OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 25

15 marks · Medium difficulty · Structured Questions

Outline the reactions of propene including polymerisation, addition with HBr, and electrophilic addition of bromine, along with questions on alkynes including homologue series, equations, isomers, and IUPAC structures.

Practise this question

Question

A multi-part chemistry question about unsaturated hydrocarbons. Part (a) shows propene reacting in boxes to form a polymer repeat unit and two haloalkane isomers with HBr. Part (b) asks to outline the mechanism for the reaction of propene with bromine using curly arrows, dipoles, and products. Part (c) provides a table of alkynes (ethyne, propyne, but-1-yne) and asks questions about homologous series, general formula, an equation for propyne reacting with bromine, drawing two structural isomers of C4H6, and drawing 2,5-dimethylhept-3-yne.
Question text

25 This question is about unsaturated hydrocarbons.

(a) Two reactions of propene are shown below.

In the boxes, show the structures of the organic products of the reactions.

H CH3

polymerisation

C C

H H

one repeat unit

HBr

+

[3]

(b) Propene also reacts with bromine.

Outline the mechanism for the reaction of propene with bromine, Br2.

The structure of propene has been provided.

Show curly arrows, relevant dipoles and product(s).

H CH3

C C

H H

[4]

(c) The ‘alkynes’ is a homologous series of hydrocarbons.

The table shows three alkynes.

Alkyne Structural formula Molecular formula

ethyne HC≡CH C2H2

propyne CH3C≡CH C3H4

but-1-yne CH3CH2C≡CH C4H6

(i) Explain what is meant by the term: homologous series.

… [2]

(ii) Suggest the general formula of the alkynes.

… [1]

(iii) Propyne reacts with bromine to form a saturated compound.

Write an equation for the reaction, showing the structure of the organic product.

[2]

(iv) But-1-yne is a structural isomer of C4H6.

Draw the structures of 2 other structural isomers of C4H6.

[2]

(v) Draw the structure of 2,5-dimethylhept-3-yne.

[1]

Mark scheme

Show the mark scheme The mark scheme provides the expected answers, including structural formulas for the addition and polymerisation products of propene, the step-by-step electrophilic addition mechanism with curly arrows and dipoles, definitions for homologous series, general formulas, equations, and structural isomer drawings.

AO

Question Answer Marks Guidance

element

25 (a) 3 2.5×3 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

For repeat unit,

• ‘side bonds’ required on either side of

repeat unit from C atoms

• DO NOT ALLOW > one repeat unit

IGNORE brackets

• IGNORE n

ALLOW in either order

(b) 4 ANNOTATE ANSWER

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 f rom, OR be traced back to any point

across width of C=C

1st curly arrow

Curly arrow from double bond to Br of Br–Br 1.2

2nd curly arrow must

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

of δ+Br–Brδ– bond

2nd curly arrow • AND go to Br δ–

Correct dipole on Br–Br

AND curly arrow for breaking of Br–Br bond 1.2

13 AO

element

3rd curly arrow

Correct carbocation with + charge on C with 3

bonds X

AND curly arrow from Br– to C+ of carbocation

3rd curly arrow must

• go to the C+ of carbocation

DO NOT ALLOW δ+ on C of carbocation

AND

• start f rom, OR be traced back to any point

across width of lone pair on :Br–

• OR start f rom – charge on Br– ion

OR 2.5

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 2.5

ALLOW bromonium ion

ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

DO NOT ALLOW half headed or double headed arrows NOTE: For a mechanism with HBr,

but allow ECF if seen more than once ALLOW all marks EXCEPT for final product

(c) (i) (series of organic compounds with the) same functional 2 1.1×2 IGNORE reference to physical properties

group IGNORE same general formula

OR same/similar reactions / chemical properties DO NOT ALLOW same empirical OR molecular

formula

each successive member differs by CH2 Differs by CH2is not sufficient (no successive)

ALLOW differs by CH2 each time AW

AO

Question Answer 14 Marks Guidance

element

(c) (ii) CnH2n–2 1 3.2 ALLOW CnH2(n–1)

(c) (iii) 2 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

2.5 ALLOW C3H4 for H3CC≡CH

Left-hand side, i.e. Reactants, balanced with 2Br2 Questions asks only for structure of product

Right-hand side, i.e. Product

ALLOW H3CCBr2CHBr2 OR H3CCBr2CBr2H

2.6

(c) (iv) Any 2 structures from: 2 3.2×2 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

(c) (v) 1 2.5 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

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How to answer it

Unsaturated Hydrocarbons Study Guide

What this question tests

This question evaluates your knowledge of alkene and alkyne chemistry, addition reactions, electrophilic addition mechanisms (including curly arrows and carbocation intermediates), addition polymerisation, definitions of homologous series, general formulae, structural isomerism, and organic nomenclature.

