OCR A-Level Chemistry Unified chemistry (03), June 2018: Question 4

11 marks · Hard difficulty · Structured Questions

Determine the oxidation number of platinum in cis-platin, outline its mechanism of formation, identify functional groups in paracetamol, and calculate the mass of 4-nitrophenol required for a two-stage synthesis of paracetamol with a given percentage yield including purification details.

Practise this question

Question

A structured chemistry question about cis-platin and paracetamol. Part (a) asks for the oxidation number of Pt in cis-platin and to outline the mechanism for its formation from [PtCl3(NH3)]- using curly arrows and lone pairs in a provided box. Part (b) shows the skeletal structure of paracetamol and asks to name its functional groups, followed by a multi-step synthesis question involving a calculation of the mass of 4-nitrophenol required to produce 5.00 g of paracetamol at 40.0% yield, including reagents, intermediate, and purification details.
Question text

4 This question is about two compounds used in medicine.

(a) Cis-platin, PtCl2(NH3)2, is a complex of platinum which is used in cancer treatment.

(i) What is the oxidation number of platinum in cis-platin?

… [1]

(ii) Cis-platin is prepared in a ligand substitution reaction which takes place in multiple steps.

The equation for the final step forming cis-platin is shown below.

[PtCl (NH )]– + NH → PtCl (NH ) + Cl–

33 3 2 3 2

cis-platin

In the box, outline the mechanism for the formation of cis-platin from [PtCl (NH )]–.

Use curly arrows and lone pairs where appropriate.

Cl Cl _

Pt + Cl

H3N NH3

cis-platin

[2]

(b) Paracetamol is a solid organic compound used in tablets as a painkiller.

H

HO N

C CH3

paracetamol

O

(i) Name the functional groups present in paracetamol.

… [2]

(ii)* A chemist prepares a pure solid sample of paracetamol from 4-nitrophenol in two stages:

H

Stage 1 Stage 2

HO NO2 Intermediate HO N

CH3COCl

C CH3

4-nitrophenol paracetamol

O

Describe a two-stage synthesis of 5.00 g of pure paracetamol from 4-nitrophenol. The

overall percentage yield of paracetamol from 4-nitrophenol is 40.0%.

In your answer, include the mass of 4-nitrophenol required, the reagents and intermediate,

and details of the purification of paracetamol. [6]

Additional answer space if required.

Mark scheme

Show the mark scheme The mark scheme provides the answers: (a)(i) +2; (a)(ii) shows curly arrow mechanisms for ligand substitution; (b)(i) lists phenol and amide functional groups; (b)(ii) details the level-based mark scheme (levels 1 to 3) for the calculation of 11.5 g of 4-nitrophenol, reagents such as Sn/conic HCl, intermediate 4-aminophenol, and recrystallisation purification steps.

Question Answer Marks Guidance

4 (a) (i) +2 1 ALLOW 2+ OR +II

Sign required ALLOW Pt2+

(ii) 2 For [PtCl (NH )]– :

– IGNORE dipoles

Cl Cl

Pt IGNORE absence of – charge

Cl NH IGNORE – charge shown on atoms

ALLOW any 4 coordinate shape for [PtCl (NH )]–,

|

NH3 e.g. tetrahedral; ––Pt––

|

Curly arrow from lone pair on NH3 to Pt 1st curly arrow must

go to Pt

[PtCl (NH )]– drawn with 1 Pt, 3 Cls and 1 NH AND

33 3

AND start from, OR be traced back to any point

Curly arrow from any Pt–Cl bond in the complex across width of lone pair on N of NH3

ALLOW SN1 mechanism:

DO NOT ALLOW charge on NH3 nucleophile,

– – –

Cl Cl Cl e.g. NH3

Pt Pt

Cl NH3 Cl NH3 2nd curly arrow must start from, OR be traced

back to, any part of Pt–Cl bond and go to one of

the 3 Cl atoms

NH3

Mark curly arrows as above for SN2

Requires + on platinum intermediate

(b) (i) Phenol 2

13 IF > 2 functional groups are shown,

Amide Mark 2 groups ONLY

Mark incorrect groups first

• IGNORE attempt to classify amide, e.g. secondary

Treat carbonyl with aldehyde OR with ketone as

one functional group,

i.e.

carbonyl, aldehyde

carbonyl, ketone

carbonyl

IGNORE aryl OR alkyl group

e.g. benzene, phenyl, aryl, arene, methyl

IGNORE hydroxyl/hydroxy

(b) (ii)* Refer to marking instructions on page 5 of mark scheme for guidance on 6 Indicative scientific points may include:

marking this question. Calculation of mass of 4-nitrophenol

Using moles

Level 3 (5-6 marks) 5.00

A correct calculation of the mass of 4-nitrophenol. n(paracetamol) = 151 = 0.0331 (mol)

AND 100

Identifies the reagents AND intermediate. n(4-nitrophenol) = 0.0331 40 = 0.0828 (mol)

AND

Mass of 4-nitrophenol = 139 0.0828 = 11.5 g

A detailed description of most purification steps.

ALLOW 11.4–11.6 for small slip/rounding

There is a well-developed line of reasoning which is clear and logically

structured. The information presented is relevant and substantiated. Using mass

Theoretical mass paracetamol = 5.00 40 = 12.5 g

Level 2 (3-4 marks)

Calculates the mass of 4-nitrophenol with some errors AND suggests 12.5

Theoretical n(4-nitrophenol) = 151 = 0.0828 (mol)

reagents and intermediate with some omissions.

