OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 16

✅ Correct Answer

B, C, D AND E only

💡 Key Knowledge

• Unsaturated hydrocarbons contain at least one carbon-carbon double bond, triple bond, or an aromatic ring (arenes).
• A is a cycloalkane (saturated, single C-C bonds only).

❌ Common Errors

Including hydrocarbon A by confusing ring structures with unsaturation. Remember: rings alone (alicyclic alkanes) are saturated unless they contain a double bond or aromatic delocalised system.

✅ Correct Answer

A AND B only

💡 Key Knowledge

• Alicyclic means arranged in a non-aromatic ring (aliphatic ring), with or without side chains.
• A (methylcyclopentane) and B (1-methylcyclohexene) are both ring structures that are not aromatic.
• C is an aromatic (benzene ring), D and E are straight/branched chains (aliphatic).

❌ Common Errors

Failing to distinguish between alicyclic rings and aromatic rings (arenes like C).

✅ Correct Answer

A AND D only

💡 Key Knowledge

• CₙH₂ₙ represents cycloalkanes (like A, cyclopentane derivative) and alkenes (like D, an alkene with one double bond).
• Check ring/bond indices: Cycloalkanes and alkenes both have 1 degree of unsaturation (or ring), giving the same general formula.

❌ Common Errors

Assuming CₙH₂ₙ only applies to alkenes and missing that cycloalkanes share the exact same general formula due to ring closure.

✅ Correct Answer

1-ethyl-2,4-dimethylbenzene

💡 Key Knowledge

• Base molecule is benzene.
• Number the ring carbons to give substituents the lowest possible locants (e.g., 1, 2, 4 rather than 1, 4, 5).
• List substituents alphabetically: ethyl comes before dimethyl (prefixes like di- are ignored for alphabetical ordering).

🧠 Exam Technique

Acceptable alternative numbering formats accepted by examiners include 1,3-dimethyl-4-ethylbenzene or 2,4-dimethylethylbenzene , provided the lowest locant rule is strictly followed. Avoid phonetic misspellings like "ethy" or "dimethy".

✅ Correct Answer

1. Priority groups are on the same side of the C=C double bond.
2. Highest priority groups are CH₃ and C₂H₅ (or lowest priority groups CH₃ and H are on the same side).

💡 Key Knowledge

• Z-isomer (zusammen): higher priority groups on the same side of the restricted rotation bond.
• Use CIP priority rules based on atomic number attached directly to the C=C carbons.

🧠 Exam Technique

To secure both marks, you must explicitly state both: (1) that the priority groups are on the same side, AND (2) identify which specific groups have higher/lower priority on each carbon of the double bond.

❌ Common Errors

Referring to "molecules" being on the same side instead of "groups" or "priority groups". Also, do not confuse CIP priority with relative molecular mass ( Mr ).