AQA A-Level Chemistry Paper 2, 2021: Question 8

13 marks · Medium difficulty · State/Explain/Describe

Functional Group Identification and Structural Deduction This question focuses on the use of chemical tests to identify functional groups such as aldehydes, ketones, and carboxylic acids. Students are required to interpret qualitative results from reagents like acidified dichromate, Tollens’ reagent, and sodium carbonate, and use this evidence to deduce the structure of an unknown organic compound.

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Question text

08 This question is about making a diester from cyclohexanol.

08.1 State the type of reaction in step 1.

Give the name of the reagent needed for step 1.

[2 marks]

Type of reaction

Reagent

08.2 State the reagents needed and give equations for step 2 and step 3.

Show the structure of Compound G in your equations.

[4 marks]

Step 2 reagent

Step 2 equation

Step 3 reagent

Step 3 equation

08.3 Cyclohexane-1,2-diol reacts with ethanedioyl dichloride.

Give the name of the mechanism for this reaction.

Complete the mechanism to show the formation of one ester link in the first step of

this reaction.

[5 marks]

Mechanism name

Mechanism

08.4 Suggest why chemists usually aim to design production methods

• with fewer steps

• with a high percentage atom economy.

[2 marks]

Fewer steps

High percentage atom economy

Mark scheme

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Question Answers Additional Comments/Guidelines Mark

Dehydration Allow (acid catalysed) Elimination M1

08.1

Conc H2SO4 Allow Conc H3PO4 M2

Br2 Allow bromine (water) M1

Allow Cl2 or I2

Allow O2 if epoxide route used

allow conseq equation to H2, H2O, HBr, HCl. HI and M2

H2SO4

An epoxide is a feasible alternative that could score

08.2 here and consequentially M3 and M4

NaOH

Or KOH or other suitable strong alkali M3

Allow this equation with molecular formulae M4

– A-LEVEL CHEMISTRY – –

M1 (nucleophilic)addition-elimination Note lone pair required for M5 1

08.3 23

– A-LEVEL CHEMISTRY – –

Less energy used OR Better yield OR reduces practical losses, simpler plant, M1

08.4 Less waste OR Less pollution OR maximises the use of raw materials in the M2

process into useful products, saves resources

How to answer it

Study Guide: Diester Formation from Cyclohexanol

Q1. Step 1 – Dehydration of Cyclohexanol

Type of reaction: Dehydration (Elimination)

Reagent: Concentrated H₂SO₄ or H₃PO₄

This forms cyclohexene by removing water.

Look for alcohol to alkene transformations — dehydration is classic with acid catalysts.

Q2. Step 2 and 3 – Addition and Hydrolysis

Step 2 reagent: Br₂ (or Br₂/H₂O)

Equation: cyclohexene + Br₂ → 1,2-dibromocyclohexane

Step 3 reagent: NaOH (aqueous, strong alkali)

Equation: 1,2-dibromocyclohexane + 2NaOH → cyclohexane-1,2-diol + 2NaBr

Over 50% of students missed either correct reagents or equations here — link Br₂ to electrophilic addition and NaOH to nucleophilic substitution.

Q3. Mechanism of Ester Formation with Ethanedioyl Dichloride

Mechanism name: Nucleophilic addition–elimination

Steps:

Lone pair on O from –OH attacks carbon in C=O (nucleophilic attack)
Curly arrow from C=O bond to O
Intermediate forms: add H transfer and Cl⁻ elimination to form ester link

Draw 3 curly arrows – one from lone pair, one from C=O bond, one to eliminate Cl⁻. Include lone pairs clearly.

Q4. Green Chemistry – Fewer Steps & High Atom Economy

Fewer steps: Less energy used, better yield, simpler process
High atom economy: Less waste, better use of materials, more sustainable

Maximising atom economy and minimising waste saves cost and reduces environmental harm.

Topics

Organic Chemistry · 3.3.14 Organic Synthesis · 3.3.9 Carboxylic Acids and Derivatives · 3.3.5 Alcohols · 3.3.4 Alkenes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.