OCR A-Level Chemistry AS Depth in chemistry (02), June 2019: Question 5

16 marks · Hard difficulty · Extended Response

Analyze the reactions, preparation, purification, yield, and polymerisation of hex-1-ene including stoichiometric calculations and organic practical procedures.

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Question

Question text

5 This question is about hex-1-ene, CH3CH2CH2CH2CH=CH2.

(a) Hex-1-ene is reacted with H2, HCl and Br2 as shown in the flowchart below.

(i) Complete the flowchart to show the structures of the organic products of these reactions.

major product

Reaction 1 H2 HCl Reaction 2

hex-1-ene

Reaction 3 Br2

[3]

(ii) State the catalyst needed in reaction 1.

… [1]

(iii) What would you observe in reaction 3?

… [1]

(b) Hex-1-ene is a liquid with a boiling point of 63 °C and a density of 0.67 g cm–3.

Hex-1-ene can be prepared by refluxing hexan-1-ol (boiling point 157 °C) with an acid catalyst.

Hexan-1-ol is a liquid with a boiling point of 157 °C and a density of 0.82 g cm–3.

The equation is shown below.

+ H2O

After reflux, the resulting mixture contains unreacted hexan-1-ol, hex-1-ene and water.

The mixture is then purified.

The expected percentage yield of hex-1-ene from hexan-1-ol is 62.5%.

(i)* A student plans to prepare 4.20 g of hex-1-ene by this method.

Calculate the mass of hexan-1-ol that the student should use and explain how you could

obtain pure hex-1-ene from the mixture obtained after reflux. [6]

Additional answer space if required

(ii) Another student suggested that hex-1-ene could be prepared from hexan-2-ol by the

same method.

Would you expect the percentage yield of hex-1-ene to be greater than, less than or

about the same as when using hexan-1-ol?

Explain your answer.

… [2]

(i) Draw a section of poly(hex-1-ene) containing two repeat units.

[1]

(ii) Waste poly(hex-1-ene) can be disposed of usefully by recycling.

State two other methods of disposing of polymers that can be beneficial to the

environment.

1 …

2 …

[2]

Mark scheme

Show the mark scheme

Question Answer Marks AO Guidance

element

5 (a) (i) Product with H2 3 AO1.2 ALLOW any combination of skeletal OR structural

H H H H H H ×3 OR displayed formula as long as unambiguous

H C C C C C C H ALLOW part molecular formulae but not full

H H H H H H

Product with HCl

H H H H H H

H C C C C C C H

H H H H Cl H

Product with Br2

H H H H H H

H C C C C C C H

H H H H Br Br

(ii) Nickel/Ni 1 AO1.2 ALLOW Pt OR Pd OR Rh

(iii) (orange to) colourless 1 AO1.2 ALLOW ‘it decolourises / turns colourless’

OR IGNORE colour change

bromine is decolourised

19 element

5 (b) (i) Please refer to the marking instructions on page 4 of this 6 Indicative scientific points may include:

mark scheme for guidance on how to mark this question. AO2.8 Calculation from moles

×2 4.20

n(hex-1-ene) = = 0.0500 (mol)

Level 3 (5–6 marks) 84.0

Calculates the correct mass of hexan-1-ol. n(hexan-1-ol) needed = 0.0500 × = 0.0800 (mol)

AND 62.5

mass needed = 0.0800 × 102 = 8.16 g

Explains the purification steps, with most fine detail. 8.16 3

OR volume = 9.95 cm

0.82

There is a well-developed line of reasoning which is clear and CHECK for extent of errors by ECF.

logically structured. The information presented is relevant and AO3.3 Calculation from mass

substantiated. 100

×4 Theoretical mass hex-1-ene = 4.20 × = 6.72 g

62.5

6.72

Level 2 (3–4 marks) Theoretical n(hex-1-ene) = = 0.0800 (mol)

Attempts a calculation of the mass of hexan-1-ol which is 84

partly correct. Mass of hexan-1-ol = 102 × 0.0800 = 8.16 g

ALLOW small slip/rounding errors such as errors on Mr (e.g.

OR use of 83 instead of 84 for hex-1-ene Mr)

Outlines the purification steps, with some fine detail. --------------------------------------------------------------

Purification

There is a line of reasoning presented with some structure. The Use of a separating funnel to separate organic

information presented is relevant and supported by some and aqueous layers

evidence. Drying with an anhydrous salt

Distillation

Level 1 (1–2 marks)

Attempts the calculation but makes little progress. Fine detail

OR Collection of upper layer (less dense from

Briefly outlines the purification steps, which may be separating funnel)

incomplete. Example of drying agent, e.g. MgSO4, CaCl2

Collection of fraction distilling at 63ºC (boiling point

There is an attempt at a logical structure with a line of of hex-1-ene)

reasoning. The information is in the most part relevant. Incorrect purification method NOT creditworthy

0 marks Examples of partly correct calculations

No response or no response worthy of credit. Mass = 5.1 g from 0.0500 × 102 % yield omitted

62.5

Mass = 3.1875 g from 0.0500 × × 102 % yield inverted

H032/02 Mark Scheme 100 June 2019

Question Answer 20 Marks AO Guidance

element

5 (b) (ii) Yield of hex-1-ene is less 2 AO3.2

×2

A mixture of hex-1-ene and hex-2-ene forms ALLOW hex-2-ene also forms

1 AO2.5 ALLOW correct structural OR displayed OR

skeletal formula

Must show two repeat units

Polymer must have side links

NOTE: C4H9– is allowed for CH3CH2CH2CH2–

IGNORE brackets and use of ‘n’

ALLOW alternating side chains, i.e.

