Edexcel A-Level Chemistry Paper 3, June 2019: Question 9

16 marks · Hard difficulty · Extended Writing

Discuss optical activity, preparation yields, ligand substitution in haemoglobin involving carbon monoxide, and the synthesis of alcohols and carboxylic acids using Grignard reagents.

Practise this question

Question

A multipart exam question about organic and transition metal compounds. Part (a) shows the structure of a glycinate ion and asks about optical activity, a preparation reaction equation for copper(II) glycinate, and a discussion of percentage yields. Part (b) shows a partial diagram of haemoglobin with an Fe2+ center bonded to four nitrogen atoms of a globin ligand and O2, asking why carbon monoxide inhalation is fatal. Part (c) is an extended response question about the formation and use of Grignard reagents in organic synthesis to produce primary, secondary, and tertiary alcohols and carboxylic acids.
Question text

9 Some organic compounds contain metals.

(a) Glycinate ions are formed from the amino acid glycine.

H O

H2N C C

O–

H

glycinate ion

(i) Explain the effect, if any, of an aqueous solution containing glycinate ions on

plane-polarised monochromatic light.

(2)

(ii) A hot aqueous solution of glycine is added to a hot solution of

copper(II) ethanoate.

When the mixture is cooled, crystals of copper(II) glycinate are formed.

Write the equation for this reaction.

State symbols are not required.

(2)

(iii) In an experiment, the crystals are filtered, weighed and the percentage yield calculated.

Student 1 obtained a yield of 102.6%.

Student 2 obtained a yield of 56.4%.

The expected yield is 82% and the students carried out the calculation correctly.

Discuss possible reasons for the yields obtained by these students.

(4)

… *P58308A02432*

(b) Haemoglobin is an iron(II) complex. It carries oxygen around the body.

Part of the structure of haemoglobin is shown.

O2

N N

Fe2+

N N

globin

The four nitrogen atoms are part of a multidentate ligand in the haem group.

Explain why inhaling carbon monoxide can be fatal.

(2)

*(c) Grignard reagents contain a metal.*P58308A02532*

Discuss how Grignard reagents are formed and used in adding one or more

carbon atoms to the carbon chain in 1-bromopropane to produce primary,

secondary and tertiary alcohols and a carboxylic acid.

Include a suitable example for each reaction and give reagents, conditions and products.

You may include equations in your answer.

(6)

… 26

… *P58308A02632*

(Total for Question 9 = 16 marks)

Mark scheme

Show the mark scheme The mark scheme provides detailed guidance for marking each part of Question 9. For 9(a)(i), it awards points for stating the glycinate ion lacks an asymmetric carbon atom and has no effect on plane-polarised light. For 9(a)(ii), it gives the equation and acceptable copper species formulas. For 9(a)(iii), it lists valid reasons for high and low percentage yields such as wet crystals, impurities, incomplete reaction, or handling losses. For 9(b), points are awarded for carbon monoxide replacing oxygen and binding strongly to the Fe2+ ion. For 9(c), a levels-of-response marking grid is provided alongside indicative chemical content for Grignard reagent reactions, reagents, conditions, and products.

How to answer it

Organic Compounds Containing Metals & Transition Metal Complexes

What this question tests

This comprehensive multi-part question assesses your understanding of stereoisomerism (optical isomerism), transition metal complex formation, percentage yield evaluation, ligand exchange in biological systems (haemoglobin and carbon monoxide poisoning), and synthetic organic chemistry using Grignard reagents to extend carbon chains.

Question 9 (a)(i)

Effect of Glycinate Ions on Plane-Polarised Light

✅ Correct Answer

There will be no effect on plane-polarised monochromatic light because glycinate ions do not contain a chiral centre (asymmetric carbon atom with four different groups attached; the central carbon has two identical hydrogen atoms).

