Edexcel A-Level Chemistry Paper 2, June 2023: Question 7

✅ Correct Answer Breakdown (4 Marks)

• 1 Mark: Both ethanal and ethanoic acid can form hydrogen bonds with water.
• 1 Mark: Diagram shows hydrogen bonding clearly between ethanal (O atom) and water (H-O), with a linear or near-linear bond angle (approx. 180°).
• 1 Mark: Ethanal has a lower boiling temperature because it only possesses weak London forces and permanent dipole-dipole forces between molecules.
• 1 Mark: Ethanoic acid has a much higher boiling temperature because it forms intermolecular hydrogen bonds, which require significantly more energy to overcome.

💡 Key Knowledge

Hydrogen bonding requires an H atom covalently bonded to a very electronegative atom (N, O, or F) interacting with a lone pair on F, O, or N. Ethanoic acid can form extensive hydrogen-bonded dimers in the liquid state, effectively doubling its molecular mass influence on boiling points.

🧠 Exam Technique

When drawing hydrogen bonding, ensure your dotted line goes from a lone pair on oxygen to the hydrogen atom attached to oxygen. Label the O-H...O angle at approximately 180° to secure examiner marks.

❌ Common Errors

Students often incorrectly draw a hydrogen bond originating from the carbon-bonded hydrogen atoms of ethanal, or omit partial charges (delta positive/negative) on the carbonyl group.

✅ Correct Answer Breakdown (4 Marks)

• 1 Mark: Curly arrow from the lone pair on the carbon of the cyanide ion ( CN⁻ ) to the carbonyl carbon ( C=O ).
• 1 Mark: Dipole on C=O (or text indicating attack) and a curly arrow from the C=O double bond breaking towards oxygen, resulting in a negative charge on oxygen.
• 1 Mark: Correct intermediate structure showing the tetrahedral arrangement with an O⁻ , an H , a CN group, and the alkyl chain ( C₂H₅ ).
• 1 Mark: Curly arrow from the lone pair on the oxygen of the intermediate (or from O⁻ ) to the hydrogen of HCN , with a second curly arrow breaking the H-CN bond to regenerate CN⁻ .

🧠 Exam Technique

Mechanism arrow heads are heavily scrutinised. The arrow starting from a lone pair must begin directly on the pair (or negative charge) and point squarely at the atom receiving the electrons. Double-bond breaking arrows must start on the bond line itself.

✅ Correct Answer Breakdown (3 Marks)

• 1 Mark: State that a racemic mixture will form because the carbonyl group ( C=O ) and adjacent intermediate structure are planar.
• 1 Mark: The cyanide nucleophile ( CN⁻ ) can attack from either above or below the plane with equal probability.
• 1 Mark: This results in equal amounts (a 50:50 mixture) of the two enantiomers being produced.

❌ Common Errors

Candidates frequently lose marks by stating the whole propanal molecule is planar rather than specifically referring to the trigonal planar geometry around the C=O carbon atom.

✅ Correct Answer Breakdown (3 Marks)

• Step 1 (1 Mark): Filter under suction (using a Büchner funnel and flask).
• Step 2 (1 Mark): Recrystallise the solid (dissolve in a minimum volume of hot solvent, filter hot if necessary, cool to recrystallise, and filter again).
• Step 3 (1 Mark): Dry the solid (using a warm oven, a desiccator, or between sheets of filter paper/suction drying).

❌ Common Errors

Do NOT suggest using a drying agent (like anhydrous sodium sulfate) for a solid derivative—drying agents are strictly used for organic liquids. Also, avoid mentioning "heat to constant mass," which applies to water of crystallisation titrations, not organic melting point prep.

✅ Correct Answer Breakdown (2 Marks)

• 1 Mark: Identity of X is cyclohexanone.
• 1 Mark: Justification: The melting temperature range of its 2,4-DNPH derivative (156–158 °C) matches the experimental value (156–158 °C), AND the IR absorption at 1717 cm⁻¹ confirms the presence of a carbonyl ( C=O ) group within the standard range (1700–1720 cm⁻¹ for alkyl ketones).

✅ Correct Answer Breakdown (2 Marks)

• 1 Mark: Correctly identify the benzene ring protons and the nitrogen-attached proton environment.
• 1 Mark: Correctly identify the carbon-chain alkyl protons ( -CH₂- and -CH₃ groups).

💡 Examiner Guidance

On the provided structure, clear labeling showing distinct groups (such as numbers 1-6 or letters A-F) is accepted. Ensure you do not circle the nitrogen atom itself when labeling the adjacent -NH- proton environment.