OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 18

18 marks · Hard difficulty · Structured Questions

Explain optical isomerism of amino acids, draw 3D isomers of alanine, complete an organic synthesis flowchart involving functional group transformations to form valine, determine the molecular formula and calculate the number of molecules of ibuprofen in a tablet, and draw the ionic structures of a lysine salt of ibuprofen and explain its solubility.

Practise this question

Question

Exam question with multiple parts about organic acids, amino acids (alanine, valine, threonine, lysine), optical isomerism, a multi-step organic synthesis flowchart to form valine, structural formula and mass calculation for ibuprofen, and ionic structures for a lysine salt of ibuprofen.
Question text

18 This question is about organic acids.

(a) Table 18.1 shows the R groups in four amino acids.

Amino acid R group

alanine CH3–

valine (CH3)2CH–

threonine CH3CH(OH)–

lysine H2N(CH2)4–

Table 18.1

Most amino acids show optical isomerism.

(i) Explain the term optical isomerism.

… [1]

(ii) Draw 3-D diagrams to show the optical isomers of alanine.

[2]

(iii) How many optical isomers does threonine have?

… [1]

(b) Complete the flowchart for two synthetic routes to the amino acid valine.

ester

C7H14O3

O

HO

OH

C5H10O3

acyl chloride

C5H9O2Cl

O

H2N

… OH

haloalkane valine

C H BrO C5H11NO2

59 2

[7]

(c) Ibuprofen, shown below, is used as a painkiller.

O

OH

ibuprofen

(i) What is the molecular formula of ibuprofen?

… [1]

(ii) One ibuprofen tablet contains 400 mg of ibuprofen.

Calculate the number of ibuprofen molecules in one ibuprofen tablet.

Give your answer to 3 significant figures.

number of ibuprofen molecules = … [3]

(d) Tablets based on ibuprofen and lysine are now available from pharmacies.

These tablets are claimed to act faster than ibuprofen by being absorbed into the body more

quickly than ibuprofen alone.

One type of these tablets contains a salt of ibuprofen and the amino acid lysine

(R = –(CH2)4NH2) in a 1:1 molar proportion.

(i) Suggest the structure of each ion in this lysine salt of ibuprofen, including the position of

any charges.

– ion + ion

[2]

(ii) Suggest why tablets based on a salt of ibuprofen should act faster in the body than

ibuprofen.

… [1]

Mark scheme

Show the mark scheme Mark scheme providing the answers for the amino acids and optical isomerism questions, 3D structural diagrams for alanine enantiomers, the complete synthesis flowchart for valine with reagents, molecular formula and mole calculation for ibuprofen yielding 1.17 x 10^21 molecules, and the structures of the negative and positive ions of the lysine salt of ibuprofen.

AO

Question Answer Marks Guidance

element

18 (a) (i) Non-superimposable mirror images (about a chiral centre) 1 AO1.1

(ii) Correct groups attached to chiral C of alanine seen once e.g. 2 AO2.1

× 2

OR

Two 3D structures of alanine that are mirror images AND Each structure must have four central

correct connectivity in both bonds with at least two wedges.

i.e. For bond into paper accept:

ALLOW two 3D structures with 2 groups

swapped

e.g.

IF CH3 is shown as ‘R’ ALLOW 1 mark for

two 3D structures with correct connectivity

that are mirror images

e.g.

COOH COOH

C C

NH2 H2N

R R

H H

AO

element

(iii) 4 1 AO2.2

(b) 7 AO1.2

× 4 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

AO2.5 unambiguous

× 3

ALLOW names of reagents

DO NOT ALLOW OH– for HO–

but ALLOW ECF for subsequent use in (b)

For hydrolysis,

ALLOW dilute acid

ALLOW alkaline conditions followed by

protonation of carboxylate

i.e. NaOH(aq)/OH–(aq) AND H+(aq)/HCl(aq)

ALLOW HBr for NaBr/H SO

AO

element

(c) (i) C13H18O2 1 AO2.1 ALLOW C, H and O in any order

(ii) FIRST CHECK ANSWER ON THE ANSWER LINE 3 AO2.2

If answer = 1.17 × 1021 award 3 marks × 3

M(ibuprofen) = 206 ALLOW ECF from (c)(i)

400 ÷ 1000 Calculator: 1.941747573 × 10–3

n(ibuprofen) = = 1.94 × 10–3 (mol)

Number of molecules = 1.94 × 10–3 × 6.02 × 1023 ALLOW ECF from n(ibuprofen)

= 1.17 × 1021 to 3 SF 3 SF essential

(d) (i) 2 AO3.2 IGNORE small slip in carbon chains

× 2

ALLOW

(ii) More soluble in water 1 AO3.1 Answer must be a comparison

ALLOW dissolve faster/quicker

IGNORE absorbed more quickly (given in

question)

Total 18

How to answer it

Organic Acids, Amino Acids & Synthesis Study Guide

OCR A-Level Chemistry • Exam Question Breakdown

What this question tests

This comprehensive multi-part question assesses your understanding of stereoisomerism (optical isomerism and chiral centres), multi-step organic synthesis involving functional group interconversions (esters, acyl chlorides, carboxylic acids, haloalkanes, and amines), molecular formula determination, moles and Avogadro constant calculations, and amino acid/carboxylate ionic structures in pharmaceutical contexts.

