Organic Synthesis

Full Notes

A full revision booklet for all the organic reactions you need to know for AQA A-level chemistry can be found here and mechanisms here

Making substances in organic chemistry often requires many reaction steps and the overall route used to make a given product is referred to as a ‘synthesis’.

There is often more than one possible synthesis route to make a given compound and organic chemists try to carefully design the synthesis route, paying particular attention to the following.

Minimising the Use of Solvents

• Solvents are often volatile organic compounds (VOCs) that contribute to pollution.
• They increase waste and energy costs.
• Alternative approaches:
  • Use solid-state reactions instead of solvent-based reactions.
  • Use water or ethanol as green solvents when necessary.

Using Non-Hazardous Starting Materials

• Many organic reactants (e.g., benzene, halogenoalkanes) are toxic or carcinogenic.
• Safer alternatives reduce risks to chemists and the environment.
• Example: Using CO₂ as a raw material instead of hazardous carbon monoxide.

Reducing Steps and Increasing Atom Economy

• Each reaction step in a synthesis:
  • Increases time and cost.
  • Reduces yield due to losses in each stage.
  • May generate waste byproducts.
• Higher atom economy means:
  • More of the reactants become useful products.
  • Less waste is produced.

Example: High Atom Economy — Hydrogenation of ethene to ethanol (100% atom economy). C₂H₄ + H₂O → C₂H₅OH

Designing a Multi-Step Synthesis

• A synthesis must consider:
  • Functional group transformations.
  • Suitable reagents and conditions.
  • Reaction mechanisms.
  • Yield and purification.

Matt’s exam tip

synthesis questions can look overwhelming! Focus on one thing at a time and remember that no matter how complicated the molecules may look, the functional group conversions will only be ones you have seen and learnt about before. Focus on the functional groups in the molecules that are changing, rather than focusing on the whole molecules overall.

Ex

Worked Example: Synthesising Ethylamine (C₂H₅NH₂) from Ethene (C₂H₄)

1. Hydration of Ethene to Ethanol
   Reagents: Steam, H₃PO₄ catalyst
   C₂H₄ + H₂O → C₂H₅OH
2. Conversion of Ethanol to Bromoethane
   Reagents: HBr
   C₂H₅OH + HBr → C₂H₅Br + H₂O
3. Nucleophilic Substitution with Ammonia
   Reagents: Excess NH₃
   C₂H₅Br + NH₃ → C₂H₅NH₂ + HBr

Key Considerations: Minimising steps increases efficiency. Choosing high-yielding reactions improves practicality.

Summary

• Organic synthesis puts together functional-group transformations to make a target molecule efficiently.
• Green design aims to avoid solvents where possible, use safer starting materials, reducing the number of steps and maximise atom economy.
• Every extra step adds cost, time, and yield loss; pick routes with high-yield, high atom-economy reactions.
• Know the AQA required reactions and mechanisms so you can propose 2–4 step routes in exams.