Edexcel A-Level Chemistry Paper 2, November 2021: Question 4

12 marks · Medium difficulty · Open Response

Draw skeletal formulae of branched alkenes, identify alcohol dehydration products, complete the electrophilic addition mechanism for pent-2-ene and HBr, and calculate the pressure of pent-1-ene vapour using the ideal gas equation.

Practise this question

Question

A 4-part structured chemistry question about alkenes with molecular formula C5H10. Part (a) asks to draw skeletal formulae of three branched alkenes with this formula in three boxes. Part (b) is a multiple-choice question on alcohol dehydration to form pent-2-ene only. Part (c) asks to complete the mechanism for the reaction of pent-2-ene with HBr using curly arrows and dipoles. Part (d) provides a mass, volume, and temperature for a sample of pent-1-ene and asks to calculate the pressure inside the container using the ideal gas equation.
Question text

4 This question is about alkenes with the molecular formula C5H10.

(a) Draw the skeletal formulae of three branched chain alkenes with the

molecular formula C5H10.

(3)

(b) Which of these compounds would form pent-2-ene only, when reacted with

concentrated phosphoric acid, H3PO4?

(1)

A CH3CH(OH)CH(CH3)2

B CH2(OH)CH2CH2CH2CH3

C CH3CH2CH2CH(OH)CH3

D CH3CH2CH(OH)CH2CH3

(c) Pent-2-ene reacts with hydrogen bromide, HBr, to form two bromoalkanes.

Complete the diagram to show the mechanism for the formation of

2-bromopentane in this reaction.

Include curly arrows, and relevant lone pairs and dipoles.

(4)

H 3

*P65464A0328*

Br

(d) A sample of pent-1-ene, with a mass of 1.33g, is warmed to 60°C in a

sealed container. The volume of the container is 500 cm3.

Calculate the pressure inside the container.

Include units and give your answer to an appropriate number of significant figures.

[Gas constant (R) = 8.31 J mol−1 K−1]

(4)

*P65464A0428*

(Total for Question 4 = 12 marks)

Mark scheme

Show the mark scheme The mark scheme gives acceptable skeletal structures for part (a), identifies option D for part (b), outlines the four marking points for the electrophilic addition mechanism in part (c) showing curly arrows, dipoles, intermediate carbocation, and Br minus attack, and details the calculation steps for the ideal gas equation in part (d) yielding approximately 105 kPa.

Question Additional Guidance Mark

Answer

Number

4(a) Allow (2) for three correct displayed (3)

or structural formulae

(1) (1) Allow (1) for any two correct

displayed or structural formulae

(1)

Question

Answer Mark

Number

4(b) The only correct answer is D CH3CH2CH(OH)CH2CH3 only (1)

A is not correct because it will not form pent-2-ene

B is not correct because it will only form pent-1-ene

C is not correct because CH3CH(OH)CH2CH2CH3 will also form pent-1-ene

Question Answer Additional Guidance Mark

Number

4(c) (4)

• arrow from double bond to ∂+ H in HBr (1) Penalise lack of dipole only once in M1 or

M2

Do not award M1 if arrow from C=C to C

also shown

• arrow from bond in HBr to Br∂– (1)

• structure of carbocation (1)

• arrow from lone pair on Br– to C+ in carbocation and

final products (1)

Formation of 3-bromopropane can potentially score

M1, M2 and M4 as a TE

Question

Answer Additional Guidance Mark

Number

4(d) Example of calculation (4)

• calculation of moles of pent-1-ene (1) 1.33 / 70 = 0.019 (mol)

500 x 10–6 m3 and 333 K

• conversion of volume and temperature (1)

Allow conversion of volume to 0.5 dm3 if units for M3

and / or M4 shown as kPa

P = (nRT) / V = (0.019 x 8.31 x 333) / 500 x 10–6

• rearrangement of ideal gas equation and calculation of

p (1) = 105154.74

= 105000 Pa / 1.05 x 105 Pa / 1.1 x 105 Pa

• final answer to 2 or 3 SF and units (1) −2

Allow N m for Pa

Allow 105 kPa

Allow TE at each stage

Penalise rounding to 1SF in M1 but then allow TE

Correct answer with units and no working scores (4)

(Total Question 4 = 12 marks)

How to answer it

Alkenes, Mechanisms and Ideal Gas Calculations

What this question tests

This question assesses your mastery of organic chemistry and quantitative chemistry. You will be tested on drawing skeletal formulae of isomers, understanding alcohol dehydration pathways, drawing precise electrophilic addition mechanisms with curly arrows and dipoles, and applying the ideal gas equation (PV = nRT) with strict attention to unit conversions and significant figures.

