OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 17

6 marks · Hard difficulty · Extended Response

Plan a two-stage synthesis of 4-hydroxypentan-2-one from 4-chloropentan-2-ol, calculating the required mass of starting material given an overall percentage yield of 31.5%, and providing reagents and balanced equations for each step.

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Question

Question text

This question is about an organic synthesis.

A student plans a two-stage synthesis of compound F from compound D, as shown below.

Cl OH OH O

Stage 1 Stage 2

Compound E

Compound D Compound F

Plan a two-stage synthesis for the preparation of 2.50 g of compound F from compound D.

The overall percentage yield of compound F from compound D is 31.5%.

In your answer include the mass of compound D required, reagents and equations for each step.

[6]

Extra answer space if required.

Mark scheme

Show the mark scheme

Question Answer Mark Guidance

17* Level 3 (5–6 marks) 6 Mark additional answer space P15 as SEEN

Correct calculation of mass of D required Indicative scientific points may include:

AND

Gives correct reagents for both stages of the synthesis in the

Calculation of mass of D

correct order with both equations mostly correct

Using moles:

2.50

There is a well-developed line of reasoning which is clear and • n(F) = = 0.0245… (mol)

logically structured. The information presented is relevant and 100

substantiated. • n(D) = 0.0245… x 31.5 = 0.0778… (mol)

• m(D) = 0.0778… x 122.5 = 9.53 (g)

Level 2 (3–4 marks)

Using mass:

Correct calculation of mass of D required 100

AND • Theoretical m(F) = 2.50 x 31.5 = 7.94 (g)

Gives correct reagents for one stage of the synthesis with 7.94

equation mostly correct • Theoretical n(D) = 102 = 0.0778… (mol)

• m(D) = 0.0778… x 122.5 = 9.53 (g)

Calculation of mass of D is partly correct ALLOW small slip/rounding errors such as errors in

AND

Mr e.g. use of Mr of 101 instead of 102 for F

Gives correct reagents for both stages of the synthesis

Note: allow use of calculator value, or values rounded to

OR min. 3SF

Gives correct reagents for both stages of the synthesis, in the

correct order, with equations mostly correct Examples of partly correct calculations:

31.5

Mass = 0.946 (g) from 0.0245… x 100 x 122.5

There is a line of reasoning presented with some structure. (inverted % yield)

The information presented is relevant and supported by some Mass = 3.00 (g) from 0.0245… x 122.5

evidence. (omission of % yield)

Mass = 6.61 (g) from Mr values wrong way round

Synthesis

Level 1 (1–2 marks) In equations, ALLOW any unambiguous formulae

IGNORE conditions or solvents

Calculation of mass of D is partly correct

Stage 1 (oxidation of 2o alcohol)

OR + 2-

Gives correct reagents for both stages of the synthesis Reagents: H /Cr2O7

ALLOW K2Cr2O7 OR Na2Cr2O7 AND H2SO4

OR ALLOW names for reagents e.g. acidified dichromate, if

Describes one stage of the synthesis with reagents and no formulae given

equation mostly correct IGNORE Roman numerals e.g. (VI), unless incorrect

Equation:

Attempts to calculate the mass of D but makes little progress

AND

Correct reagent for one stage of the synthesis

Stage 2 (substitution of Cl)

There is an attempt at a logical structure with a line of Reagents: NaOH OR KOH OR OH-

reasoning. The information is in the most part relevant. ALLOW H2O

Equation:

0 marks

No response or no response worthy of credit.

ALLOW correct ionic equation/mechanism showing OH-

and Cl-

IF stage 2 (substitution of Cl) attempted first

Equation:

Then oxidation step follows

Equation:

Aspects of the communication statement might typically

have been met when:

• All information given is correct and relevant using

appropriate scientific language, for example:

o no additional incorrect reagents e.g.

NaOH and H2SO4 for substitution

o use of an inappropriate acid e.g. HCl for

oxidation

• Equations are correctly balanced

• Calculations are set out in a clear and logical way

• No small slips/rounding errors

• Final value for mass given to an appropriate

number of significant figures/decimal places.

How to answer it

Two-Stage Organic Synthesis & Percentage Yield Planning

📋 WHAT THIS QUESTION TESTS

This 6-mark Level of Response question evaluates both quantitative calculation and synthetic organic chemistry problem-solving:

Quantitative Problem Solving: Calculating the initial mass required using reacting moles, molar masses (M_r), and taking into account an overall percentage yield.
Functional Group Interconversions: Identifying selective oxidation of secondary alcohols to ketones, and nucleophilic substitution of haloalkanes to secondary alcohols.
Synthetic Route Logic & Selectivity: Deducing the correct sequence of reactions to avoid competing reactions (e.g. avoiding the formation of a symmetrical diol).
Chemical Equations: Writing clear, balanced organic equations using structural/skeletal representations or unambiguous molecular formulae and standard organic notations (e.g. [O]).

