OCR A-Level Chemistry Unified chemistry (03), June 2017: Question 5
12 marks · Hard difficulty · Structured Questions
Write expressions and equations involving thiols and carbonyl compounds, and explain chemical tests to distinguish between six organic compounds with pleasant smells.
Practise this questionQuestion
Question text
5 This question is about organic molecules that have a strong smell.
(a) Thiols are foul-smelling, organic sulfur compounds with the functional group –SH.
Butane-1-thiol, shown below, contributes to the strong smell of skunks.
SH
butane-1-thiol
(i) Thiols are weak acids.
Write the expression for the acid dissociation constant, Ka, for butane-1-thiol.
[1]
(ii) Thiols react with carboxylic acids to form thioesters.
Write an equation for the reaction of butane-1-thiol with ethanoic acid.
Use structures for all organic compounds with the functional groups clearly displayed.
[2]
(iii) When beer is exposed to light, 3-methylbut-2-ene-1-thiol is formed, which gives an
unpleasant smell and flavour to the beer.
Draw the skeletal formula for 3-methylbut-2-ene-1-thiol.
[1]
(iv) Propane-1,3-dithiol reacts with carbonyl compounds in a condensation reaction to form a
cyclic organic sulfur product.
Write an equation for the reaction of propane-1,3-dithiol with propanone.
Use structures for organic compounds.
[2]
(b)* The structures for six naturally occurring organic compounds with pleasant smells, B–G, are
shown below. The common names in brackets relate to their source and smell.
OH
B C O
(linalool) (citral)
O
OH
D
(geraniol)
E
(camphor)
O
OH
F G
(menthol) (ionone)
Explain how chemical tests would allow each compound to be distinguished from the other
compounds.
In your answer, include essential details for all test procedures and observations.
Details of apparatus and quantities are not required.
… [6]
Mark scheme
Show the mark scheme
Question Answer Marks Guidance
For 5a(i)–(iv) IGNORE poor connectivity to SH groups Given in question
5 (a) (i) [H+] [C H S–] 1 ALLOW correct structural OR skeletal OR
Ka = [C H SH] displayed formula OR mixture of the above as long
Square brackets required as non-ambiguous
(a) (ii) 2 ALLOW correct skeletal OR displayed formula OR
mixture of the above as long as non-ambiguous
ALLOW C4H9SH
ALLOW CH3COOH
Structure of thioester Thioester functional group must be fully displayed,
OR as a skeletal formula
Complete equation but allow SC4H9 in thioester
(a) (iii) 1 IF correct skeletal formula is shown, IGNORE
displayed formula in a second structure
(a) (iv) 2 ALLOW correct structural OR skeletal
OR displayed formula OR mixture of the above
as long as non-ambiguous
Reactants
Products AND balanced equation
(b)* Refer to the marking instructions on page 5 of the mark 6 Indicative scientific points may include:
scheme for guidance on marking this question.
Functional groups
Level 3 (5–6 marks) B alkene and tertiary alcohol
Develops a plan that identifies all compounds by a C alkene and aldehyde
process of elimination D alkene and primary alcohol
AND E ketone
includes essential detail for all required tests and F secondary alcohol
observations G alkene and ketone
There is a well-developed line of reasoning which is Tests
clear and logically structured B, C, D and G Bromine decolourises
Level 2 (3–4 marks) C, D and F (H+/)Cr O 2– green
Develops a plan that identifies at least half of the
compounds OR identifies the functional groups in most C, E and G 2,4-DNP orange precipitate
of the compounds C Tollens silver mirror
AND
includes detail of the required tests and observations For Tollens’ ALLOW alternative: Fehling’s solution
produces a ‘brown/brick red/orange precipitate
There is a line of reasoning with some structure. The
information is mostly relevant and supported by some For 2,4-DNP, ALLOW 2,4-DNPH and Brady’s
evidence.
B C D E F G
Level 1 (1–2 marks)
Develops a plan that attempts to identify the Bromine
compounds OR functional groups (H+/)Cr O 2–
AND 2,4-DNP
includes detail of the required tests and observations
Tollens’
There is a line of reasoning using information that is
mostly relevant.
No credit for tests on products of tests, melting
0 marks – No response or no response worthy of credit points, spectra, etc.