(ii) Combustion for energy production 2 AO1.1 For energy production,

×2 ALLOW generate electricity/heating

for production of plastics

ALLOW as an (organic) feedstock

OR other useful organic compounds

Total 16

How to answer it

Reactions, Purification, and Polymerisation of Hex-1-ene

What this question tests

This question assesses your understanding of alkene addition reactions, catalyst requirements, observation of functional group tests, multi-step organic synthesis calculations involving percentage yield and density, separation and purification techniques (separating funnels, distillation, drying agents), structural isomerism in elimination reactions, addition polymerisation, and environmental disposal methods for polymers.

Question 5 (a)

Alkene Addition Reactions & Observations

✅ Correct Answers (Flowchart Products)

Reaction 1 (with H₂): Hexane ( CH₃CH₂CH₂CH₂CH₂CH₃ )
Reaction 2 (with HCl): 2-chlorohexane ( CH₃CH₂CH₂CH₂CHClCH₃ ) — Major product
Reaction 3 (with Br₂): 1,2-dibromohexane ( CH₃CH₂CH₂CH₂CHBrCH₂Br )

💡 Key Knowledge

Reaction 1 is catalytic hydrogenation (requires Ni, Pt, Pd, or Rh catalyst).
Reaction 2 is electrophilic addition following Markovnikov's rule, yielding 2-chlorohexane as the major product.
Reaction 3 is electrophilic addition of a halogen, used as a test for unsaturation.

🧠 Exam Technique

Part (ii): State Nickel or Ni clearly. Pt, Pd, and Rh are also accepted.
Part (iii): For the bromine test, write orange to colourless or bromine is decolourised . Avoid incorrect terms like "clear".

❌ Common Errors

Drawing 1-chlorohexane as the major product in Reaction 2, ignoring Markovnikov's rule.
Writing "clear instead of "colourless" in the bromine water test.

Question 5 (i)* - 6 Mark Extended Response

Calculation of Mass & Organic Purification

📐 Step-by-Step Calculation

Find moles of hex-1-ene required:
M(hex-1-ene) = 84.0 g mol⁻¹
Moles = 4.20 g / 84.0 g mol⁻¹ = 0.0500 mol
Account for percentage yield (62.5%):
Theoretical moles of hex-1-ene needed = 0.0500 × (100 / 62.5) = 0.0800 mol
Calculate mass of hexan-1-ol starting material:
M(hexan-1-ol) = 102 g mol⁻¹
Mass = 0.0800 mol × 102 g mol⁻¹ = 8.16 g
(Alternatively using density: Volume = 8.16 g / 0.82 g cm⁻³ = 9.95 cm³)

🧠 Purification Protocol (Level 3 Detail)

Separation: Use a separating funnel to separate the organic layer from the aqueous layer. Collect the upper layer (hex-1-ene is less dense than water, density 0.67 g cm⁻³ vs water 1.0 g cm⁻³).
Drying: Add an anhydrous salt (e.g., MgSO₄ or CaCl₂ ) to remove trace water.
Distillation: Purify by simple distillation, collecting the fraction distilling at 63 °C (the boiling point of hex-1-ene).

❌ Common Calculation Traps

Inverting the percentage yield calculation (e.g., multiplying by 62.5/100 instead of 100/62.5).
Using incorrect molar masses or forgetting to factor in the stoichiometry and yield simultaneously.

Question 5 (b)(ii)

Reflux Yield Comparison

✅ Correct Answer

Yield is less and a mixture of hex-1-ene and hex-2-ene forms.

💡 Key Knowledge

Dehydration of hexan-2-ol involves elimination of a water molecule. Because the hydroxyl group is on carbon-2, elimination can occur in two directions, forming a mixture of positional isomers (hex-1-ene and hex-2-ene), thus reducing the specific yield of hex-1-ene.

Question 5 (c)

Polymerisation and Disposal

✅ Correct Answers

Part (i) Repeat Unit: Two repeating units of poly(hex-1-ene) drawn with a carbon backbone ( -C-C-C-C- ), single bonds, correct alkyl side chains ( -C₄H₉ or -CH₂CH₂CH₂CH₃ ), and open bonds indicating continuation.
Part (ii) Polymer Disposal: 1. Combustion for energy production (generating electricity/heating).
2. Production of plastics / useful organic compounds via chemical feedstock recycling.

🧠 Exam Technique

When drawing repeat units, ensure the main chain has two carbons per repeat unit with single bonds, and the double bond from the monomer is removed.
List clear, distinct environmental disposal methods to secure both marks in part (ii).

Topics

Module 1: Development of practical skills in chemistry · Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · Practical Activity Groups · PAG 1: Moles determination · PAG 5: Synthesis of an organic liquid · PAG 6: Synthesis of an organic solid · PAG 7: Qualitative analysis of organic functional groups · 2.1 Atoms and reactions · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.