AQA A-Level Chemistry Paper 2, 2022: Question 3

11 marks · Medium difficulty · State/Explain/Describe

Reactions and Polymerisation of 2-Methylbut-1-ene This question examines the reactions and polymerisation of 2-methylbut-1-ene. It involves naming and outlining the electrophilic addition mechanism with concentrated sulphuric acid, drawing and explaining the formation of the minor product, representing the skeletal formula of a functional group isomer, and identifying the type of polymerisation along with drawing the repeating unit of the polymer formed.

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Question

Question text

03 This question is about 2-methylbut-1-ene.

03.1 Name the mechanism for the reaction of 2-methylbut-1-ene with

concentrated sulfuric acid.

Outline the mechanism for this reaction to form the major product.

[5 marks]

Name of mechanism

Outline of mechanism to form major product

03.2 Draw the structure of the minor product formed in the reaction in Question 03.1

Explain why this is the minor product.

[3 marks]

Structure of minor product

Explanation

03.3 Draw the skeletal formula of a functional group isomer of 2-methylbut-1-ene.

[1 mark]

03.4 2-methylbut-1-ene can form a polymer.

State the type of polymerisation.

Draw the repeating unit for the polymer formed.

[2 marks]

Type of polymerisation

Repeating unit

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

Electrophilic addition M1

NB Allow fully displayed or other structural

formulae M2

if H2O used as electrophile – max 4 ONLY M3

(1 x AO1,

4 x AO2)

M2: must show an arrow from = of C=C towards the H atom of the

03.1 H−O bond or HO that is part of H−O−S−… on a compound with

molecular formula H2SO4

M2 could have arrow to H+ in which case M3 would be for an

independent H−O bond break on a compound with formula H2SO4

M3: must use an arrow to show the breaking of the H−O bond M3 ignore partial charges unless wrong

M4: is for the correct carbocation structure NOT M4 if primary carbocation shown.

M5: must show an arrow from a lone pair of electrons on the correct

M5 NOT HSO4

oxygen of the negatively charged ion towards the positively charged

credit as shown or as :OSO3H – in which case

carbon atom

negative charge can be shown anywhere

ECF from H2SO3 in M2

NB: The arrows are double-headed

IGNORE subsequent use of water to hydrolyse

hydrogensulfate

If tertiary shown here allow as ECF for M1 if M1

primary shown in 03.1

03.2

(major) product formed via more stable carbocation OR tertiary Must be clear refers to intermediate and not M2

carbocation more stable (than primary) product

Due to electron-releasing character / (positive) inductive effect of Primary has one e– donating alkyl group (3 x AO2)

three alkyl groups (as opposed to one)

Skeletal formula of cycloalkane 1

(AO1)

03.3

I iiiiiif

ignore structure of 2-methylbut-1-ene

Addition (polymerisation) Not additional M1

Penalise incorrect attachment of ethyl group

03.4 Must have trailing bonds (1 x AO1,

1 x AO2)

Ignore n and brackets

Ignore structure of 2-methylbut-1-ene

How to answer it

Learning Guide for 2-Methylbut-1-ene

Part (a): Mechanism for Reaction with Sulfuric Acid

What to do: Identify the mechanism as Electrophilic Addition and correctly draw the step-by-step process involving the formation of a carbocation intermediate and attack by the hydrogensulfate ion.

Common errors:

Incorrect structure of the tertiary carbocation intermediate.
Failure to represent the hydrogensulfate ion correctly or omitting its attack on the carbocation.

Why might this be wrong? Students often confuse the stability of carbocations and fail to recognise that the tertiary carbocation is the most stable due to the electron-donating alkyl groups.

Teacher Tip: Focus on stability trends of carbocations and practice curly arrow conventions to reinforce understanding.

Part (b): Minor Product Formation

What to do: Draw the correct structure of the minor product and explain why it forms less frequently by referring to the stability of the intermediate carbocation.

Common errors:

Failure to mention the number of electron-donating alkyl groups contributing to the stability of the tertiary carbocation.
Confusion between the intermediate and the final product.

Why might this be wrong? Many students struggle to link the intermediate's stability to the product's formation frequency, often assuming all products are equally likely.

Teacher Tip: Emphasise the importance of the carbocation stability hierarchy in determining major and minor products.

Part (c): Functional Group Isomer

What to do: Draw a skeletal formula of a cycloalkane as the functional group isomer of 2-methylbut-1-ene.

Common errors:

Failing to draw a skeletal structure despite the question's requirement.
Confusion between functional group isomers and structural isomers.

Why might this be wrong? Students often default to structural representations or misidentify cycloalkanes as chain isomers.

Teacher Tip: Practice converting between different types of isomers and reinforce the concept of skeletal structures.

Part (d): Polymerisation

What to do: Identify the type of polymerisation as Addition Polymerisation and draw the repeating unit with trailing bonds.

Common errors:

Labeling the polymerisation type incorrectly, e.g., as "condensation polymerisation."
Incorrect representation of the repeating unit, such as missing trailing bonds.

Why might this be wrong? Students often confuse addition and condensation polymerisation, failing to link the process to the monomer's double bond.

Teacher Tip: Reinforce the visual link between the double bond in the monomer and its breaking during polymerisation.

Final Notes

This question set required students to combine knowledge of mechanisms, carbocation stability, and isomerism. Focus on understanding the underlying principles, practicing diagrammatic representations, and refining explanations to maximise marks.

Topics

Organic Chemistry · 3.3.4 Alkenes

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.