21. Organic synthesis
- Syllabus
- 9701–2028–2029
- Section
- 21
- Level
- AS
Exam analysis
No tagged past-paper evidence yet
Published Concept pages under this syllabus area do not have tagged past-paper appearances in the selected level yet.
Recent 5 years
In this section
Topic 21.1
21.1 Organic synthesis
Objectives in this topic
Analyse each functional group before predicting a molecule with several groups
A molecule containing several functional groups can show more than one family of reactions. Identify each group from connectivity, then apply the appropriate reagent and conditions to predict properties or products.
Functional groups can interact: steric hindrance, intramolecular hydrogen bonding and reaction conditions may change which group reacts first. State assumptions instead of treating the molecule as a list of independent labels.
A hydroxycarboxylic acid contains both –OH and –COOH: it can react with sodium at O–H sites, neutralise base at –COOH, and form an ester under acid catalysis.
Do not predict every possible reaction simultaneously. The question’s reagent and conditions select the relevant functional group.
Plan a synthesis by working backward from functional-group changes
A multi-step synthesis is a sequence of reactions that changes one functional group or carbon skeleton at a time. Work backward from the target to choose a plausible immediate precursor, then reverse the route to check reagents and conditions.
Track carbon count, oxidation state and functional-group compatibility after every step. Prefer the shortest syllabus-supported route and separate preparation from purification.
To make propanoic acid from 1-bromopropane: substitution with CN⁻ gives propanenitrile, then hydrolysis and acidification gives propanoic acid.
Do not propose a reagent that changes the carbon count accidentally, and do not omit conditions such as reflux, catalyst or acid work-up.
Audit a synthetic route by naming each reaction, reagent and likely by-product
For every arrow in a synthesis, identify the starting functional group, reaction type, reagent/conditions, product functional group and any by-products. Then check atom and charge balance.
Common by-products include H₂O in condensation, HBr or HCl in substitution, and inorganic salts after neutralisation or hydrolysis. A plausible route must explain where they go.
Ethanol → bromoethane with PBr₃ is substitution; bromoethane → propanenitrile with KCN is substitution and adds one carbon; hydrolysis then gives a carboxylic acid.
A correct product name alone is not enough: a route can be impossible if the reagent cannot cause the stated functional-group change.