organic Compounds & Grignard Reactions

What this question tests

This multi-part organic synthesis question evaluates your understanding of functional group interconversions, nucleophilic addition mechanisms, reducing agent selectivities, and complex multi-step synthetic pathways involving aromatic rings and Grignard reagents.

Question 7 (a) (i)

Selectivity of Reducing Agents

✅ Correct Answer

The hydride ion ( H⁻ ) will not attack / will be repelled by regions of high electron density (the electron-rich carbon-carbon double bond / pi-bond).

❌ Common Errors

Students often incorrectly state that "alkenes cannot be reduced at all" or confuse LiAlH₄ with catalytic hydrogenation. Vague statements about "like charges not attracting" without referencing electron density or nucleophilicity frequently lose the mark.

Mark Available: 1 mark
Question 7 (a) (ii)

Reagents and Conditions for Step 1 (Reduction of Alene/Double Bond)

✅ Correct Answer

Reagent 1: Hydrogen ( H₂ )

Reagent 2 (Catalyst): Nickel ( Ni ), Platinum ( Pt ), or Palladium ( Pd )

🧠 Exam Technique

Marked independently. Ensure you list both the hydrogen gas and a valid transition metal catalyst. Ignore mentions of temperature or pressure unless specified incorrectly.

Marks Available: 2 marks (1 per point)
Question 7 (a) (iii)

Halogenoalkane Intermediates

✅ Correct Answer

1-bromobutane (or CH₃CH₂CH₂CH₂Br ). Alternatively, 1-chlorobutane or 1-iodobutane and their respective formulas are accepted.

💡 Key Knowledge

Alcohols cannot directly react with KCN to form nitriles because the hydroxyl group ( -OH ) is a poor leaving group. It must first be substituted by a halogen (using PBr₃ , HBr , etc.) to form a halogenoalkane intermediate.

Mark Available: 1 mark
Question 7 (a) (iv)

Hydrolysis of Nitriles

✅ Correct Answer

Hydrochloric acid ( HCl ) or any dilute strong acid (providing H⁺ ions).

❌ Common Errors

Do not use weak acids (such as carboxylic acids or carbonic acid), as they will not successfully protonate the intermediate species to yield the carboxylic acid efficiently under standard reflux conditions.

Mark Available: 1 mark
Question 7 (b)

Four-Step Synthesis: Benzene to Benzoyl Chloride via Grignard Reagent

💡 Reaction Pathway Breakdown

  • Step 1: Electrophilic substitution of benzene to form bromobenzene.
    Reagents: Bromine ( Br₂ ) and iron ( Fe ) or iron(III) bromide ( FeBr₃ ).
    Structure: Benzene ring with a -Br substituent.
  • Step 2: Formation of the Grignard reagent (phenylmagnesium bromide).
    Reagent: Magnesium ( Mg ) in dry ether.
    Structure: Benzene ring attached to -MgBr .
  • Step 3: Reaction with carbon dioxide to form a carboxylic acid salt, followed by acidification.
    Reagents: Carbon dioxide ( CO₂ ) followed by dilute acid.
    Structure: Benzoic acid (benzene ring attached to -COOH ).
  • Step 4: Conversion of benzoic acid to acyl chloride.
    Reagent: Phosphorus(V) chloride ( PCl₅ ) or phosphorus trichloride / thionyl chloride ( SOCl₂ ).

🧠 Exam Technique & Grading Guidance

This 7-mark synthesis question requires absolute precision. Marks are distributed between correct reagents/conditions and clear intermediate organic structures. Ensure your structural drawings clearly show attachment points to the benzene ring.

Marks Available: 7 marks total (reagents and structures marked across the 4 distinct steps)

Topics

Organic Chemistry · Topic 18: Organic Chemistry III · Topic 17: Organic Chemistry II · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.