OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 18

✅ Mark Scheme Answers

1. Polymer of 1-chlorobut-2-ene (1 mark):

–[CH(CH₃)–CH(CH₂Cl)]–[CH(CH₃)–CH(CH₂Cl)]–

Backbone has 4 carbon atoms in a row with single C–C bonds. End extension bonds must trail off at both ends.

2. Polymer of 3-hydroxybutanoic acid (2 marks):

–[O–CH(CH₃)–CH₂–C(=O)]–[O–CH(CH₃)–CH₂–C(=O)]–

• 1 mark: Correct ester link shown (–C(=O)–O–).
• 1 mark: Exactly two complete repeat units with open end bonds.

3. Types of polymerisation (1 mark for both):

• 1-chlorobut-2-ene: Addition
• 3-hydroxybutanoic acid: Condensation

💡 Key Knowledge

• Addition Polymerisation: The C=C double bond breaks. The backbone consists only of the two carbons that formed the alkene double bond; everything else becomes a branch/substituent above or below.
• Condensation Polymerisation: Monomers join with the elimination of a small molecule (H₂O). Hydroxycarboxylic acids possess both an alcohol group (–OH) and a carboxylic acid group (–COOH), so they polymerise with themselves to form a polyester.
• "Two Repeat Units": Draw the monomer unit twice connected end-to-end. Do NOT stop after just one unit!

🧠 Exam Technique: How to Draw Repeat Units

1. Identify the reacting carbons: For the alkene, identify C2 and C3. Turn them into the backbone: –C–C–. Attach –H and –CH₃ to one carbon, and –H and –CH₂Cl to the other.
2. Duplicate it immediately: Connect an identical unit directly to it.
3. Leave open ends: Ensure single covalent bonds extend beyond the final carbons through any brackets or space to show open continuation bonds.

❌ Common Errors

• Drawing only ONE repeat unit: The question explicitly asked for two repeat units. Giving just one forfeits the mark.
• Putting extra carbons in the addition backbone: Forgetting to turn –CH₃ or –CH₂Cl into side branches, accidentally creating a long zigzag chain of all 4 carbons in the backbone.
• Writing "additional": The mark scheme specifically states: "DO NOT ALLOW 'additional'"—the correct term is strictly addition.

✅ Correct Answers

(b)(i) Incineration Challenge (1 mark):

Produces toxic / poisonous fumes / products (such as HCl / hydrogen chloride / chlorinated dioxins).

(b)(ii) Biodegradability Feature (1 mark):

It contains an ester group / ester bond / ester link (or is a polyester) which can be hydrolysed.

💡 Key Knowledge

• Halogenated polymers: Burning PVC or chlorine-containing plastics releases acidic, corrosive, and toxic hydrogen chloride (HCl) gas, requiring scrubbers (bases) to neutralise flue gases.
• Condensation vs. Addition polymer degradation:
  • Polyalkenes have non-polar, unreactive C–C backbones (non-biodegradable).
  • Polyesters/polyamides contain polar bonds (C^δ+=O and C^δ+–O) susceptible to nucleophilic attack by water or enzymes (biodegradable via hydrolysis).

❌ Common Errors & Examiner Guidance

• Too vague on environmental effects: Writing that incineration is "harmful", "polluting", "bad for the environment", or "causes global warming" scores 0 marks. You must state toxic/poisonous or name HCl.
• Wrong bond identified in (b)(ii): Mentioning "hydrogen bonding" or "C–C bonds" scores 0. Hydrolysis explicitly targets the ester linkage.

🧠 Exam Technique

When asked about chlorine-containing polymers in incineration questions, train your brain to write "produces toxic HCl gas". It guarantees the mark every time.

✅ Correct Structures

Product 1 (Ester):

CH₃CO–O–CH(CH₃)–CH₂–COOH

Ethanoic anhydride acylates the alcohol group (–OH) of 3-hydroxybutanoic acid to form an ethanoate ester.

Product 2 (By-product):

CH₃COOH

Ethanoic acid is formed as the co-product.

💡 Key Chemistry: Acid Anhydrides

Acid anhydrides react with alcohols to produce an ester and a carboxylic acid:

(RCO)₂O + R'OH → RCOOR' + RCOOH

• 3-hydroxybutanoic acid is bifunctional: it contains an alcohol (–OH) and a carboxylic acid (–COOH).
• The anhydride reacts with the nucleophilic alcohol (–OH) oxygen, leaving the –COOH group intact.

📐 Step-by-Step Reaction Breakdown

1. Identify the nucleophile: The –OH group at carbon-3 has lone pairs on oxygen and attacks the carbonyl carbon of ethanoic anhydride.
2. Cleave the anhydride: One acetyl group (CH₃CO–) is attached to the oxygen:
   HO–CH(CH₃)... becomes CH₃COO–CH(CH₃)–CH₂–COOH.
3. Determine the leaving group: The remaining fragment CH₃COO⁻ picks up the proton (H⁺) lost from the alcohol group to generate CH₃COOH.

❌ Common Errors

• Reacting the wrong group: Attempting to react ethanoic anhydride with the –COOH group instead of the –OH group.
• Connectivity errors: Drawing oxygen linked to the wrong atom (e.g. writing –HO instead of –OH, or binding the acetyl group incorrectly).
• Missing the by-product: Writing H₂O instead of CH₃COOH. (H₂O is produced when acylating with a carboxylic acid, NOT with an anhydride!).