OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 25
11 marks · Medium difficulty · Structured Questions
Deduce structures of isomeric alcohols A, B and C, draw a distillation apparatus, write an equation for haloalkane hydrolysis, and explain relative rates of haloalkane hydrolysis.
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Question text
25 Three alcohols, A, B and C, are structural isomers with the molecular formula C5H12O.
(a) A, B and C take part in combustion reactions.
Complete the equation for the complete combustion of C5H12O.
C5H12O + … [2]
(b) Alcohols A, B and C are each refluxed with acidified dichromate(VI), H+/Cr O 2–.
The organic products are shown in the table below.
Complete the table to show the structures of alcohols A, B and C.
Organic product after
Alcohol Structure of alcohol + 2–
refluxing with H /Cr2O7
O
O
A OH
OH
O
O
B
C No reaction
[3]
(c) Primary alcohols can be oxidised under distillation to make aldehydes.
Draw a labelled diagram to show how you would set up apparatus for distillation.
[2]
(d) Alcohols can be prepared by the hydrolysis of haloalkanes with aqueous alkali.
(i) Write an equation for the hydrolysis of 2‑bromo‑2‑methylpropane.
Show organic compounds as structures.
[2]
(ii) A student hydrolyses a chloroalkane, RCl, a bromoalkane, RBr, and an iodoalkane, RI.
For a fair comparison, the student has chosen the same R group for each haloalkane.
Predict, with a reason, the relative rates of hydrolysis of these three haloalkanes.
… [2]
Mark scheme
Show the mark scheme
C2H5• + C2H5• → C4H10 OR 2C2H5• → C4H10 ✓ ALLOW C2H5C2H5 for C4H10 ✓
C2H5• + Br• → C2H5Br ✓
Question Answer 20 Marks Guidance
(c) Carbon 5
Bond angle Name of shape
atom
1 109.5 tetrahedral
ALLOW 109–110 for C1
2 120 trigonal planar
ALLOW 118–122 for C2
ALLOW planar triangle
2 OR 3 correct ✓
4 correct ✓ ALLOW table responses if in wrong columns
Number of electron pairs IGNORE areas of electron density
In C1/109.5º, 4 bonded pairs/bonding regions/bonds ✓
For bonded pairs
ALLOW bp, bonded groups, bonded atoms
Bonded/bonding essential
In C2/120º, 3 bonded regions/bonds ✓ For C2, ALLOW
• 3 bonded areas/environments
• 3 bonded pairs/groups/atoms
• 2 bonded pairs and 1 double bond
• 2 bonded pairs and 1 bonded region
Electron pair repulsion
Electron pairs/bonded pairs repel (as far apart as possible) ✓ DO NOT ALLOW ‘atoms repel’
Electron pairs/bonded pairs essential IGNORE
DO NOT ALLOW ‘bonded atoms’ for this mark • electrons repel
• bonds repel
• electron region OR electron density
• lone pairs repel more irrelevant here
• shapes, even if wrong
Question Answer Marks Guidance
25 (a) C5H12O + 7½ O2 → 5 CO2 + 6 H2O 2 ALLOW multiples
21 e.g. 2 C5H12O + 15 O2 → 10 CO2 + 12 H2O
CO2 AND H2O products
Complete equation balanced Watch for 15/2 OR 7.5 for 7½
(b) Alcohol Structure 3 ALLOW any combination of skeletal OR structural OR
A displayed formula as long as unambiguous
DO NOT ALLOW structure if H(s) are missing from ONE
structural formula
B ……. BUT ALLOW any further omissions as ECF
Take care with numbers of carbons, the branches and the
position of branching …. especially for C
IGNORE connectivity,
e.g.
C ALLOW
OH CH3
BUT DO NOT ALLOW –HO
(c) 2
22 DO NOT ALLOW ANY MARKS FOR A REFLUX SET UP
Workable set up IGNORE
• Flask with ‘horizontal’ OR ‘angled down’ condenser • no heat question about apparatus
• NOT a sealed system for collection vessel • no thermometer stopper is fine
• NOT open at the top above flask
Key labels for distillation set up
• Water in at bottom and out at top
• AND condenser label
(d) (i) 2 ALLOW any combination of skeletal OR structural OR
displayed formula as long as unambiguous
IGNORE state symbols
ALLOW OH– AND Br– in a balanced mechanism
Structure of BOTH organic compounds ALLOW OH– over the arrow for LHS of equation
DO NOT ALLOW if H(s) are missing
– – DO NOT ALLOW unbalanced charges,
OH on left AND Br on right e.g. OH– with Br
ALLOW NaOH/KOH with NaBr/KBr
DO NOT ALLOW H2O AND HBr
question specifies aqueous alkali.
