AQA A-Level Chemistry Paper 2, 2022: Question 7

6 marks · Medium difficulty · State/Explain/Describe

Ester Hydrolysis Mechanism This question focuses on the mechanism of ester hydrolysis using aqueous sodium hydroxide. You are asked to complete the curly arrow mechanism for the reaction, identify the type of reaction as nucleophilic addition-elimination, explain the role of CH₃O⁻ as a leaving group in step 3, and describe the use of soap as a product of the hydrolysis of vegetable oils (triesters).

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Question

AQA A-Level Chemistry Paper 2, 2022: Question 7
Question text

07 This question is about esters.

Figure 4 shows an incomplete mechanism for the reaction of an ester with

aqueous sodium hydroxide.

Figure 4

07.1 Add three curly arrows to complete the mechanism in Figure 4.

[3 marks]

07.2 Name the type of reaction shown in Figure 4.

[1 mark]

07.3 Deduce the role of the CH O− ion in step 3 shown in Figure 4.

[1 mark]

07.4 A triester in vegetable oil reacts with sodium hydroxide in a similar way.

Give a use for a product of this reaction.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2022: Question 7

Question Answers Additional Comments/Guidelines Mark

M1: Arrow from C=O bond to O

(3 x AO3)

M2 Arrow from correct C-O bond to O

M3 Arrow from O-H bond to O

07.1

(Alkaline/base) hydrolysis 1

07.2

(AO1)

Base Allow proton acceptor 1

07.3 Ignore ref to Bronsted Lowry

(AO1)

Soap only 1

07.4

(AO1)

How to answer it

Study Guide: Ester Hydrolysis and Common Errors

Question Breakdown

This guide covers the question on the mechanism of ester hydrolysis, focusing on the common errors students make and explaining why these errors occur.

Part (a): Completing the Mechanism

Marking Points:

  • M1: Curly arrow from the carbonyl C=O bond to the oxygen atom.
  • M2: Curly arrow showing the breaking of the C-O bond to form the intermediate.
  • M3: Curly arrow from O-H bond to the oxygen atom, leading to methanol elimination.

Common Errors:

  • Failing to draw the correct first curly arrow for the nucleophilic attack, missing the mechanism's initiation.
  • Incorrectly showing bond breaking in step 2, which often leads to an invalid intermediate structure.
  • Omitting the curly arrow for methanol elimination in step 3.

Explanation of Errors:

Students may struggle with identifying the exact nucleophile and electrophile roles. Additionally, an incomplete understanding of bond breaking and forming processes during intermediate stages can result in incorrect arrows.

Part (b): Reaction Type

Correct Answer: Alkaline Hydrolysis (Base Hydrolysis)

Common Errors:

  • Confusing the reaction type with condensation or esterification.

Explanation of Errors:

Some students may not recall the specific term "hydrolysis," despite the context and reaction conditions pointing to it. They may mix it up with other organic reactions involving esters.

Part (c): Role of CH3O-

Correct Answer: Acts as a base (proton acceptor).

Common Errors:

  • Describing the methoxide ion as a nucleophile instead of a base.

Explanation of Errors:

Students may focus on earlier steps of the mechanism where methoxide could act as a nucleophile and fail to consider its role in step 3 specifically.

Part (d): Hydrolysis of Vegetable Oils

Correct Answer: Soap

Common Errors:

  • Stating biodiesel or another unrelated product instead of soap.

Explanation of Errors:

While both biodiesel and soap involve fatty acid derivatives, this reaction's specific conditions (sodium hydroxide hydrolysis) favour soap formation, a fact often overlooked.

Summary Table

Part Correct Answer Common Errors Explanation
(a) Curly arrows showing nucleophilic attack, intermediate formation, and methanol elimination. Incorrect or missing arrows in the mechanism. Confusion over bond breaking/forming and the nucleophilic attack.
(b) Alkaline Hydrolysis Confusing with condensation or esterification. Lack of familiarity with specific reaction terminology.
(c) Methoxide acts as a base (proton acceptor). Described as a nucleophile. Misinterpretation of methoxide's role in step 3.
(d) Soap Stating biodiesel or other unrelated products. Confusion between different fatty acid reactions.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives

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.