AQA A-Level Chemistry Paper 2, 2017: Question 4

✅ Correct Answer

2-hydroxyhexanenitrile

💡 Key Knowledge

• Priority rule: The nitrile group ( -C≡N ) takes precedence over the alcohol ( -OH ) group.
• Carbon numbering: The nitrile carbon is always C1.
• Chain length: Including the nitrile carbon, the longest continuous carbon chain has 6 carbons ( hexanenitrile ).
• The -OH group is treated as a prefix: 2-hydroxy.

❌ Common Errors

• Forgetting the nitrile carbon in the main chain: Naming it 2-hydroxypentanenitrile (only counting 5 carbons).
• Wrong suffix: Calling it hexan-2-ol-nitrile or hexanonitrile.
• Missing the 'e': Writing 2-hydroxyhexannitrile instead of 2-hydroxyhexanenitrile.

🧠 Exam Technique

Always draw out the skeletal or structural formula if you find condensed formulas tricky:

C6(H₃)-C5(H₂)-C4(H₂)-C3(H₂)-C2(H)(OH)-C1≡N

Notice how C1 is the nitrile carbon. Count total carbons carefully before picking the stem name!

✅ Correct Answer

• Pass plane-polarised light through separate samples / use a polarimeter. [1 mark]
• The enantiomers will rotate the plane of polarised light by equal angles in opposite directions. [1 mark]

💡 Key Knowledge

• Enantiomers share identical physical properties (boiling point, density, solubility, refractive index).
• The only non-biological ways they differ are:
  1. Their interaction with plane-polarised light.
  2. Their reaction with other chiral molecules.

❌ Examiner Pitfalls & Guidance

• "Different directions" is NOT enough: The mark scheme specifically states "not different alone". You must state opposite directions (one clockwise, one anticlockwise).
• Missing "plane": Stating just "polarised light" is often allowed, but writing "light" alone gets 0 marks. Always write plane-polarised light.
• No mention of rotation: You must state that they rotate the light, not just "absorb" or "refract" it.

🧠 Model Phrasing

"Pass plane-polarised light through each sample; one enantiomer will rotate the plane of polarisation clockwise, and the other will rotate it by the same amount anticlockwise (in opposite directions)."

✅ 3-Step Marking Breakdown

1. Mark 1: The planar carbonyl group (or planar C=O group).
2. Mark 2: Nucleophile (CN⁻) can attack from either side (above or below the plane).
3. Mark 3: With equal probability (producing equal amounts / a 50:50 mixture of the two enantiomers).

💡 Mechanism Context

• The carbonyl carbon is sp² hybridised with trigonal planar geometry (120° bond angles around the C=O group).
• Because the aldehyde is unsymmetrical (R-CHO where R ≠ H), attack from the top face forms one enantiomer, while attack from the bottom face forms its non-superimposable mirror image.
• Since neither side is sterically hindered over the other, attack from either face is equally likely.

❌ Common Errors & Mark Losses

• Saying "planar molecule": The rest of the molecule is tetrahedral and NOT planar! Stating "the molecule is planar" loses Mark 1 immediately. You must specify the planar C=O group or planar carbonyl group.
• Saying "planar bond": A bond is linear, not planar.
• Vague probabilities: Writing "it can attack both sides" without adding that attack occurs with equal probability or gives equal amounts / 50:50 mixture forfeits Mark 3.

🧠 Exam Golden Rule for "Explain Racemic Mixture"

Always hit these three precise trigger phrases:

1. "Planar C=O group"

2. "Attack from either side / above or below"

3. "With equal probability / forming equimolar amounts"

✅ Correct Answer

Structure: 3-hydroxypentane-3-nitrile

(Skeletal or displayed formula showing central C bonded to two ethyl groups, an -OH, and a -CN). [1 mark]

Justification:

• The product does not contain a chiral centre / no asymmetric carbon atom.
• OR The central carbon is not attached to 4 different groups (it has two identical ethyl groups, -CH₂CH₃). [1 mark]

💡 Chemical Reasoning

• Formula of pentanal: C₅H₁₀O.
• Its isomers include aldehydes and ketones with 5 carbons.
• To produce a non-stereoisomeric (achiral) hydroxynitrile, the starting carbonyl must be a symmetrical ketone!
• Symmetrical 5-carbon ketone = pentan-3-one (CH₃CH₂COCH₂CH₃).
• When CN⁻ attacks pentan-3-one, the central carbon is attached to:
  • -OH
  • -CN
  • -CH₂CH₃
  • -CH₂CH₃
• Because two attached groups are identical (two ethyl groups), the carbon is achiral.

❌ Common Errors

• Drawing the starting carbonyl instead of the product: The question asks to "Draw the structure of the compound formed" (the hydroxynitrile), NOT pentan-3-one itself. (Note: If pentan-3-one is drawn, M2 is allowed only if described as a symmetrical ketone, but M1 is lost!).
• Drawing pentan-2-one derivative: Attaching CN to pentan-2-one gives a carbon with -CH₃ and -CH₂CH₂CH₃ (still 4 different groups, so still chiral!).
• Imprecise justification: Saying "it's planar" or "it cancels out" gets 0 marks for justification. Explicitly mention the lack of 4 different groups or the presence of two identical ethyl groups.

🧠 Diagram Description for Skeletal Drawings

If drawing a skeletal formula for the product:

Draw a horizontal 5-carbon zigzag chain. From the central carbon (carbon 3), add a bond going straight up to an -OH group, and a bond going straight down to a -C≡N group.