OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 20

8 marks · Hard difficulty · Extended Response

Calculate the mass of cyclopentanol needed to prepare 4.00 g of cyclopentene given a 64.0% percentage yield, describe the purification of the organic layer, and explain the IR spectrum of the product.

Practise this question

Question

Question about the preparation of cyclopentene from cyclopentanol via acid-catalyzed dehydration using a distillation setup, followed by purification and an IR spectrum analysis question.
Question text

20 Cyclopentanol can be reacted to form cyclopentene.

Cyclopentene is a liquid with a boiling point of 44 °C and a density of 0.74 g cm−3.

A student plans to prepare 4.00 g of cyclopentene by reacting cyclopentanol (boiling point 140 °C)

with an acid catalyst.

Equation

OH

+ H2O

The expected percentage yield of cyclopentene is 64.0%.

Method

The student carries out the preparation using apparatus set up for distillation, as shown below.

1 The reaction mixture is heated gently, and a distillate containing impure cyclopentene is

collected.

cyclopentanol

and water

concentrated out

phosphoric

acid

water in

water bath

distillate

ice-water

mixture

2 The distillate has an aqueous layer and an organic layer.

The student purifies the cyclopentene from the distillate.

(a)* Calculate the mass of cyclopentanol that the student should use and explain how pure

cyclopentene could be obtained from the distillate. [6]

Additional answer space if required

(b) The organic layer in the distillate was analysed by IR spectroscopy.

The IR spectrum is shown below.

Transmittance(%) 50

4000 3000 2000 1500 1000 500

Wavenumber / cm–1

Explain how the IR spectrum of the organic layer suggests that cyclopentene has been

formed and that the reaction is incomplete.

… [2]

Mark scheme

Show the mark scheme Mark scheme showing level-based marking for part (a) involving mass calculation and purification steps, and specific wavenumber ranges for C=C and O-H peaks for part (b).

AO

Question Answer Marks Guidance

element

20 (a)* Refer to marking instructions on page 4 of mark scheme 6 2.8×2 Indicative scientific points may include:

for guidance on marking this question. 3.3×4 Calculation of mass of cyclopentanol

Using moles

Level 3 (5-6 marks) 4.00

• n(cyclopentene) = = 0.0588…. (mol)

A correct calculation of the mass of cyclopentanol 68

AND 100

• n(cyclopentanol) = 0.0588 × = 0.0919…(mol)

A detailed description of most purification steps 64

• Mass of cyclopentanol = 86 × 0.0919 = 7.90 g

There is a well-developed line of reasoning which is

Using mass

clear and logically structured. The information presented 100

is relevant and substantiated. • Theoretical mass cyclopentene= 4.00 × = 6.25 g

6.25

Level 2 (3-4 marks) • Theoretical n(cyclopentanol) = = 0.0919 (mol)

Calculates the mass of cyclopentanol with some errors • Mass of cyclopentanol = 86 × 0.0919 = 7.90 g

AND

A detailed description of some purification steps ALLOW for small slip in Mr / rounding errors

OR

A correct calculation of the mass of cyclopentanol Examples of some calculation errors

AND Incorrect inverse ratio:

A detailed description of a few purification steps 64

• 0.0588 × = 0.0376…. (mol)

There is a line of reasoning presented with some • Mass = 86 × 0.0376 = 3.24 g

structure. The information presented is relevant and

Ignoring % yield gives:

supported by some evidence. 4.00

• = 0.0588…. (mol)

Level 1 (1-2 marks) • Mass = 86 × 0.0588 = 5.06 g

Calculates the mass of cyclopentanol with some errors

OR Purification

A detailed description of some purification steps • Add a neutralising agent by formula or name e.g.