❌ Common Errors

  • Stating that the solution is a "racemic mixture" (glycinate ions are achiral, so enantiomers do not exist here).
  • Claiming the light itself will not be polarised (the light starts polarised; the solution simply fails to rotate it).
Mark breakdown: 2 marks total. 1 mark for stating/explaining lack of chirality, 1 mark conditional on the first for stating there is no effect on the plane-polarised light.
Question 9 (a)(ii)

Equation for Copper(II) Glycinate Formation

✅ Correct Answer

(CH₃COO)₂Cu + 2NH₂CH₂COOH → (NH₂CH₂COO)₂Cu + 2CH₃COOH

(Accept ionic or simplified formulae for copper(II) ethanoate and copper(II) glycinate).

💡 Key Knowledge

Ligand exchange occurs where the bidentate glycinate ligand replaces ethanoate ligands, forming a stable chelate complex coordination compound.

Mark breakdown: 2 marks total. 1 mark for correct formula of copper(II) products/reactants, 1 mark for balancing the full equation. State symbols are not required.
Question 9 (a)(iii)

Evaluating Experimental Percentage Yields

✅ Correct Answer (Select 4 valid points)

  • Student 1 (102.6%): Crystals were wet/still damp when weighed, or impurities remained in the crystals.
  • Student 2 (56.4%): The reaction was incomplete, some copper(II) glycinate remained dissolved (failed to crystallise fully upon cooling), or practical handling losses occurred during filtration/transfer.

🧠 Exam Technique

Always link yields over 100% to water retention or trapped impurities, and yields significantly below expected values to incomplete reactions, equilibrium limitations, or transfer losses.

Mark breakdown: 4 marks total (1 mark per valid, distinct reason provided, up to a maximum of 4).
Question 9 (b)

Haemoglobin and Carbon Monoxide Toxicity

✅ Correct Answer

  • Carbon monoxide replaces (ligand exchange) the oxygen molecule coordinated to the Fe²⁺ ion in haemoglobin.
  • CO binds strongly/irreversibly to the Fe²⁺ ion due to a higher binding affinity/stability constant.
  • This prevents oxygen transport around the body, leading to cellular oxygen starvation.

❌ Common Errors

Vaguely stating that "carbon monoxide bonds more strongly to haemoglobin" without referencing the central iron(II) ion or ligand exchange mechanics can occasionally miss marking points depending on exact phrasing.

Mark breakdown: 2 marks total. Any two valid points from replacement/ligand exchange, strong/irreversible binding to Fe²⁺, and consequence on oxygen delivery.
Question 9 (c)*

Grignard Reagents: Chain Extension & Alcohol Synthesis

💡 Key Knowledge & Reactions

Grignard reagents ( RMgX ) are prepared by reacting a haloalkane with magnesium metal in dry ether. They act as strong nucleophiles and strong bases, extending the carbon skeleton when reacted with carbonyl compounds or carbon dioxide.

  • Primary Alcohol: React Grignard with methanal (formaldehyde), followed by dilute acid hydrolysis.
  • Secondary Alcohol: React Grignard with any aldehyde (other than methanal) + acid.
  • Tertiary Alcohol: React Grignard with a ketone + acid.
  • Carboxylic Acid: React Grignard with solid/gaseous carbon dioxide (CO₂), followed by acid hydrolysis.

🧠 Extended Response Structure (Level of Response)

To access top-tier marks (Level 3, 5–6 marks), your answer must be logically structured and address:

  1. Preparation of a Grignard reagent starting from 1-bromopropane ( CH₃CH₂CH₂MgBr ).
  2. Specific organic reagents and conditions (dry ether solvent, followed by aqueous acid like dilute HCl).
  3. Clear chemical pathways/examples showing how carbon chain length increases to form primary, secondary, tertiary alcohols, and a carboxylic acid.
Mark breakdown: 6 marks total (Levels of response rubric combining chemical accuracy and logical communication structure).

Topics

Organic Chemistry · Inorganic Chemistry · Physical Chemistry · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III · Topic 15: Transition Metals · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.