Part 1 of 6Key Conversions
A synthesis route is a sequence of reactions that converts a starting material into a target compound.
Alkane to haloalkane
- Chlorine or bromine with UV light substitutes a hydrogen atom by a halogen.
Alkene to haloalkane
- Hydrogen halide adds across the C=C bond at room temperature.
Alkene to alcohol
- Steam with a phosphoric acid catalyst adds water across the double bond.
Haloalkane to alcohol
- Warming with aqueous sodium hydroxide substitutes OH for the halogen.
Alcohol to alkene
- Heating with concentrated sulfuric or phosphoric acid dehydrates the alcohol.
Part 2 of 6More Conversions
More Conversions
Primary alcohol to aldehyde
- Distil with acidified potassium dichromate to remove the aldehyde as it forms.
Primary alcohol to acid
- Reflux with excess acidified potassium dichromate to give the carboxylic acid.
Secondary alcohol to ketone
- Heat with acidified dichromate to give the ketone.
Haloalkane to nitrile
- Heating with ethanolic KCN adds a carbon atom and gives a nitrile.
Haloalkane to amine
- Heating with excess ammonia in ethanol gives a primary amine.
| From | To | Reagents and conditions |
|---|---|---|
| Haloalkane | Alcohol | NaOH(aq), heat |
| Alkene | Alcohol | Steam, H₃PO₄, 300 °C, 60–70 atm |
| Alcohol | Alkene | Conc. H₂SO₄ or H₃PO₄, heat |
| Primary alcohol | Carboxylic acid | K₂Cr₂O₇/H₂SO₄, reflux |
| Haloalkane | Nitrile | KCN in ethanol, reflux |
Part 3 of 6Planning a Route
1
Identify the functional group in the starting material and in the target compound.
2
Work backwards from the target to find a reaction that makes that functional group.
3
Continue until you reach the starting material, listing reagents and conditions for each step.
4
Check that each intermediate is correct, and that conditions such as reflux or distillation are right for the product.
5
Write balanced equations for each step if required, using [O] for oxidations.
Part 4 of 6Overall Yield
Lost material
- Each step loses some product through side reactions, incomplete reactions and losses during purification.
Overall yield
- The overall percentage yield is found by multiplying the fractional yields for each step.
Many steps
- A long route with several steps can have a low overall yield even if each step is efficient.
Atom economy
- Routes with fewer steps and addition reactions waste fewer atoms and are often chosen in industry.
Part 5 of 6Worked Example
Question Suggest a two-step route from 1-bromopropane to propanoic acid, and find the overall yield if step 1 gives 80% and step 2 gives 70%.
- Step 1: heat 1-bromopropane with aqueous NaOH to give propan-1-ol: CH₃CH₂CH₂Br + OH⁻ → CH₃CH₂CH₂OH + Br⁻.
- Step 2: heat propan-1-ol under reflux with acidified potassium dichromate to give propanoic acid: CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O.
- Overall fractional yield = 0.80 × 0.70 = 0.56.
- Convert to a percentage.
Answer Overall yield = 56%
Part 6 of 6Exam Checklist & Tips
4.6.2 – Key Points for the Exam
- Synthetic route A sequence of reactions that converts a starting material into a target compound.
- Reflux Heating with a condenser so that volatile liquids return to the reaction flask.
- Percentage yield Actual yield divided by theoretical yield, expressed as a percentage.
- Intermediate A compound formed in one step of a synthesis and used in the next.
Common Mistakes
- Giving 'heat' without naming the reagents, which loses marks in synthesis questions.
- Confusing distillation and reflux for oxidation of primary alcohols.
- Adding yields from separate steps instead of multiplying them.
- Choosing a reaction that fails for the functional group present, such as oxidising a tertiary alcohol.