📋 Structured Questions
Question 1: Recrystallisation and Purification
6 marks(a) Outline the steps required to recrystallise an impure sample of solid organic product (such as aspirin) to obtain pure crystals. [4]
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- Dissolve the impure solid in the minimum volume of hot solvent [1]
- Filter the hot solution through fluted filter paper (to remove insoluble impurities) [1]
- Cool the filtrate slowly (at room temperature, then in ice) to allow crystals to reform [1]
- Filter the pure crystals under reduced pressure using a Buchner funnel, wash with a minimum volume of ice-cold solvent, and dry (in a warm oven or desiccator) [1]
(b) Explain why washing the crystals with ice-cold solvent is necessary, and why the solvent must be ice-cold. [2]
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- Washing rinses off any soluble impurities remaining on the surface of the crystals [1]
- The solvent must be ice-cold to minimize the amount of purified solid product that dissolves, maximizing the final percentage yield [1]
Question 2: Designing a Multi-Step Synthesis
6 marks(a) Propose a two-step synthesis to prepare butan-1-amine starting from 1-bromopropane. Identify the intermediate compound, and state the reagents and conditions for both steps. [4]
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- Step 1 Reagent & Conditions: Potassium cyanide (KCN) in aqueous ethanol, heating under reflux [1]
- Intermediate Structure/Name: Butanenitrile (CH3CH2CH2CN) [1]
- Step 2 Reagent & Conditions: Hydrogen gas (H2) in the presence of a Nickel catalyst (or LiAlH4 in dry ether followed by dilute acid) [1]
- Equation/Conversion: Correct description or drawing showing the reduction of the nitrile to the primary amine (CH3CH2CH2CH2NH2) [1]
(b) Explain why potassium cyanide (KCN) is preferred over hydrogen cyanide (HCN) in laboratory syntheses of nitriles. [2]
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- Hydrogen cyanide (HCN) is a volatile, extremely toxic gas which is highly dangerous to handle in a typical school/college laboratory [1]
- Potassium cyanide (KCN) is a solid salt, making it safer to weigh, dissolve, and handle, while still providing a high concentration of nucleophilic CN- ions in solution [1]
Question 3: Distillation and Reflux Setup
4 marks(a) Contrast the setup of heating a reaction mixture under reflux with that of simple distillation. Describe when each technique is used. [3]
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- Reflux: The condenser is fitted vertically directly above the reaction flask, returning condensed vapors back into the flask. Used to carry out a reaction at its boiling point over a long period without losing volatile substances [1]
- Distillation: The condenser is fitted at an angle with a side-arm adaptor, directing the condensed vapor into a separate collection flask. Used to separate liquids with different boiling points, or to remove a volatile product as it is formed (e.g. distilling an aldehyde from a primary alcohol) [1]
- Condenser connection: In both cases, water enters the cooling jacket at the bottom (closest to the heat source) and exits at the top to ensure the jacket is fully filled and cooling is efficient [1]
(b) State one essential safety precaution that must be taken when assembling glass apparatus for heating under reflux or distillation. [1]
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Do not seal the apparatus (ensure the system is open to the atmosphere at the end of the condenser) to prevent pressure building up inside the glass, which could cause the apparatus to explode upon heating [1]
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