OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 7
12 marks · Hard difficulty · Extended Response
Write a balanced equation for the preparation of 2-chloro-2-methylpropane from 2-methylpropan-2-ol, explain the purification of the organic liquid, calculate the expected mass from percentage yield, name structural isomer A, and write the oxidation equation for structural isomer B.
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Question text
7 2-Chloro-2-methylpropane, (CH3)3CCl, is an organic liquid with a boiling point of 50 °C.
A student prepares (CH3)3CCl by reacting 2-methylpropan-2-ol, (CH3)3COH, with concentrated
hydrochloric acid.
(a) Write a balanced equation for this reaction.
Use skeletal formulae for organic compounds.
[2]
(b)* The student’s method for the preparation is outlined below.
• Add 10.0 cm3 (7.70 g) of (CH ) COH and 30 cm3 concentrated hydrochloric acid (an
excess) to a round-bottom flask. Stopper the flask.
• Shake the flask until the mixture separates into two layers.
Densities: (CH ) CCl : 0.85 g cm–3; concentrated HCl : 1.18 g cm–3
After purification, the percentage yield of (CH3)3CCl is 76%.
Explain how the student could obtain a pure, dry sample of (CH3)3CCl from the mixture in the
flask and calculate the mass of pure (CH3)3CCl that would be expected from this preparation.
Additional answer space if required.
… [6]
(c) Compounds A and B are structural isomers of (CH3)3COH.
(i) Compound A is a secondary alcohol.
What is the systematic name of compound A?
… [1]
(ii) Compound B is a branched primary alcohol.
Compound B is refluxed with acidified potassium dichromate(VI) as an oxidising agent.
Write the equation for the reaction that takes place.
Use structures for organic compounds and [O] for the oxidising agent.
[3]
Mark scheme
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How to answer it
Organic Synthesis, Purification, and Isomerism
This multi-step synoptic organic chemistry question tests your ability to translate chemical descriptions into balanced equations using skeletal formulae, design practical purification steps for separating organic liquids (using separating funnels, drying agents, and distillation), perform complex multi-step chemical calculations involving density, moles, theoretical yield, and percentage yield, and deduce structural isomerism and oxidation reactions of alcohols.
Part (a): Equation for the Preparation
Write a balanced equation using skeletal formulae
✅ Correct Answer
Reacting skeletal formula of 2-methylpropan-2-ol plus HCl yielding 2-chloro-2-methylpropane and H₂O .
(CH₃)₃COH + HCl → (CH₃)₃CCl + H₂O
💡 Key Knowledge
When drawing skeletal formulae, carbon vertices and terminal lines represent methyl groups. Ensure functional groups like -OH and -Cl are clearly attached.
Part (b): Purification and Percentage Yield Calculation
Describe purification steps and calculate expected mass
🧠 Exam Technique (Levels of Response)
This is a 6-mark extended response question assessed via levels. To hit Level 3 (5–6 marks), you must provide a logical, detailed sequence for purification and perform a fully correct calculation of the expected mass.
📐 Step-by-Step Calculation
- Find moles of reactant: Mass = Density × Volume. Mass of (CH₃)₃COH = 0.85 × 10.0 = 7.70 g. Molar mass = 74.0 g mol⁻¹. Moles = 7.70 / 74.0 = 0.10405 mol.
- Relate reacting ratios: 1:1 molar ratio, so theoretical moles of (CH₃)₃CCl = 0.10405 mol.
- Account for percentage yield: Actual yield is 76%. Expected (actual) moles = 0.10405 × (76 / 100) = 0.0791 mol.
- Convert to mass: Molar mass of (CH₃)₃CCl = 92.5 g mol⁻¹. Mass = 0.0791 × 92.5 = 7.315 g (rounds to 7.3 g or 7.32 g ).
💡 Purification Procedure
- Step 1: Use a separating funnel to remove the lower aqueous layer from the organic layer. Note: organic layer is the top layer.
- Step 2: Add an anhydrous drying agent (e.g., anhydrous MgSO₄ or CaCl₂ ) to remove trace water.
- Step 3: Purify by distillation, collecting the fraction boiling around 50 °C .
❌ Common Calculation Traps
A major error is inverting the percentage yield ratio (multiplying by 100 / 76 instead of 76 / 100 ), which incorrectly inflates the expected mass to 12.7 g. Always double-check whether you are calculating a theoretical maximum or an expected yield!
Part (c): Isomerism and Oxidation of Alcohols
Identify isomers and write oxidation equations
✅ (i) Systematic Name for A
Compound A is a secondary alcohol isomer of (CH₃)₃COH with a 4-carbon chain.
Answer: Butan-2-ol
✅ (ii) Equation for B
Compound B is a branched primary alcohol ( 2-methylpropan-1-ol ). Refluxing a primary alcohol with acidified potassium dichromate(VI) oxidises it fully to a carboxylic acid.
(CH₃)₂CHCH₂OH + 2[O] → (CH₃)₂CHCOOH + H₂O
❌ Common Errors in (ii)
Students often forget to balance the oxidising agent, writing [O] instead of 2[O] for the conversion of a primary alcohol straight to a carboxylic acid under reflux, or omitting water ( H₂O ) on the product side.
Topics
Module 1: Development of practical skills in chemistry · Module 4: Core organic chemistry · Practical Activity Groups · 1.1 Practical skills assessed in a written examination · 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 Depth in chemistry (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.