OCR A-Level Chemistry AS Depth in chemistry (02), June 2023: Question 5

✅ Correct Answer

3-methylpentan-2-ol

💡 Key Knowledge

• Find the longest continuous carbon chain containing the principal functional group (the -OH group). Here, it is 5 carbons long (pentan).
• Number the chain from the end that gives the principal functional group the lowest possible number (carbon 2).
• Identify substituents and their positions (methyl group at carbon 3).

❌ Common Errors

• Incorrect numbering giving 2-methylpentan-3-ol or 3-methylpentan-4-ol .
• Incorrect spelling of prefixes/suffixes (e.g., missing hyphens or commas). Examiners are lenient on hyphens/commas, but strict on number locants.

✅ Correct Answer

Structural or skeletal equation showing alcohol A losing water ( H₂O ) to form alkene D ( CH₃CH₂CH=CHCH₃ or CH₃CH₂CH₂CH=CH₂ depending on isomer, yielding 3-methylpent-2-ene or 3-methylpent-1-ene ) plus H₂O .

🧠 Exam Technique

• Ensure formulas represent valid chemical structures. Skeletal, structural, or displayed formulas are all accepted as long as connectivity is unambiguous.
• Do not include catalysts like H⁺ or [O] inside the balanced equation as reactants unless asked; keep it strictly as reactant yielding organic product + H₂O .

✅ Correct Answer

• Priority groups are on the same side of the double bond.
• High(est) priority groups are -CH₂CH₃ (ethyl) and -CH₃ (methyl) [OR low priority groups are -CH₃ and -H ].

💡 Key Knowledge

• CIP Rules: Assign priorities based on atomic number attached directly to the carbon atoms of the C=C double bond. Higher atomic number = higher priority.
• Z (Zusammen): Higher priority groups are on the same side (zame side) of the double bond.
• E (Entgegen): Higher priority groups are on opposite sides.

❌ Common Errors

• Vague answers referencing "heavier groups" or "larger molecules" without specifying atoms attached directly to the C=C carbons.
• Confusing E and Z assignments.

✅ Correct Answer

Polymer section (two repeat units):
A carbon-carbon single bond backbone with alternating side chains containing -CH₂OH and -H groups, with clear, open continuation bonds at both ends.

Reason for solubility:
The -OH (alcohol) groups form hydrogen bonds with water molecules.

🧠 Exam Technique

• Watch the repeat unit count! The question explicitly asks for two repeat units. Drawing only one unit will lose structural credit.
• Continuation bonds must extend outside the brackets or past the terminal carbons (solid or dotted lines cutting through the brackets/ends).
• For intermolecular forces, always state what forms bonds with what (e.g., "-OH groups form hydrogen bonds with water"). Avoid shorthand like "it forms hydrogen bonds" without naming the species involved.

❌ Common Errors

• Leaving out continuation bonds at the ends of the polymer chain.
• Writing -HO instead of -OH for the alcohol group connection.

✅ Correct Answer

Any two of the following:

• Recycled (to make other plastic materials / melted and reshaped).
• Combustion to generate energy / electricity (used as a fuel).
• As an organic feedstock (for cracking / producing chemicals or plastics).

💡 Key Knowledge

• Disposing of polyalkenes is challenging due to their unreactivity (non-polar C-C and C-H bonds).
• Useful chemical processing strategies recover value from waste plastics rather than sending them straight to landfill.