OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 9

✅ Correct Answer: A

Option A correctly identifies the two monomers:

• Diol: Ethane-1,2-diol (HO-CH₂-CH₂-OH) providing a 2-carbon chain.
• Dicarboxylic Acid: Butanedioic acid (HOOC-CH₂-CH₂-COOH) providing a 4-carbon chain between the carbonyl groups.

💡 Key Knowledge

• Polyesters are formed by condensation polymerization between a diol and a dicarboxylic acid (or a hydroxycarboxylic acid).
• An ester linkage is formed by the reaction: -COOH + HO- → -COO- + H₂O .
• To find monomers, split the repeat unit across the oxygen atoms of the ester bonds ( -O- and -CO- ) and add back -H and -OH respectively.

🧠 Exam Technique

• Count the carbon atoms in each section of the repeat unit between the functional groups before looking at the options.
• The diol section in the repeat unit has two carbon atoms ( -O-CH₂-CH₂-O- ), meaning the diol must be ethane-1,2-diol.
• The dicarboxylic acid section has four carbon atoms total between the carbonyl oxygens ( -OC-CH₂-CH₂-CO- ), pointing to butanedioic acid.

❌ Common Errors

• Counting chain lengths incorrectly: Students often miscount the number of -CH₂- units in the backbone, leading them to confuse propane-1,3-diol with ethane-1,2-diol.
• Confusing monomer types: Mistaking dicarboxylic acids for diols or selecting hydroxycarboxylic acid options when the repeat unit clearly originates from two separate symmetrical monomer components.