Question 4(a) - 3 Marks

Drawing Branched Alkenes

✅ Correct Answers

You need to draw the skeletal formulae for three different branched isomers of C₅H₁₀ :

  • 2-methylbut-2-ene ( CH₃C(CH₃)=CHCH₃ )
  • 2-methylbut-1-ene ( CH₂=C(CH₃)CH₂CH₃ )
  • 3-methylbut-1-ene ( CH₂=CHCH(CH₃)₂ )

❌ Common Errors

  • Including straight-chain alkenes: Drawing pent-1-ene or pent-2-ene will lose marks because the question explicitly specifies branched chain isomers.
  • Incorrect notation: Failing to use proper skeletal format (e.g., drawing out carbons explicitly when requested to show skeletal, though structural/displayed is accepted as an alternative allowance).
Mark breakdown: 3 marks total. 3 correct = (3), 2 correct = (1), 1 or 0 correct = (0).
Question 4(b) - 1 Mark

Dehydration of Alcohols

✅ Correct Answer

D: CH₃CH₂CH(OH)CH₂CH₃ (Pentan-3-ol)

💡 Key Knowledge

Dehydration using concentrated H₃PO₄ removes an -OH group and an adjacent hydrogen to form a C=C double bond. Pentan-3-ol is symmetrical, so eliminating water from either side exclusively yields pent-2-ene.

❌ Common Errors & Distractor Analysis

  • Compound C ( CH₃CH₂CH₂CH(OH)CH₃ ): Pentan-2-ol forms a mixture of pent-1-ene and pent-2-ene due to different adjacent hydrogens.
Mark breakdown: 1 mark for selecting option D.
Question 4(c) - 4 Marks

Electrophilic Addition Mechanism

🧠 Exam Technique & Mechanism Steps

  1. First Curly Arrow: Must start from the C=C double bond and point directly to the δ+ H atom of HBr. (Do not point to the bond between H and Br).
  2. Second Curly Arrow: Must start from the H-Br bond and point to the Br atom, showing heterolytic fission to form a bromide ion ( Br⁻ ).
  3. Dipoles: Label δ+ on H and δ- on Br in the HBr molecule.
  4. Intermediate Carbocation: Draw the correct secondary carbocation structure with a positive charge ( + ) located on the correct carbon atom.
  5. Final Arrow: Must start from a lone pair of electrons on the Br⁻ ion and point directly to the positive carbon ( C⁺ ) in the intermediate.

❌ Common Pitfalls

  • Starting the first curly arrow from a carbon atom instead of the middle of the C=C double bond.
  • Omitting lone pairs or formal charges on the bromide ion in the final step.
Mark breakdown: 4 marks total mapped across arrow 1, arrow 2, carbocation intermediate, and final nucleophilic attack arrow.
Question 4(d) - 4 Marks

Ideal Gas Equation Calculation

📐 Step-by-Step Calculation

  1. Calculate Molar Mass of pent-1-ene ( C₅H₁₀ ):
    5 × 12.0 + 10 × 1.0 = 70.0 g mol⁻¹
  2. Calculate moles ( n ) of pent-1-ene:
    n = mass / Mr = 1.33 / 70.0 = 0.0190 mol
  3. Convert units for volume and temperature:
    Volume: 500 cm³ = 500 × 10⁻⁶ m³ (or 0.500 dm³ )
    Temperature: 60 °C + 273 = 333 K
  4. Rearrange PV = nRT to solve for pressure ( P ):
    P = (nRT) / V
    P = (0.0190 × 8.31 × 333) / (500 × 10⁻⁶)
    P = 105154.74 Pa
  5. Apply Significant Figures & Units:
    Final answer = 105000 Pa (or 1.05 × 10⁵ Pa , or 105 kPa ). Must be given to 2 or 3 significant figures with correct units.

❌ Common Calculation Traps

  • Forgetting to add 273: Using 60 instead of 333 K for temperature.
  • Volume conversion errors: Forgetting to convert cm³ to m³ ( ×10⁻⁶ ) when using R = 8.31 J mol⁻¹ K⁻¹ .
  • Sig Fig penalties: Giving an unrounded calculator value or rounding to 1 significant figure.
Mark breakdown: (1) for moles calculation, (2) for unit conversions (V and T), (3) for rearranging and calculating P, (4) for correct 2-3 SF and valid units.

Topics

Organic Chemistry · Physical Chemistry · Topic 6: Organic Chemistry I · Topic 5: Formulae, Equations and Amounts of Substance

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.