AQA A-Level Chemistry Paper 2, 2023: Question 3

12 marks · Medium difficulty · State/Explain/Describe

Reaction Mechanisms This question covers intermediates in mechanisms, including nucleophilic addition–elimination, electrophilic substitution, and nucleophilic addition, requiring completion of diagrams with intermediates and curly arrows.

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Question

AQA A-Level Chemistry Paper 2, 2023: Question 3
Question text

03 This question is about intermediates in reaction mechanisms.

03.1 Figure 3 shows an intermediate formed in the first step of a

nucleophilic addition–elimination mechanism.

Figure 3

Complete Figure 3 to show the structures of the two reactant species with curly

arrows and relevant lone pairs of electrons involved in the formation of the

intermediate.

Draw curly arrows and relevant lone pairs of electrons on the intermediate to show

how the final products are formed.

[4 marks]

03.2 Figure 4 shows an intermediate formed in the first step of a reaction mechanism of

methylbenzene.

Figure 4

Complete Figure 4 to show the reactant species and any curly arrows involved in the

formation of the intermediate.

*06* Draw a curly arrow on the intermediate to show how the product is formed.

Give the name of the reaction mechanism.

[4 marks]

Name of mechanism

03.3 Figure 5 shows the reactant species involved in the first step of a mechanism.

Figure 5

Complete Figure 5 to show the structure of the intermediate formed with curly arrows

involved in its formation.

Give the name of the reaction mechanism.

[4 marks]

Name of mechanism

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2023: Question 3

Question Answers Additional Comments/Guidelines Mark

M1 Structures of reactant species

M2 Arrow from lp on O to C

M3 Arrow from C=O bond to O

03.1 M4 3 curly arrows and lp on intermediate

(4 x AO1)

Allow full marks for candidates who draw a second

intermediate formed after formation of C=O and

loss of Cl– then loss of H+

Ignore any attempt to show the final products

M1 Structures of reactant species including + on N

of +NO

M2 Arrow from ring (inside hexagon) to N or + on

N

03.2 M3 Arrow from C-H bond into hexagon

(4 x AO1)

M4 Electrophilic substitution

Apply list principle

M1 Arrow from lp on hydride to C

O O

H M2 Arrow from C=O to O

M3 Intermediate structure / Allow displayed or

abbreviated structures 4

03.3

Ignore any attempt to show further steps if correct (4 x AO1)

Penalise further incorrect steps

M4 Nucleophilic addition

Apply list principle

How to answer it

Reaction Mechanisms and Intermediates

Question

(a) Complete the diagram for the nucleophilic addition-elimination mechanism in Figure 1. Add curly arrows and lone pairs to show reactants and products.

(b) Complete the diagram for the electrophilic substitution mechanism in Figure 2. Add curly arrows and lone pairs, and name the mechanism.

(c) Complete the diagram for the nucleophilic addition mechanism in Figure 3. Add curly arrows and lone pairs, and name the mechanism.

Answer: Part (a)

Mechanism: Nucleophilic addition-elimination.

Steps:

  1. Draw the structure of the reactant species: CH3CH2COCl and H3C-OH.
  2. Use curly arrows:
    • Arrow from the lone pair on the oxygen of H3C-OH to the carbonyl carbon of CH3CH2COCl.
    • Arrow from the carbonyl C=O bond to the oxygen atom.
  3. Add lone pairs on the oxygen of the intermediate.
Misconception: Students often forget to include lone pairs or curly arrows. These are critical for showing the electron flow in the mechanism.

Answer: Part (b)

Mechanism: Electrophilic substitution.

Steps:

  1. Draw the structure of the reactant species: CH3 on a benzene ring and NO2+.
  2. Use curly arrows:
    • Arrow from the benzene ring to the nitrogen of NO2+.
    • Arrow from a C-H bond in the intermediate into the benzene ring.
  3. Add lone pairs and charges where appropriate.
Tip: In electrophilic substitution, the benzene ring is a key electron donor. Ensure arrows start inside the ring and point towards the electrophile.

Answer: Part (c)

Mechanism: Nucleophilic addition.

Steps:

  1. Draw the structure of the reactant species: CH3CHO and H-.
  2. Use curly arrows:
    • Arrow from the lone pair on H- to the carbonyl carbon.
    • Arrow from the C=O bond to the oxygen atom.
  3. Draw the intermediate with lone pairs on oxygen and negative charge.
Misconception: Students may forget the direction of curly arrows, especially from nucleophile to electrophile. Practice drawing mechanisms systematically.

Final Notes

Key points to remember:

  • Always include lone pairs and charges where applicable.
  • Curly arrows must accurately represent the movement of electrons.
  • Practice recognising common mechanisms like nucleophilic addition, electrophilic substitution, and addition-elimination.

Topics

Organic Chemistry · 3.3.10 Aromatic Chemistry

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.