Na2CO3

There is an attempt at a logical structure with a line of • In separating funnel, organic layer is on top

reasoning. The information is in the most part relevant. • Drying with an anhydrous salt by formula or

name, e.g. MgSO4, Na2SO4, CaCl2

0 marks No response or no response worthy of • Redistil at approx. 44ºC

credit. Examples of detail in bold (NOT INCLUSIVE)

18 AO

element

(b) C=C/alkene peak in region 1620-1680 cm–1 2 3.2×2 LOOK ON THE SPECTRUM for labelled peaks

which can be given credit

O–H/alcohol peak in region 3200-3600 cm–1

IGNORE references to C-O at 1000cm-1

How to answer it

Preparation and Purification of Cyclopentene from Cyclopentanol

What this question tests

This synoptic organic chemistry question tests your mastery of practical chemistry techniques, stoichiometry, and analytical spectroscopy. Specifically, it assesses: calculating reactant quantities using percentage yields, executing multi-step organic purification procedures (neutralisation, separation, drying, and redistillation), and interpreting infrared (IR) spectra to confirm functional groups and monitor reaction completion.

Question Part (a)

Calculation of Reactant Mass & Organic Purification Procedure

6 Marks total (AO2: Calculation / AO3: Practical Procedures)

📐 Step-by-Step Calculation

  1. Find moles of product needed:
    Molar mass of cyclopentene (C₅H₈) = (5 × 12.0) + (8 × 1.0) = 68.0 g mol⁻¹ .
    Moles of cyclopentene = Mass ÷ Mr = 4.00 ÷ 68.0 = 0.0588 mol .
  2. Account for percentage yield:
    Since the yield is 64.0%, you need to scale up the theoretical moles of cyclopentanol required.
    Moles of cyclopentanol = 0.0588 × (100 ÷ 64.0) = 0.0919 mol .
  3. Calculate mass of cyclopentanol:
    Molar mass of cyclopentanol (C₅H₁₀O) = (5 × 12.0) + (10 × 1.0) + 16.0 = 86.0 g mol⁻¹ .
    Mass = Moles × Mr = 0.0919 × 86.0 = 7.90 g .

✅ Purification Steps Required

  • Neutralise: Add a suitable neutralising agent with formula or name (e.g., sodium carbonate, Na₂CO₃ ) to remove unreacted acid catalyst.
  • Separate: Use a separating funnel. Run off the lower aqueous layer; retain the upper organic layer (cyclopentene has a density lower than water).
  • Dry: Add an anhydrous salt (e.g., anhydrous MgSO₄ , Na₂SO₄ , or CaCl₂ ) to remove trace water.
  • Redistil: Collect the fraction boiling at approximately 44 °C .

❌ Common Calculation Traps

  • Inverting the yield ratio: Multiplying by 64/100 instead of 100/64 leads to an incorrect mass of 3.24 g .
  • Ignoring percentage yield entirely: Failing to scale up for the 64.0% yield results in 5.06 g and loses critical marks.

🧠 Level of Response Guidance

To achieve a Level 3 (5–6 marks), you must provide both the correct numerical calculation ( 7.90 g ) and a detailed, logically ordered description of the purification steps mentioning specific reagents (like naming the drying agent or carbonate).

Question Part (b)

Infrared Spectroscopy Analysis

2 Marks total (AO3: Analysis & Interpretation)

💡 Key Knowledge: IR Absorptions

  • Alkene C=C stretch: Appears at 1620–1680 cm⁻¹ , confirming that cyclopentene has been formed.
  • Alcohol O-H stretch: Appears as a broad absorption at 3200–3600 cm⁻¹ , proving that unreacted cyclopentanol remains in the mixture, confirming the reaction is incomplete.

🧠 Examiner Tip

Always quote wavenumber ranges directly from your data booklet when answering spectroscopy questions. Examiners look for both the specific absorption range and the structural deduction tied to it (e.g., stating the range and linking it to the presence of C=C or O-H bonds).

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · Practical Activity Groups · 4.2 Alcohols, haloalkanes and analysis · 6.3 Analysis · PAG 5: Synthesis of an organic liquid · PAG 7: Qualitative analysis of organic functional groups

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.