36.1 Organic synthesis
- Syllabus
- 9701–2028–2029
- Topic
- 36.1
- Level
- A2
Learning objectives
36.1.1For 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 reactions36.1.2Devise multi-step synthetic routes• Devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus36.1.3Analyse• Analyse a given synthetic route in terms of type of reaction and reagents used for each step of it, and possible by-products
Audit every functional group before predicting a multifunctional molecule's chemistry
For a molecule containing several functional groups: 1) mark every group and its exact location; 2) name the reaction or test from the syllabus that identifies each one; 3) predict physical/acid-base properties from all groups together; 4) test each proposed reagent against every group, not only the intended site.
| Audit question | What it prevents |
|---|---|
| Which atoms define each functional group? | confusing an ester with separate ether and ketone groups |
| What observation or product identifies it? | naming a group without reaction evidence |
| Which groups are acidic, basic, polar or hydrogen-bonding? | predicting properties from only one group |
| Which groups react under the chosen conditions? | ignoring competing oxidation, reduction, hydrolysis or acid-base reactions |
A molecule containing both C=C and a primary alcohol can decolourise bromine water by addition at C=C and can be oxidised at the alcohol. If an oxidant is used, first check whether the alkene or any other group is also susceptible under those conditions.
Do not assign one overall functional group or assume a reagent is perfectly selective. Predictions must use the exact structure, reagent, conditions and all other groups present.
Plan a multi-step synthesis backwards, then validate it forwards
Retrosynthesis: 1) compare start and target carbon skeletons; 2) choose the final syllabus transformation and immediate precursor; 3) work backwards until the start is reached. Forward validation: 4) write every intermediate; 5) give reagent and condition at each arrow; 6) check carbon count, oxidation level, selectivity and survival of other functional groups.
| Required skeleton change | Useful syllabus route | Carbon accounting |
|---|---|---|
| add one carbon and form a carboxylic acid | halogenoalkane -> nitrile with CN-, then hydrolyse nitrile | nitrile carbon becomes carboxyl carbon |
| add one carbon and form a primary amine | halogenoalkane -> nitrile, then reduce nitrile | nitrile carbon becomes CH2 bonded to NH2 |
| keep carbon count but change oxidation level | alcohol <-> aldehyde/ketone -> carboxylic acid, or carbonyl reduction | no carbon inserted or removed |
Example: 1-bromopropane -> butanoic acid. Step 1: heat/reflux with KCN in ethanol to form butanenitrile by nucleophilic substitution (the chain gains the CN carbon). Step 2: heat under reflux with dilute aqueous acid to hydrolyse the nitrile to butanoic acid.
A short route is not valid unless each arrow is a syllabus reaction. Never jump directly between structures, lose the nitrile carbon, or choose conditions that destroy another required group without a protection strategy supplied by the syllabus.
Analyse every synthetic arrow as a complete evidence row
For each arrow, compare the structures immediately before and after, then record: functional-group change, reaction type, reagent, condition, intended product and possible by-product. Only after every row is complete should the route be judged for feasibility or efficiency.
| Field | Evidence to extract |
|---|---|
| structural change | bonds/groups gained, lost or changed; carbon count |
| reaction type | addition, elimination, substitution, oxidation, reduction, hydrolysis, condensation or acid-base step |
| reagent and condition | concentration, solvent, temperature/reflux/distillation and catalyst where specified |
| by-product or competing product | H2O/HCl/salt, further oxidation, elimination versus substitution, or over-substitution where applicable |
Primary alcohol -> aldehyde: oxidation with acidified K2Cr2O7 and distil the aldehyde as it forms; reflux would favour further oxidation to the carboxylic acid, a possible competing product. Acyl chloride + alcohol -> ester: addition-elimination/condensation at room temperature with HCl formed.
Do not classify from a reagent name alone. The same reagent under different conditions can produce a different outcome, and a correct reaction type without the correct reagent, condition or by-product is an incomplete route analysis.