Part (a): Reactions of Propene

Addition Polymerisation and Hydrohalogenation [3 Marks]

✅ Correct Answers

  • Polymerisation product: Poly(propene) repeat unit showing a carbon-carbon single backbone -[CH(CH₃)-CH₂]- with open bonds extending through the brackets.
  • HBr products: Two possible haloalkanes formed as major and minor products: 2-bromopropane ( CH₃-CH(Br)-CH₃ ) and 1-bromopropane ( CH₃-CH₂-CH₂Br ).

💡 Key Knowledge

  • Addition polymerisation breaks the C=C double bond to form a long saturated carbon chain.
  • Reaction with HBr is an electrophilic addition. The major product forms via the more stable secondary carbocation intermediate.

❌ Common Errors

  • Drawing repeat units with double bonds still present in the backbone.
  • Omitting the essential side bonds extending outside the square brackets of the repeat unit.
  • Including a subscript n incorrectly inside the single repeat unit box when asked for "one repeat unit".
Marks available: 3 marks (1 for polymer repeat unit, 2 for the two HBr reaction products).

Part (b): Mechanism of Electrophilic Addition

Propene with Bromine ( Br₂ ) [4 Marks]

✅ Correct Answers

  • 1st curly arrow: Starts from the C=C double bond and points precisely to the δ+ Br atom.
  • Dipole & Bond breaking: Correct δ+ Br - δ- Br dipole shown, with a second curly arrow showing the Br-Br bond breaking, going to the δ- Br atom.
  • Intermediate: A carbocation intermediate with a positive charge on the correct carbon atom, plus a third curly arrow from the lone pair of the bromide ion ( Br⁻ ) to the positive carbon.
  • Final product: 1,2-dibromopropane .

🧠 Exam Technique

Carefully anchor your curly arrows: the tail must originate from the electron source (either the bond line or a lone pair/charge) and the head must point directly to the atom receiving the electron pair. Examiners heavily penalise floating arrows.

❌ Common Errors

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  • Placing the partial positive ( δ+ ) charge on a carbon atom instead of the bromine molecule.
  • Drawing the first curly arrow pointing to the space between the bromine atoms rather than the δ+ bromine.
  • Forgetting to show the curly arrow coming from the bromide ion to the carbocation.
Marks available: 4 marks (allocated across curly arrows, dipoles, intermediate, and final structure).

Part (c): Homologous Series and Alkynes

Definitions, General Formulae, Equations, and Isomerism

💡 (i) Homologous Series Definition [2 Marks]

A family of organic compounds with the same functional group, where each successive member differs by a -CH₂- group (and share similar chemical properties).

✅ (ii) General Formula [1 Mark]

General formula for alkynes: CnH₂n₋₂

📐 (iii) Equation for Propyne with Bromine [2 Marks]

Propyne reacts with excess bromine ( 2Br₂ ) to form a saturated tetrabromocompound or dibromocompound depending on stoichiometry. For 1 mole of propyne reacting with excess bromine:

CH₃C≡CH + 2Br₂ → CH₃C(Br)₂(CHBr₂) or structural equivalent showing saturation.

✅ (iv) Structural Isomers of C₄H₆ [2 Marks]

Any 2 valid structural isomers of but-1-yne, such as:

  • CH₃-C≡C-CH₃ (but-2-yne)
  • CH₂=C=CH-CH₃ (buta-1,2-diene)
  • CH₂=CH-CH=CH₂ (buta-1,3-diene)

✅ (v) Organic Nomenclature [1 Mark]

2,5-dimethylhept-3-yne: A 7-carbon main chain with a triple bond starting at carbon 3, and methyl branches at carbons 2 and 5.

CH₃-CH(CH₃)-C≡C-CH(CH₃)-CH₂-CH₃

Total marks for (c): 8 marks across sub-parts (i) to (v).

Topics

Module 4: Core organic chemistry · Module 2: Foundations in chemistry · 4.1 Basic concepts and hydrocarbons · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.