OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 21

8 marks · Medium difficulty · Structured Questions

Name the alkene (CH3)3CCH=CH2, outline its reaction mechanism with bromine including curly arrows and the intermediate, draw a repeat unit of its polymer, and state an advantage and disadvantage of polymer combustion.

Practise this question

Question

Question text

21 The alkene, (CH3)3CCH=CH2, is used to make some perfumes.

(a) (i) What is the systematic name for (CH3)3CCH=CH2?

… [1]

(ii) (CH3)3CCH=CH2 decolourises bromine.

Outline the reaction mechanism for the reaction of (CH3)3CCH=CH2 and bromine.

The structure of (CH3)3CCH=CH2 has been provided.

Include curly arrows and relevant dipoles, the structure of the product and the name of

the mechanism.

(CH3)3C H

C C

H H

name of mechanism … [5]

(b) The alkene (CH3)3CCH=CH2 can be polymerised to form a polymer.

(i) Draw one repeat unit for this polymer.

[1]

(ii) State one advantage and one disadvantage of using combustion as a method for the

disposal of a polymer after it has exceeded its useful life.

Advantage …

Disadvantage …

[1]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

21 (a) (i) 3,3-dimethylbut-1-ene 1 AO1.2 IGNORE lack of hyphens, or addition of commas

×1 or spaces

CARE: Look for dimethyl

ALLOW full stops or spaces between numbers

e.g. 3.3 dimethyl but-1-ene

DO NOT ALLOW meth OR methy

(ii) ANNOTATE ANSWER WITH TICKS AND CROSSES 5

For curly arrows, ALLOW straight or snake-like

arrows and small gaps (see examples):

1st curly arrow must

• go to a Br atom of Br–Br

AND

• start from, OR be traced back to any point

across width of C=C

1st curly arrow (from ANY alkene)

Curly arrow from double bond to Br of Br–Br AO1.2 2nd curly arrow must

DO NOT ALLOW partial charge on C=C • start from, OR be traced back to, any part of

δ+Br–Brδ– bond

• AND go to Br δ–

2nd curly arrow

Correct dipole on Br–Br

AND curly arrow for breaking of Br–Br bond AO1.2

11 AO

element

3rd curly arrow IGNORE connectivity of CH3 groups in carbocation

Correct carbocation with + charge on C with 3 and product and ALLOW C4H9

bonds

AND curly arrow from Br– to C+ of carbocation

3rd curly arrow must

DO NOT ALLOW δ+ on C of carbocation • go to the C+ of carbocation

AND

• start from, OR be traced back to any point

across width of lone pair on :Br–

• OR start from – charge on Br– ion

AO2.5

i.e. ALLOW carbonium + on either C atom (Lone pair NOT needed if curly arrow shown

from – charge on Br–)

Correct product to match mechanism/intermediate

ALLOW bromonium ion

AO2.5

ALLOW any combination of skeletal OR structural

OR displayed formula as long as unambiguous

NOTE: For a mechanism with HBr, ALLOW all

DO NOT ALLOW half headed or double headed arrows marks EXCEPT for final product mark

but allow ECF if seen more than once

Name of mechanism: Electrophilic addition

AO1.1

(b) (i) 1 For repeat unit,

12 AO

element

• ‘side bonds’ required on either side of

repeat unit from C atoms

• ALLOW more than one repeat unit

• ALLOW C4H9 for C(CH3)3

IGNORE brackets

AO2.5 • IGNORE n

Correct polymer with side links ×1

IGNORE connectivity of C(CH3)3 group

(b) (ii) Advantage: 1 AO1.1

Energy/electricity (produced) ×1 ALLOW reduced use of fossil fuels

IGNORE produced CO2 and H2O

AND

ALLOW less landfill / less harm to wildlife or

environment (not just harmful)

Disadvantage:

CO2 produced

OR gases causing global warming/climate change

OR greenhouse gases, e.g CO2 ALLOW toxic/poisonous (waste) products/gases,

e.g. CO

BOTH advantage and disadvantage

IGNORE harmful/dangerous

How to answer it

Alkenes, Electrophilic Addition and Polymers

What this question tests

This question assesses your core organic chemistry knowledge regarding alkenes (AS Level). Specifically, it evaluates IUPAC nomenclature, the step-by-step curly arrow mechanism for electrophilic addition with halogens (including dipoles, intermediate carbocations, and name of mechanism), addition polymerisation repeat units, and environmental considerations regarding polymer disposal via combustion.

Question 21 (a) (i)

Systematic Nomenclature

✅ Correct Answer

3,3-dimethylbut-1-ene

Marks: 1

💡 Key Knowledge

Find the longest carbon chain containing the double bond (4 carbons = butane/but-ene).
Number the chain from the end nearest the functional group (double bond gets position 1).
Identify substituent groups ( methyl groups at carbon 3).

❌ Common Errors

Using prefixes like methy instead of dimethyl .
Incorrect numbering giving the double bond a higher locant (e.g., but-3-ene).

Question 21 (a) (ii)

Electrophilic Addition Mechanism

✅ Correct Answer

Name of mechanism: Electrophilic addition

Mechanism details:

Curly arrow from the C=C double bond to the Br atom with the partial positive charge ( Br^δ+ ).
Induced dipole clearly shown on the Br₂ molecule ( Br^δ+–Br^δ– ).
Second curly arrow showing the heterolytic fission of the Br–Br bond to the Br^δ– atom.
Intermediate carbocation showing the positive charge on the correct carbon atom with 3 alkyl groups, and a third curly arrow from the lone pair on the bromide ion ( Br^– ) to the C⁺ carbocation.
Correct final product structure: 1,2-dibromo-3,3-dimethylbutane .

Marks: 5

🧠 Exam Technique

Curly arrow precision: The first arrow must start from the bond (C=C line), not floating in space, and point directly at the Br atom.
Dipole: Always label the attacking molecule with partial charges ( δ+ and δ– ).
Carbocation charge: The positive charge must be centered on the carbon atom ( C⁺ ).

❌ Common Errors

Starting the first curly arrow from a single carbon atom rather than from the double bond.
Omitting the Br–Br dipole label, resulting in the loss of the second mark point.
Placing the positive charge incorrectly on the wrong carbon atom of the intermediate.

Question 21 (b) (i)

Polymerisation Repeat Unit

✅ Correct Answer

A repeat unit showing the carbon-carbon single backbone with the side groups attached, flanked by extension bonds (side bonds) breaking the brackets or ending abruptly.

Marks: 1

💡 Key Knowledge

Addition polymers break the C=C double bond to form a repeating single-bonded carbon chain –C–C– .
Ensure extension bonds cross the outer square brackets or are clearly left open-ended.

❌ Common Errors

Leaving the double bond in the repeat unit structure.
Forgetting to include the extension bonds outside the bracket/repeat unit.

Question 21 (b) (ii)

Polymer Disposal via Combustion

✅ Correct Answer

Advantage: Energy/electricity is produced (from the heat of combustion).

Disadvantage: CO₂ is produced causing global warming/climate change (or toxic/greenhouse gases produced).

Marks: 1 (requires BOTH an advantage and a disadvantage)

🧠 Exam Technique

Read the question carefully: it asks for one advantage and one disadvantage. Both must be correct to secure the single mark allocated.

❌ Common Errors

Vague statements like "it is harmful" without specifying CO₂ generation or toxic gas release.
Only providing an advantage or a disadvantage when both are strictly required for the mark.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.