AQA A-Level Chemistry Paper 2, 2022: Question 8

7 marks · Medium difficulty · State/Explain/Describe

Electrophilic Substitution with Benzene This question involves the electrophilic substitution reaction of benzene with methanoyl chloride (HCOCl) in the presence of a catalyst. It asks you to write the reaction equation, name the organic product (benzaldehyde), identify the catalyst (anhydrous AlCl₃) used to generate the electrophile ([HCO]⁺), and outline the mechanism showing how benzene reacts with this electrophile to form the final product.

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Question

Question text

08 Benzene reacts with methanoyl chloride (HCOCl) in the presence of a catalyst.

08.1 Give an equation for the overall reaction when benzene reacts with

methanoyl chloride.

Name the organic product.

[2 marks]

Equation

Name

08.2 Identify the catalyst needed in this reaction.

Give an equation to show how the catalyst is used to form the electrophile, [HCO]+

[2 marks]

Catalyst

Equation

08.3 Outline the mechanism for the reaction of benzene with the electrophile, [HCO]+

[3 marks]

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

C6H6 + HCOCl → C6H5CHO + HCl 1

Or shown as structural formulae

Allow phenyl methanal 1

08.1 Benzaldehyde Allow Benzenealdehyde or Benzene carbaldehyde (2 x AO2)

If ethanoyl chloride used allow ecf for name :

phenyl ethanone

AlCl3 Allow Aluminium chloride 1

Allow Iron (III) chloride / bromide or formulae

08.2 HCOCl + AlCl → [HCO]+ + [AlCl ]- Allow + on C or O in equation 1

Can score M1 in equation (1 x AO1,

1 x AO2)

H M1 Arrow from inside hexagon to C or + on C M1

C+ C

M2 Structure of intermediate M2

O H • horseshoe centred on C1 and must not extend

beyond C2 and C6, but can be smaller

• + in intermediate not too close to C1 (allow on or

08.3 “below” a line from C2 to C6)

M3 Arrow from bond into hexagon (Unless Kekule) M3

• Can allow M3 arrow independent of wrong M2 (3 x AO2)

structure

• + on H in intermediate loses M2 not M3

28 – –

• Ignore Cl and AlCl4 used in M3

How to answer it

Learning Guide for Electrophilic Substitution of Benzene

Part (a)

What to do: Write the equation for the reaction of benzene with methanoyl chloride and name the organic product.

C₆H₆ + HCOCl → C₆H₅CHO + HCl

Organic product: Benzaldehyde

Common errors:

Writing the wrong product, e.g., phenylmethanal.
Forgetting to balance the equation.

Misconception: Confusing aldehyde and ketone naming conventions.

Part (b)

What to do: Identify the catalyst and write an equation to show the formation of the electrophile.

Catalyst: AlCl₃

HCOCl + AlCl₃ → [HCO]⁺ + [AlCl₄]^-

Common errors:

Using the wrong catalyst, e.g., FeCl₃.
Failing to show the correct charges in the equation.

Teacher Tip: Memorise the use of AlCl₃ as a catalyst in Friedel-Crafts reactions.

Part (c)

What to do: Outline the mechanism for the reaction of benzene with the electrophile.

Steps:

Benzene donates electrons from the π-system to the electrophile, forming a carbocation intermediate.
The intermediate is stabilised by delocalisation (horseshoe delocalisation).
The hydrogen atom is lost, restoring aromaticity.

Common errors:

Incorrectly drawing the intermediate, e.g., missing the horseshoe delocalisation.
Not showing the correct placement of positive charges.

Teacher Tip: Practise drawing mechanisms step by step to avoid errors in electron movement.

Final Notes

Electrophilic substitution is a core concept in aromatic chemistry. Focus on the steps of the mechanism, naming products correctly, and understanding the role of catalysts in generating electrophiles. Review common mistakes to avoid them in exams!

Topics

Organic Chemistry · 3.3.10 Aromatic Chemistry

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.