OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 23

12 marks · Hard difficulty · Structured Questions

Synthesize compound H from compound G using reflux, write neutralization equations, evaluate a reagent substitution, calculate percentage yield, and describe the purification of the liquid product.

Practise this question

Question

A chemistry exam question detailing the synthesis of compound H from compound G. Part (a) asks why reflux is used. Part (b)(i) asks for equations for the removal of sulfuric acid and unreacted compound G using sodium carbonate. Part (b)(ii) evaluates the use of sodium hydroxide instead. Part (c) requires calculating the percentage yield of compound H to 3 significant figures. Part (d) describes how to purify the impure compound H from two layers.
Question text

23 Oil of wintergreen is a liquid used in medicine to relieve muscle pain.

Compound H is a component in oil of wintergreen and can be synthesised from compound G, as

shown below. The boiling point and density of compound H are stated.

O O

OH O

OH OH

Compound G Compound H

boiling point = 222°C

density = 1.174 g cm−3

A student prepares a sample of compound H by the method below.

Step 1 Reflux 8.97 g of compound G for 30 minutes with an excess of methanol in the

presence of a small amount of sulfuric acid as a catalyst.

Step 2 Add an excess of aqueous sodium carbonate, Na2CO3(aq).

Two layers are obtained.

Step 3 Purify the impure compound H that forms from the resulting mixture.

The student follows this method and obtains 5.32 g of pure compound H.

(a) Why does the student use reflux in Step 1?

… [1]

(b)

(i) In Step 2, Na2CO3(aq) removes the sulfuric acid catalyst and any unreacted compound G from

the mixture.

Write equations for this removal.

Removal of sulfuric acid

Removal of unreacted compound G

O O

OH O

OH OH

Compound G Compound H

27boiling point = 222°C [3]

density = 1.174 g cm−3

(ii) Another student suggests that adding aqueous sodium hydroxide would be more effective in

removing the sulfuric acid catalyst than Na2CO3(aq).

Comment on whether the student’s suggestion is an improvement for the preparation of

compound H.

… [1]

(c) Calculate the percentage yield of compound H.

Give your answer to three significant figures.

percentage yield = … % [3]

(d) Describe how to purify the impure compound H from the two layers in Step 2.

… [4]

Mark scheme

Show the mark scheme The mark scheme for the question providing model answers for reflux, balanced equations for reaction with sulfuric acid and compound G with sodium carbonate, evaluation of sodium hydroxide, percentage yield calculation steps resulting in 53.8 percent, and the step-by-step purification procedure involving a separating funnel, drying with an anhydrous salt, and distillation at 222 degrees Celsius.

Question Answer Marks Guidance

23 (a) Idea that reflux is used to prevent loss by evaporation IGNORE responses related to rate of reaction

1 IGNORE responses related to ensuring complete reaction

e.g. prevents reaction mixture boiling dry

e.g. prevents loss of (volatile) compounds / products / DO NOT ALLOW reference to incorrect reaction e.g.

reactants oxidation, combustion (flammability)

e.g. prevent methanol escaping

23 (b) (i) 3 ALLOW multiples in both equations

IGNORE state symbols

Reaction with H2SO4 ALLOW Na2CO3 + 2H2SO4 → 2NaHSO4 + CO2 + H2O

ALLOW ionic equation CO 2- + 2H+ → CO + H O

32 2

Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O ALLOW H2CO3 instead of CO2 + H2O

Reaction with excess G ALLOW –COO- (Na+) for product structure mark

ALLOW ionic equation

O O

OH ONa

2 + Na2CO3 2 + CO2 + H2O

OH OH

ALLOW

Correct organic product structure

Correct balanced equation

ALLOW H2CO3 instead of CO2 + H2O

ALLOW correct Kekulé representation of benzene

Question Answer 29 Marks Guidance

23 (b) (ii) 1 IGNORE comment about whether it improves or not

(NaOH) reacts with phenol / -OH (in compound G / H) DO NOT ALLOW (NaOH) reacts with alcohol

OR (NaOH) would hydrolyse the ester / compound H

(c) FIRST CHECK ANSWER ON ANSWER LINE 3 ALLOW ECF for each step

If answer = 53.8 (%) award 3 marks TAKE CARE as value written down may be truncated but

---------------------------------------------------------------------------- with value stored in calculator, depending on rounding,

Theoretical moles either can be credited.

n(H) OR n(G)

8.97 IGNORE trailing zeroes e.g. 0.065 for 0.0650

OR 0.065(0) (mol) ✓

138.0

Actual moles

5.32

n(methyl salicylate) = OR 0.035(0) (mol) ✓

152.0

0.035

% yield = 100 = 53.8% to 3 SF ✓ DO NOT ALLOW ECF for final mark if value is ≥100%

