3.3 Advanced Organic Chemistry · Year 13
3.3.14 Organic Synthesis
Plan efficient multi-step routes using only the reaction chemistry required by AQA.
What you need to know
Open a line for a quick recap. If it feels obvious, move straight to the linked practice.
3.3.14 Explain why greener process design may avoid solvents and favour less hazardous starting materials. Quick revision
A greener route may reduce solvent use and avoid hazardous reagents because solvents and auxiliary chemicals can dominate waste and exposure even when they do not appear in the product equation.
When you compare two proposed routes, include the solvent and auxiliary chemicals in your judgement even though they may be absent from the balanced product equation. Their mass, toxicity and disposal can dominate the process.
3.3.14 Explain why fewer synthetic steps and higher atom economy can improve the efficiency/sustainability of manufacture. Quick revision
When you compare synthetic routes, every extra step can cost you reagent, solvent, energy, purification effort and yield. I’d therefore favour fewer high-yield, high-atom-economy steps when the rest of the process chemistry also makes sense.
If a four-step route gives 80% yield at every step, the overall yield is 0.8⁴ ≈ 41%. That is why cutting unnecessary steps can matter so much even when each individual reaction looks reasonably efficient.
3.3.14 Devise an organic synthesis of up to four steps using only reaction chemistry required elsewhere in the AQA course. Quick revision
I normally work backwards from the target functional group until I reach the starting material, choosing only transformations from this course. Then I reverse the route and write it forwards with reagent and conditions on every arrow.
Check the carbon skeleton at each step as well as the functional group. Nitrile formation adds a carbon, whereas many substitution/oxidation/reduction steps retain the carbon count; that often determines whether a proposed route can work.
Devise a route from chloroethane to N-propylethanamide in no more than four steps.
- CH₃CH₂Cl → CH₃CH₂CN using KCN in ethanol and heating under reflux. This adds one carbon.
- CH₃CH₂CN → CH₃CH₂CH₂NH₂ by reduction, for example H₂/Ni.
- React propylamine with ethanoyl chloride, CH₃COCl, to form the amide.
Answer CH₃CH₂Cl → CH₃CH₂CN → CH₃CH₂CH₂NH₂ → CH₃CONHCH₂CH₂CH₃.