OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 21

6 marks · Medium difficulty · Extended Response

Describe the formation of carbon–carbon bonds in aliphatic compounds by two different mechanisms, including reaction mechanisms for each.

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Question

Question text

Carbon-carbon bond formation is used in synthesis to increase the length of a carbon chain.

Describe the formation of carbon–carbon bonds in aliphatic compounds by two different

mechanisms.

Your answer should include mechanisms for each aliphatic compound. [6]

Additional answer space if required.

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

21 Refer to marking instructions on page 5 of mark scheme for 6 AO1.2 Indicative scientific points may include:

guidance on marking this question. ×4

Reactions of aliphatic compounds and mechanisms

Level 3 (5–6 marks)

Describes, in detail, reactions of two aliphatic compounds that AO2.5 – –

• Haloalkane, RX and CN → RCN + X

form a C–C bond ×2 Reagents: NaCN and ethanol

AND mechanisms for the two aliphatic reactions. Reaction: Nucleophilic substitution

Mechanism:

There is a well-developed line of reasoning which is clear and

logically structured. The information presented is relevant and

substantiated.

Level 2 (3–4 marks)

Describes a reaction of one aliphatic compound that forms a C–C

bond with few omissions/errors.

AND mechanism for one aliphatic reaction.

Describes reactions of two compounds that forms a C–C bond • Aldehyde or ketone and HCN

AND attempts a mechanism for one of the reactions e.g. RCHO + HCN → RCH(OH)CN

Reagents: NaCN and H+

There is a line of reasoning presented with some structure. The Reaction: Nucleophilic addition

information presented is relevant and supported by some Mechanism:

evidence.

Level 1 (1–2 marks)

Selects suitable reagents for reactions of two compounds that

form a C–C bond.

Attempts to describe a reaction and mechanism of one compound

that forms a C–C bond, with omissions/errors.

There is an attempt at a logical structure with a line of reasoning.

The information is in the most part relevant. OR H O instead of H+ for 2nd stage

0 marks No response or no response worthy of credit. If alternative reactions are shown contact your TL

e.g. radical substitution, polymerisation

Total 6

How to answer it

C-C Bond Formation Mechanisms in Aliphatic Compounds

📌 What this question tests

This 6-mark extended response question assesses your knowledge of synthetic chemistry pathways used to increase carbon chain length. Specifically, it tests your ability to name reagents, state reaction types, and draw complete curly arrow mechanisms for two distinct aliphatic reactions: haloalkanes reacting with cyanide ions, and carbonyl compounds (aldehydes/ketones) reacting with hydrogen cyanide.

Question Breakdown & Mark Scheme Overview

Total Marks: 6 (Level of Response Question)

💡 Level 3 Criteria (5–6 Marks)

Describes in detail reactions of two aliphatic compounds forming a C–C bond.
Includes accurate curly arrow mechanisms for both aliphatic reactions.
Well-developed, clear, and logically structured line of reasoning.

🧠 Exam Technique

This is a Level of Response question. Examiners look for comprehensive coverage rather than just isolated bullet points.
Clearly separate your two examples with headings to make the structure obvious.
Include reaction conditions/reagents alongside every mechanism.

Part 1: Nucleophilic Substitution (Haloalkane + Cyanide)

✅ Correct Answer & Reagents

Reagents: Potassium cyanide ( KCN ) or sodium cyanide ( NaCN ) in ethanol (heated under reflux).

Reaction Type: Nucleophilic substitution.

Equation: RCH₂Br + CN⁻ → RCH₂CN + Br⁻ (or using general formula RX ).

💡 Mechanism Description

The cyanide ion acts as a nucleophile, donating an electron pair to the carbon bonded to the halogen.

Partial charges: Cᵰ⁺–Brᵰ⁻ due to electronegativity difference.
Curly arrow from lone pair on carbon of :CN⁻ to the δ⁺ carbon.
Curly arrow breaking the C–Br bond, moving both electrons to the bromine atom to form Br⁻ .

❌ Common Errors

Omitting partial charges ( δ⁺ and δ⁻ ) on the polar bond.
Drawing the curly arrow starting from the minus sign instead of the lone pair on the carbon of the CN⁻ ion.
Forgetting solvent details (ethanol/water mixture is required).

Part 2: Nucleophilic Addition (Aldehyde/Ketone + Hydrogen Cyanide)

✅ Correct Answer & Reagents

Reagents: Acidified potassium cyanide ( KCN + dilute H⁺ / H₂SO₄ ) or HCN generated in situ.

Reaction Type: Nucleophilic addition (forming a hydroxynitrile / cyanohydrin).

Equation: RCHO + HCN → RCH(OH)CN

💡 Mechanism Description

Involves a planar carbonyl carbon attacked by a nucleophile, followed by protonation of the intermediate.

Partial charges: Cᵰ⁺=Oᵰ⁻ .
First step: Curly arrow from lone pair on :CN⁻ to the δ⁺ carbonyl carbon.
Simultaneous curly arrow breaking the C=O π -bond, moving the electron pair onto oxygen to form a negatively charged intermediate ( alkoxide ion ).
Second step: Curly arrow from the lone pair on the oxygen to a hydrogen ion ( H⁺ or H₂O ) to form the –OH group.

❌ Common Errors

Failing to show the intermediate species with its negative charge on the oxygen atom.
Incorrect arrow direction during protonation (arrow must start from the electron pair on oxygen towards H⁺ , not the other way around).
Using HCN directly without mentioning safety/in situ generation via KCN + acid.

Examiner Insight: To achieve Level 3 (5–6 marks), top responses feature immaculately drawn curly arrows starting precisely from lone pairs/bonds, clearly articulated reagents, and correct identification of both reaction types alongside clean structural representations of R groups.

Topics

Module 6: Organic chemistry and analysis · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.