21.1 Organic synthesis
- Syllabus
- 9701–2028–2029
- Topic
- 21.1
- Level
- AS
Learning objectives
21.1.1For an organic molecule containing several• For an organic molecule containing several functional groups:: (a) identify organic functional groups using the reactions in the syllabus; (b) predict properties and reactions21.1.2Devise multi-step synthetic routes• Devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus21.1.3Analyse• Analyse a given synthetic route in terms of type of reaction and reagents used for each step of it, and possible by-products
Read every functional group before predicting a multifunctional molecule
First mark every functional-group connectivity in the structure. For each group, link only a reaction or property from the syllabus, then use the stated reagent and conditions to decide which group actually responds.
| Structural feature | Syllabus reaction evidence / likely behaviour |
|---|---|
| C=C | decolourises aqueous bromine by addition |
| halogenoalkane C–X | hydrolyses; released X⁻ forms AgX with aqueous AgNO₃/ethanol |
| primary/secondary alcohol | acidified dichromate oxidation, orange → green |
| aldehyde C=O | 2,4-DNPH positive and Tollens’/Fehling’s positive |
| ketone C=O | 2,4-DNPH positive but Tollens’/Fehling’s negative |
| –COOH | reacts with carbonate to release CO₂ |
Polar O–H and C=O groups create stronger intermolecular attractions than a hydrocarbon region and can raise boiling point; O–H groups can hydrogen bond with water. A larger non-polar carbon skeleton can still reduce water solubility, so consider the whole molecule rather than one label.
A molecule may satisfy more than one test, but the reagent selects the observed reaction. Do not make all groups react simultaneously or import an unlisted reaction to identify a familiar-looking structure.
Plan a multi-step synthesis backward, then verify it forward
- Compare target and starting structures: carbon count, functional groups and oxidation level. 2. Work backward one syllabus transformation at a time to a plausible precursor. 3. Reverse the sequence and attach exact reagents, solvent, catalyst, temperature and work-up. 4. Check carbon count and structure after every arrow.
Use KCN substitution only when a one-carbon chain extension is wanted. Use oxidation/reduction, addition, substitution, elimination or hydrolysis when the carbon skeleton should be retained; select apparatus such as distillation or reflux from the required oxidation endpoint.
| Step | Conversion | Reagent and conditions | Carbon count |
|---|---|---|---|
| 1 | bromoethane → propanenitrile | KCN in ethanol, heat | 2 → 3 |
| 2 | propanenitrile → propanoic acid | dilute acid, heat | 3 → 3 |
Starting from 1-bromopropane in step 1 would give four-carbon butanenitrile, not propanenitrile. A short route is useful only when every transformation and condition is supported by the syllabus.
Audit every synthetic arrow for reaction, conditions and by-products
For each arrow, record: starting functional group → product functional group; reaction type; complete reagent and conditions; intended organic product; and chemically required by-product(s). Then verify atoms, charge and any carbon-count change.
| Step | Functional-group change | Reaction type and conditions | By-product / coproduct |
|---|---|---|---|
| 1 | ethanol → bromoethane | substitution with HBr(g) | H₂O |
| 2 | bromoethane → propanenitrile | nucleophilic substitution, KCN in ethanol, heat | KBr |
| 3 | propanenitrile → propanoic acid | hydrolysis, dilute acid and heat | NH₄⁺ in acidic solution |
Possible by-products follow the chemistry: radical halogenation may continue to further substitutions; elimination may compete with substitution under ethanolic base/heat; an unsymmetrical addition may give a minor regioisomer. Name one only when the given substrate and conditions justify it.
A correct final product does not validate an impossible intermediate. Do not use PBr₃ or another unlisted reagent when an assessed route is required, and do not assign a generic HX by-product to every substitution.