AQA A-Level Chemistry Paper 3, 2017: Question 5

1 mark · Easy difficulty · Multiple Choice

Identify which compound has the highest boiling point among propan-1-ol, propanal, propanone, and methyl ethanoate.

Practise this question

Question

Multiple choice question asking: 'Which compound has the highest boiling point?' Option A is CH3CH2CH2OH, Option B is CH3CH2CHO, Option C is CH3COCH3, and Option D is CH3COOCH3. Each option has a selection box beside it. The question is worth 1 mark.
Question text

05 Which compound has the highest boiling point?

[1 mark]

A CH3CH2CH2OH

B CH3CH2CHO

C CH3COCH3

D CH3COOCH3

Mark scheme

Show the mark scheme Mark scheme table row showing question number 5 with the correct answer corresponding to letter A.

5 A

How to answer it

Boiling Points & Intermolecular Forces

📌 What this question tests

This question assesses your ability to identify organic functional groups, deduce the strongest type of intermolecular force operating between their molecules (van der Waals, permanent dipole-dipole, or hydrogen bonding), and relate intermolecular strength to relative boiling points.

Question 05 • Multiple Choice [1 Mark]

Identifying the Compound with the Highest Boiling Point

Analysis of organic compounds: alcohol vs aldehyde vs ketone vs ester

✅ Correct Answer

A: CH₃CH₂CH₂OH (propan-1-ol)

Award 1 mark for selecting option A.

💡 Key Knowledge

  • Hydrogen bonding: Occurs when hydrogen is bonded directly to a highly electronegative atom (N, O, or F) with a lone pair. Only CH₃CH₂CH₂OH possesses an -O-H group.
  • Permanent dipole-dipole forces: Operate between polar molecules like propanal ( CH₃CH₂CHO ), propanone ( CH₃COCH₃ ), and methyl ethanoate ( CH₃COOCH₃ ) due to the polar C=O bond.
  • Hierarchy of strength: For molecules of similar size/Mᵣ, Hydrogen bonding > Permanent dipole-dipole > Van der Waals (induced dipole-dipole) forces.

📐 Molecule Comparison Breakdown

  • A: CH₃CH₂CH₂OH (Mᵣ = 60.0): Alcohol → Has Hydrogen bonds (as well as dipole-dipole and van der Waals forces). Boiling point ≈ 97 °C.
  • B: CH₃CH₂CHO (Mᵣ = 58.0): Aldehyde → Only permanent dipole-dipole + van der Waals forces (no O-H bond). Boiling point ≈ 49 °C.
  • C: CH₃COCH₃ (Mᵣ = 58.0): Ketone → Only permanent dipole-dipole + van der Waals forces. Boiling point ≈ 56 °C.
  • D: CH₃COOCH₃ (Mᵣ = 74.0): Ester → Only permanent dipole-dipole + van der Waals forces. Despite higher Mᵣ, boiling point (≈ 57 °C) is significantly lower than the alcohol.

❌ Common Errors

  • Assuming carbonyls form hydrogen bonds with themselves: Aldehydes, ketones, and esters have oxygen with lone pairs, but NO hydrogen atom covalently bonded to oxygen ( O-H ). Therefore, they cannot form intermolecular hydrogen bonds with each other.
  • Choosing D based purely on Mᵣ: Methyl ethanoate has a higher Mᵣ (74.0 vs 60.0), but the presence of hydrogen bonding in propan-1-ol easily outweighs the modest difference in van der Waals forces.

🧠 Exam Technique & Examiner Insight

When asked for the highest boiling point among small organic molecules, follow this 3-step checklist:

  1. Scan the list immediately for carboxylic acids ( -COOH ) or alcohols ( -OH ), as these exhibit hydrogen bonding between their molecules.
  2. Check that the molecular sizes ( Mᵣ ) are comparable. Here, options A, B, and C all have 3 carbons, making comparison direct and straightforward.
  3. Conclude: More energy is required to overcome the much stronger hydrogen bonds between alcohol molecules than the weaker dipole-dipole and van der Waals forces in aldehydes, ketones, and esters.

Topics

Physical Chemistry · Organic Chemistry · 3.1.3 Bonding · 3.3.5 Alcohols

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.