OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 20

✅ Correct Answer

A clearly labelled diagram showing:

• A hydrogen bond represented by a dashed or dotted line between the lone pair on a oxygen atom (either C=O or O-H) and a delta-plus hydrogen (H) attached to oxygen.
• Explicit statement or labeling indicating the presence of a hydrogen bond.
• Origin of the hydrogen bond must stem from a lone pair on a delta-minus oxygen and point to a delta-plus hydrogen.

💡 Key Knowledge

• Short-chain carboxylic acids are soluble because they form hydrogen bonds with polar water molecules.
• Both the carbonyl oxygen (C=O) and hydroxyl group (-OH) can act as hydrogen bond acceptors/donors with water.

❌ Common Errors

• Drawing a hydrogen bond originating from a hydrogen atom attached to a carbon (e.g., from the C-H of HCOOH). This loses all marks instantly.
• Failing to label the hydrogen bond or missing partial charges (delta+ and delta-).
• Pointing the hydrogen bond directly to the oxygen atom's nucleus instead of its lone pair.

✅ Correct Answer

ethyl 4-methylpentanoate (Accept hyphenated or unhyphenated variants, e.g., ethyl4-methylpentanoate ).

🧠 Exam Technique

• Split the ester name into two parts: the alkyl group attached to the oxygen ( ethyl ) and the carboxylate chain derived from the carboxylic acid ( pentanoate ).
• Number the longest carbon chain starting from carbon-1 of the ester carbonyl carbon. Locate the methyl branch at carbon-4.

❌ Common Errors

• Writing 1-ethyl-4-methylpentanoate (incorrect numbering for the ethyl chain).
• Spelling errors like methy , ethly , or combining words incorrectly.

✅ Correct Answer

Step 1 Reagent: SOCl₂ (or PCl₅ , PCl₃ )

Step 1 Equation:
(CH₃)₂CHCH₂CH₂COOH + SOCl₂ → (CH₃)₂CHCH₂CH₂COCl + SO₂ + HCl

Step 2 Equation:
(CH₃)₂CHCH₂CH₂COCl + CH₃CH₂OH → (CH₃)₂CHCH₂CH₂COOCH₂CH₃ + HCl

💡 Key Knowledge

• Carboxylic acids are converted to more reactive acyl chlorides using chlorinating agents like thionyl chloride ( SOCl₂ ).
• Acyl chlorides react smoothly and rapidly with alcohols to form esters and hydrogen chloride gas without requiring an acid catalyst or equilibrium setup.

❌ Common Errors

• Omitting side products ( SO₂ , HCl ) when balancing equations.
• Using an incorrect alcohol chain length in Step 2; Ester F contains an ethyl ester group derived from ethanol ( CH₃CH₂OH ).

✅ Correct Answer

The two required monomers are:

• Benzene-1,4-dicarboxylic acid: HOOC-C₆H₄-COOH (or its corresponding diacyl chloride / dianhydride derivative).
• Ethane-1,2-diol: HO-CH₂CH₂-OH (or the 4-carbon chain shown in the diagram core: HO-CH₂CH₂CH₂CH₂-OH butane-1,4-diol depending on exact repeat unit alignment).

🧠 Exam Technique

• "Slice" the ester linkage ( -COO- ) within the repeat unit skeleton.
• Add an -OH group back to the carbonyl carbon ends to form the dicarboxylic acid, and add -H back to the oxygen ends to form the diol monomers.

❌ Common Errors

• Leaving open bonds ( - ) on the ends of the monomer structures instead of capping them with -H or -OH functional groups.

✅ Correct Answer

Products formed from the complete alkaline hydrolysis of the bi-functional compound:

• Methanol / Methoxide ion: CH₃OH (from the ester cleavage).
• Carboxylate salts: Both the original ester and amide groups are cleaved/converted under strong alkaline conditions. The ester converts to a carboxylate salt ( -COO⁻ Na⁺ ) and the cyclic/amide structure opens or hydrolyses to form the corresponding amino/carboxylate species.

💡 Key Knowledge

• Under alkaline conditions ( NaOH(aq) ), ester hydrolysis yields carboxylate salts and alcohols rather than free carboxylic acids due to the presence of OH⁻ .
• Amides also undergo base hydrolysis upon prolonged heating with strong alkali, yielding carboxylate salts and amines/ammonia derivatives.

❌ Common Errors

• Drawing neutral carboxylic acid groups ( -COOH ) instead of carboxylate sodium salts ( -COO⁻ Na⁺ or -COONa ) under alkaline conditions.
• Failing to hydrolyse both functional groups present in the molecule.