Section 1: Quantitative Analysis (Mass of Compound D Required)

Working backwards from target mass through overall percentage yield

📐 Step-by-Step Calculation

Determine Molecular Formulae and M_r:
• Compound F (4-hydroxypentan-2-one): C₅H₁₀O₂
M_r(F) = (5 × 12.0) + (10 × 1.0) + (2 × 16.0) = 102.0 g mol⁻¹
• Compound D (4-chloropentan-2-ol): C₅H₁₁ClO
M_r(D) = (5 × 12.0) + (11 × 1.0) + 35.5 + 16.0 = 122.5 g mol⁻¹
Calculate Actual Moles of F Needed:
n(F) = 2.50 g / 102.0 g mol⁻¹ = 0.02451 mol
Account for 31.5% Percentage Yield:
Since 1 mol of D theoretically forms 1 mol of F (1 : 1 stoichiometry):
Theoretical n(D) = 0.02451 × (100 / 31.5) = 0.07781 mol
Calculate Mass of D:
Mass of D = 0.07781 mol × 122.5 g mol⁻¹ = 9.53 g (to 3 sig figs).

❌ Calculation Traps & Common Errors

Inverting the Yield: Multiplying by (31.5 / 100) instead of dividing gives 0.946 g . If you need 2.50 g of product from an inefficient reaction, you must start with more reactant, not less!
Omitting Percentage Yield: Calculating 0.02451 × 122.5 = 3.00 g forgets that the yield is only 31.5%.
Miscounting Carbons in Skeletal Structures: Both compounds have a 5-carbon chain (pentane backbone). Miscounting as 6 carbons gives an incorrect M_r.
Rounding Intermediate Values Prematurely: Keeping intermediate values in your calculator prevents final rounding drift away from 9.53 g.

Section 2: Organic Synthetic Route (D → E → F)

Deducing reagents, intermediates, and reaction sequence

💡 Why Reaction Order Matters: Synthetic Selectivity

Compound D contains a secondary alcohol group ( -CH(OH)- ) and a chloroalkane group ( -CH(Cl)- ). Target F has a ketone ( C=O ) and a secondary alcohol ( -CH(OH)- ).

Correct Order (Oxidation first, then Substitution):
Oxidising compound D converts the alcohol into a ketone while leaving the chloro group untouched, producing 4-chloropentan-2-one (Compound E). Subsequent nucleophilic substitution with aqueous hydroxide replaces the -Cl with -OH , cleanly yielding target F.
Why the Reverse Order is Flawed:
If you substitute first, you hydrolyse the -Cl to form pentane-2,4-diol. This molecule contains two identical secondary alcohol groups. Acidified dichromate cannot differentiate between them and would oxidise both into a diketone (pentane-2,4-dione).

✅ Stage 1: Oxidation of Secondary Alcohol

Intermediate E: 4-chloropentan-2-one ( CH₃COCH₂CH(Cl)CH₃ )

Reagents & Conditions: Acidified potassium dichromate(VI): K₂Cr₂O₇ / H₂SO₄ (or H⁺ / Cr₂O₇²⁻ ), heated under reflux.

Equation:

CH₃CH(Cl)CH₂CH(OH)CH₃ + [O] → CH₃CH(Cl)CH₂COCH₃ + H₂O

Skeletal Equation: Draw compound D + [O] → Compound D with C=O in place of C-OH + H₂O.

✅ Stage 2: Nucleophilic Substitution of Chloroalkane

Target F: 4-hydroxypentan-2-one ( CH₃COCH₂CH(OH)CH₃ )

Reagents & Conditions: Aqueous sodium hydroxide ( NaOH(aq) or KOH(aq) ), warm/heat under reflux.

Equation:

CH₃CH(Cl)CH₂COCH₃ + NaOH → CH₃CH(OH)CH₂COCH₃ + NaCl

Ionic Equation:

CH₃CH(Cl)CH₂COCH₃ + OH⁻ → CH₃CH(OH)CH₂COCH₃ + Cl⁻

Section 3: Examiner Commentary & Maximising Marks

Level of Response Criteria (Level 3: 5–6 Marks)

🧠 Structuring for Level 3 (5–6 Marks)

Line of Reasoning: Organise your response clearly under separate subheadings: Mass Calculation, Stage 1, and Stage 2.
Reagent Precision: Always specify the acid when quoting dichromate (e.g. H₂SO₄ ). Stating only "dichromate" or using the wrong acid (like HCl , which gets oxidised to toxic Cl₂!) loses communication credit.
Balance By-Products: Remember that oxidation using [O] produces a molecule of H₂O , and hydrolysis with NaOH produces NaCl (or Cl⁻ if using ionic form).

📋 Mark Scheme Breakdown

Level 3 (5–6 marks): Correct mass of D (9.53 g) AND correct reagents for both stages in the correct order with both balanced equations mostly correct. Clear, well-developed line of reasoning.
Level 2 (3–4 marks): Correct mass of D with one stage correct, OR partly correct calculation with both stages correct, OR correct reagents in correct order with mostly correct equations.
Level 1 (1–2 marks): Partly correct calculation OR correct reagents for both stages OR one stage fully correct with reagents and equation.

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 2.1 Atoms and reactions · 4.2 Alcohols, haloalkanes and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.