DO NOT ALLOW 2nd mark if a CON reagent is present,
e.g. an acid
(d) (ii) Rates of hydrolysis of all 3 haloalkanes 2 IGNORE reactivity of halogens
Fastest RI > RBr > RCl slowest
ALLOW unambiguous comparison of two haloalkanes
e.g. RI is fastest AND RCl is slowest
Bond enthalpies/strength of bonds for any one RX ALLOW C–Cl is strongest bond
Strongest: C–Cl > C–Br > C–l weakest
MUST refer to C–X bond in some way ALLOW R–Cl, etc BUT NOT RCl > RBr > Rl no bonds
DO NOT ALLOW just ‘strongest bond enthalpy
iodine has weakest bond
How to answer it
Alcohols, Oxidation, and Haloalkane Hydrolysis
What this question tests
This question assesses core organic chemistry concepts from AS Level: writing balanced combustion equations for alcohols, deducing structures of structural isomers based on oxidation reactions, drawing standard laboratory apparatus for distillation, writing nucleophilic substitution equations for haloalkanes, and explaining the relative rates of hydrolysis linked to carbon-halogen bond enthalpies.
Complete Combustion of C₅H₁₂O
✅ Correct Answer
C₅H₁₂O + 7½ O₂ ➔ 5 CO₂ + 6 H₂O
(Accepts multiples such as doubling the entire equation: 2 C₅H₁₂O + 15 O₂ ➔ 10 CO₂ + 12 H₂O)
💡 Key Knowledge
- Complete combustion of any organic molecule containing C, H, and O always yields carbon dioxide (CO₂) and water (H₂O).
- Balance carbon atoms first, hydrogen atoms second, and oxygen atoms last.
❌ Common Errors
- Forgetting to account for the oxygen atom already present inside the alcohol molecule C₅H₁₂O when balancing the O₂ on the left side.
Deducing Alcohol Structures from Oxidation Products
✅ Correct Answers
- Alcohol A: 2-methylbutan-1-ol (primary alcohol oxidised under reflux to 2-methylbutanoic acid).
- Alcohol B: 3-methylbutan-2-ol (secondary alcohol oxidised under reflux to 3-methylbutan-2-one).
- Alcohol C: 2-methylpropan-2-ol (tertiary alcohol, which gives No reaction with acidified dichromate(VI)).
🧠 Exam Technique
Examine the given organic product structures carefully. Work backwards: a carboxylic acid indicates a primary alcohol, a ketone indicates a secondary alcohol, and "No reaction" strictly indicates a tertiary alcohol.
Drawing Set-up for Distillation
✅ Correct Diagram Requirements
- Round-bottom or pear-shaped flask with a workable setup and a horizontal/downward angled condenser.
- Not a sealed system (collection vessel must be open at the top, or clearly not stoppered airtight).
- Key Labels: Condenser, water in (at the bottom), water out (at the top).
❌ Common Errors
- Drawing a reflux set-up instead (vertical condenser with water flowing into the top) results in 0 marks.
- Reversing the water flow direction in the condenser (water must enter from the bottom to ensure the jacket stays completely filled).
Nucleophilic Substitution of 2-bromo-2-methylpropane
✅ Correct Equation / Mechanism
Showing the conversion of the haloalkane to an alcohol using aqueous alkali:
(CH₃)₃CBr + OH⁻ ➔ (CH₃)₃COH + Br⁻
(Structures can be skeletal, structural, or displayed as long as they are completely unambiguous).
💡 Key Knowledge
Tertiary haloalkanes undergo hydrolysis via an SN1 mechanism involving a stable tertiary carbocation intermediate. The nucleophile is the hydroxide ion ( OH⁻ ).
Explaining Reactivity Trends of Haloalkanes
✅ Correct Answer & Explanation
- Rate order (fastest to slowest): RI > RBr > RCl
- Reason: The C-I bond enthalpy is the lowest (weakest bond), requiring the least energy to break, whereas the C-Cl bond is the strongest. Therefore, iodoalkanes react the fastest.
🧠 Exam Technique
You must explicitly mention the C-X bond enthalpy or bond strength in your answer. Simply stating "iodine is bigger" or "reactivity increases down the group" without linking it to bond energy often loses the second marking point.
Topics
Module 4: Core organic chemistry · Practical Activity Groups · 4.2 Alcohols, haloalkanes and analysis · PAG 5: Synthesis of an organic liquid
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.