Part (a): Optical Isomerism in Amino Acids

(i) Definition of optical isomerism

✅ Correct Answer

Non-superimposable mirror images (about a chiral centre).

💡 Key Knowledge

A chiral centre (asymmetric carbon) must have four different groups attached to it. The resulting stereoisomers rotate plane-polarized light in opposite directions.

❌ Common Errors

Saying "molecules that are mirror images" without mentioning that they are non-superimposable. Simple mirror images can sometimes be rotated to look identical (achiral).

(ii) Drawing 3D structures of optical isomers of alanine

✅ Correct Answer

Two 3D structures of alanine showing tetrahedral arrangement around the central carbon atom. Must feature at least two wedge/dash bonds per structure representing 3D spatial orientation, showing mirror-image relationship and correct connectivity ( CH₃ , COOH , NH₂ , H ).

🧠 Exam Technique

Draw a central C with one standard vertical bond, one standard horizontal bond, one wedge bond, and one dash bond. Draw the mirror image clearly beside it, swapping positions of two groups to demonstrate enantiomerism.

(iii) Number of optical isomers for threonine

✅ Correct Answer

4

💡 Key Knowledge

Threonine contains two chiral centres ( C-2 and C-3 with an -OH group). Using the formula 2ⁿ where n = number of chiral centres, 2² = 4 optical isomers.

Mark breakdown: (i) 1 mark | (ii) 2 marks | (iii) 1 mark

Part (b): Synthetic Routes to Valine

Complete the flowchart for synthetic routes

✅ Correct Answers & Reagents

  • Top reaction (Ester to Carboxylic Acid): Hydrolysis using dilute acid ( H⁺/H₂O (aq) or HCl (aq) ) or alkaline hydrolysis followed by acidification.
  • Right reaction (Acyl Chloride to Carboxylic Acid): Addition of H₂O .
  • Bottom-left reaction (Carboxylic Acid to Haloalkane): Substitution using NaBr / H₂SO₄ or HBr .
  • Bottom-right reaction (Haloalkane to Valine Amine): Nucleophilic substitution using excess NH₃ in ethanol .

❌ Common Errors

Writing OH⁻ instead of H₂O for the hydration of an acyl chloride, or omitting ethanol when reacting haloalkanes with ammonia (which leads to unwanted elimination/alkene formation).

Mark breakdown: 7 marks total (Allocated across intermediate structures and synthetic reagents).

Part (c): Ibuprofen Calculations

(i) Molecular Formula

✅ Correct Answer

C₁₃H₁₈O₂

🧠 Exam Technique

Count vertices carefully on the skeletal structure for carbons, add hydrogens based on carbon's tetravalency (accounting for the benzene ring and degree of unsaturation), and count oxygen atoms from the carboxylic acid group.

(ii) Number of Ibuprofen Molecules Calculation

📐 Step-by-Step Calculation

  1. Find Molar Mass of Ibuprofen ( C₁₃H₁₈O₂ ):
    (13 × 12.0) + (18 × 1.0) + (2 × 16.0) = 206 g mol⁻¹
  2. Calculate Moles of Ibuprofen in 400 mg:
    Convert mg to g: 400 mg = 0.400 g (or 400 × 10⁻³ g)
    Moles = Mass / Molar Mass = (0.400) / 206 = 1.9417... × 10⁻³ mol
  3. Calculate Number of Molecules:
    Multiply moles by Avogadro's constant ( 6.02 × 10²³ ):
    Number = 1.9417 × 10⁻³ × 6.02 × 10²³ = 1.1689... × 10²¹
  4. Apply Significant Figures:
    Round to exactly 3 significant figures: 1.17 × 10²¹

❌ Common Calculation Traps

  • Forgetting to convert milligrams (mg) to grams (g) by dividing by 1000.
  • Failing to give the final answer to the requested 3 significant figures, which loses a standalone mark point.
Mark breakdown: (i) 1 mark | (ii) 3 marks

Part (d): Ibuprofen-Lysine Salt Formulation

(i) Structure of ions in the lysine salt

✅ Correct Answers

  • Negative ion (Ibuprofenate): The ibuprofen skeleton with the carboxylic acid group deprotonated to form -COO⁻ .
  • Positive ion (Lysinated amino acid): The lysine skeleton with the terminal aliphatic amine protonated to form -NH₃⁺ (the carboxylic acid end can remain -COOH or be zwitterionic/deprotonated depending on accepted variants, but standard protonation of the basic amine is essential).

💡 Key Knowledge

Acid-base reactions between carboxylic acids and basic amino acid side chains (like the butyl-amine side chain in lysine, R = -(CH₂)₄NH₂) form ionic salts that enhance water solubility.

(ii) Why salt tablets act faster

✅ Correct Answer

The salt is more soluble in water than neutral ibuprofen.

🧠 Exam Technique

Always ensure your answer is framed as a comparison ("more soluble" rather than just "soluble"). Examiner mark schemes strictly require comparative language.

Mark breakdown: (i) 2 marks | (ii) 1 mark | Total for question: 18 marks

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.