Edexcel A-Level Chemistry Paper 3, June 2018: Question 2
9 marks · Hard difficulty · Open Response
Devise a two-step synthesis of lactic acid starting from ethanal, and identify the type of polymerisation and repeat unit for poly(lactic acid).
Practise this questionQuestion
Question text
2 This question is about lactic acid (2-hydroxypropanoic acid), CH3CH(OH)COOH.
Lactic acid is used to make biodegradable polymers.
(a) Lactic acid can be made in a two-step synthesis starting from ethanal, CH3CHO.
Devise a reaction scheme for a two-step synthesis.
Include in your answer all reagents and conditions, the type of reaction occurring
at each step, and a balanced equation for each reaction.
State symbols are not required.
(7)
(b) Polymerisation of lactic acid forms poly(lactic acid) as shown in the diagram.
O O CH3 H O CH3 H
HO C polymerisation O C O C
OH O C O C
H CH3 H CH3 O H CH3 O
(i) State the type of polymerisation occurring in this reaction.
(1)
(ii) On the diagram, draw a circle around the repeat unit of the polymer.
(1)
(Total for Question 2 = 9 marks)
Mark scheme
Show the mark scheme
Br + 2I- 2Br- + I (1) Allow multiples
Ignore state symbols even if incorrect
use of ONLY two correct experiments as above (1)
(Total for Question 1 = 10 marks)
Question
Acceptable Answers Additional Guidance Mark
Number
2(a) An answer that makes reference to the following points: Ignore references to other conditions / solvent in (7)
(1st Step) step 1
HCN (and KCN) (1) Allow HCN and CN− / H+ and CN− / H+ and KCN
or
KCN and H2SO4 / KCN and HCl
or
HCN at pH 8 – 9
M1 can be scored for the appearance of HCN in M3
Nucleophilic addition (1) Do not award additional incorrect reaction types e.g.
nitrification
CH3CHO + HCN → CH3CH(OH)CN (1) Allow skeletal formulae in equations
(2nd Step) M4, 5 & 6 dependent on the formation of any nitrile
in step 1
Any identified (dilute) strong acid / H+ (1) Allow sodium hydroxide followed by acid
Do not award conc. acid / just “acidify” / just “acid”
Heat (under reflux) / reflux (1) Allow warm
Hydrolysis (1) Do not award additional incorrect reaction types
Allow two equations involving NaOH and H+
How to answer it
Synthesis and Polymerisation of Lactic Acid
What this question tests
This question assesses your knowledge of carbonyl chemistry reactions (specifically nucleophilic addition of hydrogen cyanide), functional group interconversions via nitrile hydrolysis, balancing organic equations, and identifying polymerisation types and repeat units from structures.
Two-Step Synthesis from Ethanal to Lactic Acid
💡 Key Knowledge
- Step 1: Converting an aldehyde (ethanal, CH₃CHO ) into a hydroxynitrile requires HCN (often generated in situ using KCN/H⁺ ). The mechanism is nucleophilic addition.
- Step 2: Converting a nitrile into a carboxylic acid requires acid hydrolysis using a dilute strong acid (e.g., HCl or H₂SO₄ ) under reflux.
✅ Correct Answer Breakdown
- Reagent 1: HCN (or KCN + H⁺ / KCN + HCl / KCN + H₂SO₄ ) (1 mark)
- Reaction Type 1: Nucleophilic addition (1 mark)
- Equation 1: CH₃CHO + HCN → CH₃CH(OH)CN (1 mark)
- Reagent 2: Identified dilute strong acid / H⁺ (e.g., HCl(aq) or H₂SO₄(aq) ) (1 mark)
- Condition 2: Heat under reflux (or reflux) (1 mark)
- Reaction Type 2: Hydrolysis (1 mark)
- Equation 2: CH₃CH(OH)CN + 2H₂O + H⁺ → CH₃CH(OH)COOH + NH₄⁺ (or using HCl producing NH₄Cl , or full balanced equation including water) (1 mark)
Note: The 7th mark is typically awarded for the complete second step equation or integrated stoichiometry.
🧠 Exam Technique
- Always state reagents clearly as a pair where necessary (e.g., KCN followed by dilute acid) since HCN is a toxic gas often generated safely in solution.
- Make sure "reflux" is explicitly stated for the hydrolysis step; writing just "heat" is often penalised or restricted to "warm" depending on strictness, but reflux ensures volatile components don't escape.
❌ Common Errors
- Writing contradictory reaction types like "nitrification" or "electrophilic addition".
- Forgetting to specify dilute acid or writing "conc. acid" which can cause unwanted dehydration or side reactions.
- Failing to link Step 2 correctly to the intermediate nitrile formed in Step 1 (marks for step 2 are dependent on a valid nitrile intermediate).
Type of Polymerisation
✅ Correct Answer
Condensation (polymerisation)
💡 Key Knowledge
Lactic acid is a monomer containing both a carboxylic acid group ( -COOH ) and an alcohol group ( -OH ). When these react across multiple molecules, an ester link is formed with the elimination of a small molecule ( H₂O ), which is the definition of condensation polymerisation (specifically making a polyester).
Identifying the Repeat Unit
✅ Correct Answer
A circle drawn enclosing one full repeating unit of the polymer chain in the displayed formula. Typically: -O-CH(CH₃)-CO-
🧠 Exam Technique
Carefully count the atoms between repeating segments in the polymer chain. For polyesters formed from hydroxy acids, look for the ester linkage -COO- or -O-CO- backbones and isolate the monomer residue unit accurately without missing terminal bonds.
❌ Common Errors
Drawing the circle too large (enclosing two units) or too small (cutting through functional groups midway), which fails to isolate the exact minimal repeat unit.
Topics
Organic Chemistry · Topic 17: Organic Chemistry II · Topic 18: Organic Chemistry III
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.