Edexcel A-Level Chemistry Paper 2, June 2024: Question 2
6 marks · Medium difficulty · Short Open Response
Identify properties, reactions, and synthesis involving Grignard reagents including solvents, reaction type, organic product with water, and carbonyl compounds forming tertiary alcohols.
Practise this questionQuestion
Question text
2 This question is about Grignard reagents.
A Grignard reagent is formed by reacting bromoethane with magnesium
under reflux.
CH3CH2Br + Mg → CH3CH2MgBr
(a) The most suitable solvent for this reaction is
(1)
A cyclohexane
B ether
C ethyl ethanoate
D hexane
(b) State the use of Grignard reagents in organic synthesis.
(1)
(c) In their reactions, the Grignard reagent is best described as
(1)
A a carbocation
B an electrophile
C a nucleophile
D a radical
(d) Grignard reagents must be kept dry.
Predict the organic product that forms when CH3CH2MgBr reacts with water.
Justify your answer by considering the polarity of both CH3CH2MgBr and water.
(2)
(e) Which compound will form a tertiary alcohol when it reacts with a
Grignard reagent, followed by acid hydrolysis?
(1) 5
A CO2 *P76896A0532*
B HCHO
C CH3CHO
D CH3COCH3
(Total for Question 2 = 6 marks)
Mark scheme
Show the mark scheme
Question
Answer Mark
Number
2(a) The only correct answer is B (ether) (1)
A is not correct because cyclohexane is not a good solvent for Grignard reagents as it is not polar
C is not correct because ethyl ethanoate would react with Grignard reagents
D is not correct because hexane is not a good solvent for Grignard reagents as it is not polar
Question
Answer Additional Guidance Mark
Number
2(b) An answer that makes reference to the following point: (1)
• to increase the length of a carbon chain (in a molecule) Accept to form a (new) carbon- carbon (single)
/ extend the carbon chain bond
Allow to increase the number of carbon(s)
(atoms) (in the starting material) /
Ignore to increase chain length / add to the
carbon chain
Ignore adding alkyl groups
Question
Answer Mark
Number
2(c) The only correct answer is C (a nucleophile) (1)
A is not correct because the reactive carbon has a partial negative charge
B is not correct because the reactive carbon has a partial negative charge
D is not correct because the reactive carbon has a partial negative charge
Question
Answer Additional Guidance Mark
Number
2(d) An answer that makes reference to the following points: (2)
• ethane (1) Accept CH3CH3 / C2H6
• (partially) negative carbon (in Grignard reagents) is
(1) Allow use of symbols e.g. δ− C, δ+ H
attracted to / reacts with the (partially) positive
hydrogen (in water) Allow use of full negative charges on C and H
Question
Answer Mark
Number
2(e) The only correct answer is D (CH3COCH3) (1)
A is not correct because CO2 would form a carboxylic acid with a Grignard reagent
B is not correct because HCHO would form a primary alcohol with a Grignard reagent
C is not correct because CH3CHO would form a secondary alcohol with a Grignard reagent
(Total for Question 2 = 6 marks)
How to answer it
Edexcel A-Level Chemistry Study Guide: Grignard Reagents
What this question tests
This question assesses your understanding of Grignard reagents (organometallic compounds), including their preparation solvents, synthetic utility, nucleophilic nature, reactivity with protic solvents (like water), and their reactions with carbonyl compounds to form specific classes of alcohols.
Solvent Selection for Grignard Formation
✅ Correct Answer
B (ether)
Dry ether (such as ethoxyethane) is required to stabilize and dissolve the organometallic Grignard reagent through coordinate bonding.
❌ Common Errors
Choosing cyclohexane or hexane (A & D) fails because they are non-polar alkanes and cannot stabilize the polar reagent. Choosing ethyl ethanoate (C) fails because esters contain a reactive carbonyl group which the Grignard reagent would instantly attack.
Synthetic Utility of Grignard Reagents
✅ Correct Answer
To increase the length of a carbon chain (or extend the carbon chain) in a molecule by forming a new carbon-carbon single bond.
🧠 Exam Technique
Keep your answer concise and direct. Examiners accept "forming a new carbon-carbon bond" or "extending the carbon chain". Avoid vague phrases like "adding alkyl groups" without context.
Reactivity Profile of Grignard Reagents
✅ Correct Answer
C (a nucleophile)
Because carbon is more electronegative than magnesium, the carbon bonded to Mg carries a partial negative charge (δ⁻). This makes the carbon electron-rich and a powerful nucleophile.
💡 Key Knowledge
Grignard reagents act as carbanion equivalents. Never describe them as electrophiles or carbocations, as their reactive carbon centre is electron-rich, not electron-deficient.
Reactivity with Water (Protic Solvents)
✅ Correct Answer & Marking Points
- Organic product: Ethane ( CH₃CH₃ or C₂H₆ ). (1 mark)
- Justification: The partially negative carbon in the Grignard reagent is attracted to and reacts with the partially positive hydrogen in water. (1 mark)
❌ Common Errors
Students often lose the second mark by failing to explicitly link the polarity of the species (δ⁻ carbon reacting with δ⁺ hydrogen). Simply stating "water reacts with it" is far too vague for A-Level credit.
Formation of Alcohols from Carbonyl Compounds
✅ Correct Answer
D ( CH₃COCH₃ - propanone)
Reacting a Grignard reagent with a ketone (like propanone) followed by acid hydrolysis yields a tertiary alcohol.
💡 Key Knowledge Breakdown
- A (CO₂): Forms a carboxylic acid.
- B (HCHO - methanal): Forms a primary alcohol.
- C (CH₃CHO - ethanal): Forms a secondary alcohol.
- D (CH₃COCH₃ - propanone): Forms a tertiary alcohol.
Topics
Organic Chemistry · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.