OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 19

✅ Mark Scheme Breakdown (3 Marks)

• Mark 1: Curly arrow starting from the lone pair on the C of :CN⁻ (or from the negative charge on C) pointing to the electron-deficient carbon atom of the C–Br bond.
• Mark 2: Dipoles correctly labelled on the polar bond (C^δ+—Br^δ−) AND a curly arrow from the C–Br bond going directly to the Br atom.
• Mark 3: Correct organic product structure ((CH₃)₂CHCN or skeletal equivalent with attached –CN group) AND the inorganic leaving group shown as Br⁻.

🧠 Mechanism Drawing Guide

Exactly how to draw the S_N2 mechanism:

• Start with the given skeletal structure of 2-bromopropane: add δ+ above the central carbon and δ− above the attached Br atom.
• Draw the cyanide ion as :C⁻≡N (or :CN⁻). Ensure the lone pair or negative charge is clearly positioned on the Carbon atom, not Nitrogen.
• Draw Arrow 1 starting exactly from the lone pair on :C⁻ and pointing directly to the central C^δ+.
• Draw Arrow 2 starting from the middle of the C–Br bond and pointing directly to the Br atom.
• Draw the products: skeletal structure with –CN replacing –Br (2-methylpropanenitrile) plus Br⁻ with its negative charge.

❌ Common Errors to Avoid

• Wrong atom on CN⁻: Starting the curly arrow from Nitrogen or drawing the charge on N. Cyanide acts as a nucleophile via its carbon lone pair to extend the carbon chain!
• Imprecise arrows: Arrow starting in blank space rather than directly from the lone pair/negative charge, or missing the C atom.
• Missing the halide ion: Writing "KBr" as a product instead of showing the free ion Br⁻.
• Arrow originating from Br: The second arrow must start from the bond pair between C and Br, never from the Br atom itself.

💡 Alternative S_N1 Mechanism Accepted

The mark scheme also allows an S_N1 route because 2-bromopropane is a secondary haloalkane:

• Step 1: Dipole on C^δ+—Br^δ− with arrow from bond to Br, forming a secondary carbocation (CH₃)₂C⁺H + Br⁻.
• Step 2: Arrow from lone pair on :CN⁻ to the carbocation C⁺ atom, yielding the nitrile.

✅ Correct Answer

Nucleophilic substitution

Both words are required for the 1 mark.

💡 Why is it classified this way?

• Nucleophilic: The attacking species (:CN⁻) donates an electron pair to an electron-deficient carbon.
• Substitution: The bromide atom (–Br) is replaced by the nitrile group (–CN).

✅ Correct Reagents

NaBr AND H₂SO₄

Also accepted:

• KBr AND H₂SO₄
• HBr (formed in situ by reacting NaBr with acid)
• PBr₃
• H₃PO₄ in place of H₂SO₄
• Full chemical names: sodium bromide and sulfuric acid

❌ Common Reagent Traps

• Writing just "H⁺" or "acid": The identity of the acid must be specific (e.g., H₂SO₄).
• Using HCl: You cannot use hydrochloric acid as it would introduce chloride ions, competing to form 1-chloropropane.
• Omitting the acid: Writing only "NaBr" or "KBr" scores 0 because the halide salt alone is unreactive toward alcohols without an acid catalyst/proton source.

✅ Step-by-Step Practical Sequence (4 Marks)

• 1 Separation: Place the mixture into a separating funnel.
• 2 Collect bottom layer: Collect the lower / bottom organic layer (since 1-bromopropane has a density of 1.35 g cm⁻³, which is denser than the aqueous layer at 1.04 g cm⁻³).
• 3 Drying: Add an anhydrous drying agent (specifically anhydrous MgSO₄ or CaCl₂ ) until the liquid turns from cloudy to clear.
• 4 Distillation: (Re)distil the dried liquid and collect the pure fraction at its boiling point of 71 °C (allowable range: 70–72 °C).

❌ Major Traps Identified by the Examiner

• Describing recrystallisation: If you mention dissolving in minimum hot solvent, filtering, or cooling to form crystals, zero marks are awarded for subsequent steps. 1-Bromopropane is a liquid!
• Identifying the wrong layer: The question gives densities: organic (1.35 g cm⁻³) vs aqueous (1.04 g cm⁻³). The denser organic layer is at the bottom, not the top.
• Vague drying agents: Simply stating "salt" or naming soluble salts like NaCl / CuSO₄ will not score. It must be named as an anhydrous inorganic salt (e.g. anhydrous MgSO₄ or CaCl₂).
• Missing the collection temperature: Stating "distil" without linking it to the data table boiling point (71 °C) misses the final purification mark.