Planning a synthetic route
Planning a synthetic route
- A target molecule is built in several steps.
- The trick is to work backwards from the target.
- You also analyse a given route step by step.
Planning a route
- compare the target with the starting material — what changed (the group, the number of carbons)?
- work backwards: which single reaction could make the target, and from what?
- repeat until you reach the starting material.
- write each step with its reagent and conditions.
- To add a carbon, the KCN step is the key (the only chain-lengthening AS reaction).
Practice
The best way to plan a multi-step synthesis is to:
Working backwards from the target finds which reaction makes it, and from what, step by step.
Practice
If your route needs to add a carbon atom, the key step is:
KCN is the only AS reaction that lengthens the carbon chain.
Analysing a route
- For each step, state the type of reaction (oxidation, reduction, substitution, addition, elimination) and the reagent.
- Think about by-products — e.g. making an amine from a halogenoalkane also gives further-substituted amines, so the yield of the simple amine is low.
Practice
When analysing each step of a given route, you should state:
Identify the reaction type (oxidation, substitution, etc.) and the reagent/conditions for each step.
Practice
Why is the yield of a simple amine from a halogenoalkane low?
The amine is itself a nucleophile, so it reacts again — a by-product mixture lowers the yield.
You've got it
Key idea
- plan by working backwards from the target (compare groups and carbon count)
- write each step's reagent + conditions; use KCN to add a carbon
- to analyse a route, give each step's reaction type and reagent, and watch for by-products