0.065

DO NOT ALLOW 59.3% IF no moles have been

calculated for final mark

e.g. masses used 5.32/8.97 x 100 = 59.3%

Calculator = 53.84615385 BUT 3 SF required for % yield

Alternative method using mass

1. Theoretical moles = 0.065(0) mol ✓

2. Theoretical mass = 0.065 152.0 OR 9.88 g ✓

5.32

3. % yield = 100 = 53.8% ✓

9.88

23 (d) Steps must be given in correct order: 4 Mark each step in order but then don’t mark any

further if response refers to purification of a solid

e.g. dissolve in minimum amount of hot solvent, evaporate

Step 1 off water to allow solid to crystalise

(Add to) separating funnel ✓

(Use of) bottom layer (containing H / organic) ✓ IGNORE use of carbonate (to remove excess acid) OR

(saturated) NaCl

ALLOW (remove) aqueous layer on the top

ALLOW description that aqueous layer can be determined

by adding water and seeing which layer increases in size

IGNORE distillation OR filtration prior to step 1 OR step 2

Step 2

Dry with an anhydrous salt ALLOW ‘to remove water’ instead of ‘dry’

OR IGNORE any other named salt, e.g. ‘an anhydrous salt

Dry with MgSO4 /magnesium sulfate OR CaCl2 /calcium chloride ✓ e.g. CaCO3’ is acceptable

IGNORE filtration to remove anhydrous salt after step 2

Step 3 ALLOW termperature range of 220-224oC for distillation

(Re)Distil at 222 ºC ✓ DO NOT ALLOW if forms a solid product

How to answer it

Synthesis and Purification of Oil of Wintergreen

What this question tests

This multi-part organic synthesis question assesses your practical and theoretical knowledge of esterification, reflux apparatus functions, acid-base neutralization equations, reagent selection implications, percentage yield calculations, and liquid-liquid purification techniques (separating funnels, drying agents, and distillation).

Part (a) [1 mark]

Purpose of Reflux

✅ Correct Answer

Reflux is used to prevent loss by evaporation of volatile reactants or products (specifically preventing methanol or the organic mixture from boiling dry/escaping).

❌ Common Errors

Students frequently lose this mark by discussing reaction rates, ensuring complete reactions generally, or incorrectly referencing flammability and combustion.

Part (b)(i) [3 marks]

Neutralization Equations

✅ Correct Answers

  • Removal of sulfuric acid:
    Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O
  • Removal of unreacted compound G (salicyl acid):
    2C₆H₄(OH)(COOH) + Na₂CO₃ → 2C₆H₄(OH)(COONa) + CO₂ + H₂O

💡 Key Knowledge

Sodium carbonate ( Na₂CO₃ ) is a weak base used to neutralize strong acid catalysts and react with carboxylic acids to form water-soluble carboxylate salts, leaving the neutral ester layer intact.

Part (b)(ii) [1 mark]

Reagent Selection: NaOH vs Na₂CO₃

✅ Correct Answer

The suggestion is not an improvement because sodium hydroxide ( NaOH ) is a strong base that would react with the phenol group ( -OH ) and hydroalyse the ester (compound H).

🧠 Exam Technique

Keep your answer direct and focused on chemical reactivity. Examiners explicitly look for mention of ester hydrolysis or reaction with the phenolic/hydroxyl group.

Part (c) [3 marks]

Percentage Yield Calculation

📐 Step-by-Step Calculation

  1. Find moles of limiting reactant (Compound G):
    Molar mass of C₇H₆O₃ (Compound G) = 138.0 g mol⁻¹.
    Moles = 8.97 g / 138.0 g mol⁻¹ = 0.065 mol (Theoretical moles).
  2. Find actual moles of product (Compound H):
    Molar mass of methyl salicylate ( C₈H₈O₃ , Compound H) = 152.0 g mol⁻¹.
    Actual moles = 5.32 g / 152.0 g mol⁻¹ = 0.035 mol .
  3. Calculate percentage yield:
    (0.035 / 0.065) × 100 = 53.846...% = 53.8% (to 3 sig fig).

❌ Common Calculation Traps

  • Using mass/mass ratios incorrectly without converting to moles first (which yields an incorrect figure like 59.3%).
  • Failing to round the final answer strictly to 3 significant figures as requested by the prompt.
Part (d) [4 marks]

Purification Procedure

✅ Correct Answer (Sequential Steps)

  1. Step 1: Transfer the mixture into a separating funnel, shake, allow layers to settle, and run off/collect the lower organic layer (containing compound H).
  2. Step 2: Dry the organic layer using an anhydrous salt (e.g., MgSO₄ or CaCl₂ ).
  3. Step 3: Purify via re-distillation, collecting the fraction boiling at 222 °C.

🧠 Exam Technique

Sequence matters! If you attempt to dry the liquid before separating the aqueous layer, or attempt to distil without separating layers first, you will lose sequential processing marks. Clearly state the use of an anhydrous salt and the exact boiling point.

Topics

Module 6: Organic chemistry and analysis · Practical Activity Groups · Module 2: Foundations in chemistry · PAG 5: Synthesis of an organic liquid · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.