OR Mass of 4-nitrophenol = 139 0.0828 = 11.5 g

Calculates the mass of 4-nitrophenol with some errors AND describes 100

NOTE: Incorrect inverse ratio of 40 gives:

some purification steps, with some detail.

OR

Question Answer 14 Marks Guidance

Suggests reagents and intermediate with some omissions AND describes 40

some purification steps, with some detail. 0.0331 100 = 0.0132 (mol)

Mass = 139 0.0132 = 1.84 g

There is a line of reasoning presented with some structure. The information

presented is relevant and supported by some evidence.

Reagents and intermediate

Reagents: Sn + (conc) HCl (then NaOH)

Level 1 (1-2 marks)

Intermediate: 4-aminophenol or structure

Attempts to calculate the mass of 4-nitrophenol

OR

Purification

Suggests reagents OR intermediate but may be incomplete

Dissolve impure solid in minimum volume of hot

OR

solvent

Describes few purification steps.

Cool solution and filter solid

There is an attempt at a logical structure with a line of reasoning. The Scratch with glass rod

information is in the most part relevant. Wash with cold solvent/solvent and dry

Examples of detail in bold (NOT INCLUSIVE)

0 marks No response or no response worthy of credit.

NOTE: ‘Recrystallisation’ on its own is NOT a detailed

description

Total 11

How to answer it

Medicinal Compounds: Cis-platin & Paracetamol

What this question tests

This multi-topic question assesses transition metal chemistry (oxidation states and ligand substitution mechanisms), organic functional group identification, synthetic multi-stage reaction routes (reduction followed by acylation), percentage yield stoichiometry, and practical organic purification techniques (recrystallisation).

Part (a)(i): Oxidation Number in Cis-platin

Question Part (a)(i) • 1 Mark

✅ Correct Answer

+2 (or +II or Pt²⁺ )

Mark: 1 mark for correct oxidation number with the sign included.

❌ Common Errors

Writing 2 without the + sign will lose the mark. Examiners strictly penalise missing signs in oxidation numbers.

Part (a)(ii): Ligand Substitution Mechanism

Question Part (a)(ii) • 2 Marks

✅ Correct Answer (SN2 pathway shown)

  • First curly arrow: Starts from the lone pair on the nitrogen of NH₃ and points directly to the Pt atom.
  • Second curly arrow: Starts from the bond pair of any Pt-Cl bond in the complex and goes to the Cl atom, breaking the bond heterolytically.

💡 Key Knowledge

Square planar platinum(II) complexes typically undergo ligand substitution via an associative (SN2) mechanism involving a 5-coordinate intermediate, or a dissociative pathway. Ensure curly arrows originate precisely from electron sources (lone pairs or bonds).

🧠 Exam Technique

When drawing curly arrows for nucleophilic attack, make sure the arrowhead touches the target atom (Pt), and the tail begins right at the nitrogen lone pair. Do not add incorrect charges to neutral nucleophiles like NH₃.

Part (b)(i): Functional Groups in Paracetamol

Question Part (b)(i) • 2 Marks

✅ Correct Answers

1. Phenol (or hydroxyl attached directly to a benzene ring)

2. Amide (or substituted amide)

Mark: 1 mark per correct functional group. Max 2 if more than 2 groups are stated (incorrect groups evaluated first).

❌ Common Errors

Writing "alcohol" instead of "phenol", or writing "carbonyl" and "amine" separately instead of recognising the combined -CONH- linkage as an amide. Ignore descriptors like "secondary" unless asked.

Part (b)(ii): Synthesis, Yield Calculations & Purification

Question Part (b)(ii) • 6 Marks (Level of Response)

📐 Step-by-Step Calculation (Mass of 4-nitrophenol)

  1. Moles of paracetamol desired:
    n(paracetamol) = Mass / Mᵣ = 5.00 g / 151 g mol⁻¹ = 0.0331 mol
  2. Moles of 4-nitrophenol required (accounting for 40.0% yield):
    Since yield is 40%, theoretical moles needed = 0.0331 × (100 / 40) = 0.0828 mol
  3. Mass of 4-nitrophenol:
    Mass = Moles × Mᵣ = 0.0828 mol × 139 g mol⁻¹ = 11.5 g (Accept 11.4 – 11.6 g)

💡 Reagents, Intermediate & Purification

Stage 1 Reagents: Sn / concentrated HCl (followed by NaOH to liberate the free amine).

Intermediate: 4-aminophenol (or its structure).

Purification (Recrystallisation steps):

  • Dissolve impure solid in a minimum volume of hot solvent.
  • Cool the solution and allow crystals to form.
  • Filter under suction (Buchner funnel), wash with cold solvent, and dry.

❌ Common Calculation Traps

Multiplying by 40/100 instead of 100/40 in the yield step is the most common error, resulting in an incorrectly small mass (1.84 g) and dropping marks into Level 1.

Topics

Module 5: Physical chemistry and transition elements · Module 6: Organic chemistry and analysis · Practical Activity Groups · 5.3 Transition elements · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis · PAG 6: Synthesis of an organic solid

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.