H432/03 with no compounds identified Mark Scheme For other tests seen, contact TL for adviceJune 2017
Total 12
Appendix for Q5b Level of Response
Results of tests
B C D E F G
Bromine
(H+/)Cr O 2–
2,4-DNP
Tollens
Possible processes of elimination (not inclusive)
BCDEFG with 2,4 DNP CEG orange ppt
CEG with Tollens C silver mirror
EG with bromine G decolourises E no change
BDF with (H+)/Cr O 2– DF green B no colour change
DF with bromine D decolourises F no change
---------------------------------------------------------------------------------------------------------------------
BCDEFG with (H+)/Cr O 2– CDF green
CDF with Tollens/2,4DNP C silver mirror/orange ppt
DF with bromine D decolourises F no change
BEG with 2,4 DNP EG orange ppt B no change
EG with bromine G decolourises E no change
---------------------------------------------------------------------------------------------------------------------
BCDEFG with bromine BCDG decolourise EF no change
EF with 2,4-DNP/(H+/)Cr O 2– E orange ppt/F green
BCDG with Tollens’ C silver mirror BDG no change
BDG with H+/Cr O 2– D green BG no change
BG with 2,4-DNP G orange ppt B no change
How to answer it
Organic Molecules with a Strong Smell
What this question tests
This question assesses your knowledge of sulfur-containing organic compounds (thiols), acid dissociation constant expressions, equation writing for organic reactions (thioester formation and condensation reactions), interpreting skeletal formulae, and applying chemical test reagents in a systematic separation or identification plan for six organic compounds.
Acid Dissociation Constant Expression for Butane-1-thiol
✅ Correct Answer
Ka = [H⁺][C₄H₉S⁻] / [C₄H₉SH]
💡 Key Knowledge
- The acid dissociation constant (Ka) follows the general equilibrium expression: Ka = [H⁺][A⁻] / [HA].
- Square brackets strictly denote equilibrium concentrations. Omitting them loses the mark.
❌ Common Errors
Forgetting to use square brackets around the species, or writing division incorrectly.
Reaction of Butane-1-thiol with Ethanoic Acid
✅ Correct Answer
CH₃CH₂CH₂CH₂SH + H₃C-COOH ⟶ H₃C-CO-S-CH₂CH₂CH₂CH₃ + H₂O
(Note: The thioester functional group must be fully displayed showing the C=O bonded to the S atom).
🧠 Exam Technique
Ensure structural or skeletal formulae clearly display the functional groups. The mark scheme explicitly states that the thioester functional group must be clearly shown.
❌ Common Errors
Failing to balance water as a byproduct, or incorrectly attaching the alkyl chain to oxygen instead of sulfur (forming a normal ester instead of a thioester).
Skeletal Formula of 3-methylbut-2-ene-1-thiol
✅ Correct Answer
A skeletal formula showing a 4-carbon chain with a double bond between carbons 2 and 3, a methyl branch at carbon 3, and an -SH group attached to carbon 1.
💡 Key Knowledge
Translating IUPAC nomenclature into skeletal drawings requires careful counting of the longest carbon chain and positioning substituents at the correct locants.
Reaction of Propane-1,3-dithiol with Propanone
✅ Correct Answer
Reactants: Propanone + Propane-1,3-dithiol ⟶ Products: Cyclic thioacetal (or cyclic sulfur product) + H₂O
🧠 Exam Technique
Check reactant and product stoichiometry carefully. Condensation reactions between dithiols and carbonyl compounds eliminate water.
Distinguishing Between Compounds B, C, D, E, F, and G
💡 Key Knowledge & Functional Groups
- B: Alkene and tertiary alcohol
- C: Alkene and aldehyde
- D: Alkene and primary alcohol
- E: Ketone (cyclic)
- F: Secondary alcohol (cyclic)
- G: Alkene and ketone
🧠 Exam Technique (Level of Response Strategy)
To achieve a Level 3 response (5–6 marks), you must develop a clear, logical step-by-step plan of elimination using specific chemical reagents and their characteristic observations.
- Bromine water: Decolourises with alkenes (B, C, D, G). No change with non-alkenes (E, F).
- Acidified potassium dichromate (H⁺/Cr₂O₂²⁻): Turns green with primary/secondary alcohols (D, F) and aldehydes (C). No change with tertiary alcohol (B) or ketones (E, G).
- 2,4-DNP (Brady's reagent): Forms an orange precipitate with carbonyl compounds (C, E, G). No reaction with alcohols (B, D, F).
- Tollens' reagent: Forms a silver mirror with aldehydes (C only).
❌ Common Errors
Vague observations (e.g., writing "it reacts" instead of stating specific colour changes like "orange precipitate" or "solution turns green"). Failing to test systematically so that every single compound is uniquely isolated.
Topics
Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.3 Analysis
Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.