AQA questions can ask you to devise a synthesis of up to four steps. A reliable method:1. Compare the starting material and the target: note the functional groups and count the carbon atoms.
2. Chain length: if the target has one more carbon, include a cyanide step, KCN with a haloalkane (giving a nitrile) or with an aldehyde or ketone (giving a hydroxynitrile).
3. Work backwards from the target: which group could become the target group in one step? What makes that?
4. Link up the backward chain with the starting material.
5. For each step give the reagent(s), conditions and the name or structure of the intermediate.Worked example. Devise a synthesis of propylamine, CH3CH2CH2NH2, from ethene.
Compare: ethene has 2 carbons; the target has 3 carbons and an amine group, so a cyanide step is needed.
Backwards: RCH2NH2 comes from reducing a nitrile, so the previous compound is propanenitrile, CH3CH2CN. This comes from bromoethane and KCN, and bromoethane comes from ethene and HBr.
Forwards:
Step 1: ethene + HBr → bromoethane (electrophilic addition)
Step 2: bromoethane + KCN in aqueous ethanol, heat under reflux → propanenitrile
Step 3: propanenitrile + H2 with Ni catalyst (or LiAlH4) → propylamineMaking an amine directly from bromoethane with ammonia would give ethylamine, not propylamine, because the chain length would not change.Common pitfalls
• Missing conditions (aqueous or ethanolic, reflux or distil).
• Forgetting that an unsymmetrical alkene with HBr gives a major and a minor product.
• Using a reaction that is not in the specification.
2. Chain length: if the target has one more carbon, include a cyanide step, KCN with a haloalkane (giving a nitrile) or with an aldehyde or ketone (giving a hydroxynitrile).
3. Work backwards from the target: which group could become the target group in one step? What makes that?
4. Link up the backward chain with the starting material.
5. For each step give the reagent(s), conditions and the name or structure of the intermediate.Worked example. Devise a synthesis of propylamine, CH3CH2CH2NH2, from ethene.
Compare: ethene has 2 carbons; the target has 3 carbons and an amine group, so a cyanide step is needed.
Backwards: RCH2NH2 comes from reducing a nitrile, so the previous compound is propanenitrile, CH3CH2CN. This comes from bromoethane and KCN, and bromoethane comes from ethene and HBr.
Forwards:
Step 1: ethene + HBr → bromoethane (electrophilic addition)
Step 2: bromoethane + KCN in aqueous ethanol, heat under reflux → propanenitrile
Step 3: propanenitrile + H2 with Ni catalyst (or LiAlH4) → propylamineMaking an amine directly from bromoethane with ammonia would give ethylamine, not propylamine, because the chain length would not change.Common pitfalls
• Missing conditions (aqueous or ethanolic, reflux or distil).
• Forgetting that an unsymmetrical alkene with HBr gives a major and a minor product.
• Using a reaction that is not in the specification.