Organic synthesis: core exam idea

· Organic synthesis = planning and analysing routes to make an organic product from simpler starting materials.
· For CIE 21.1, you must use reactions from the syllabus to connect functional groups in a logical sequence.
· Exam questions usually test three skills: identify functional groups, predict reactions/properties, and devise or analyse multi-step routes.
· Focus on the functional group changing, not the whole molecule.
· A good route must show the starting compound → intermediate(s) → final product, with reagents and conditions for each step.

This map shows how common organic functional groups can be interconverted. It is useful for planning multi-step synthesis routes because each arrow represents a possible reaction step. Source

Identifying functional groups in multi-functional molecules

· First scan for C=C, –X, –OH, –CHO, C=O, –COOH, –COOR, –NH₂, –CN.
· A molecule can contain several functional groups, so predict reactions for each group separately.
· Use diagnostic reactions from the syllabus to confirm functional groups.
· Alkenes: decolourise aqueous bromine; oxidised by KMnO₄.
· Alcohols: react with Na, undergo oxidation, dehydration, and substitution to halogenoalkanes.
· Aldehydes/ketones: give orange precipitate with 2,4-DNPH; aldehydes are oxidised by Tollens’/Fehling’s.
· Carboxylic acids: react with carbonates to give CO₂, and form esters with alcohols.
· Halogenoalkanes: undergo nucleophilic substitution or elimination.
· Nitriles: can be hydrolysed to carboxylic acids or reduced to amines.

This chart helps students recognise functional groups quickly from structural or skeletal formulae. In synthesis questions, identifying the functional group is the first step before choosing reagents and conditions. Source

Predicting properties and reactions

· Functional groups control reactivity: molecules with the same functional group usually react in similar ways.
· C=C: reactive towards electrophilic addition and oxidation.
· –OH: may undergo oxidation, dehydration, esterification, or substitution.
· C=O in aldehydes/ketones: undergoes nucleophilic addition and reduction.
· –COOH: acidic; undergoes neutralisation, esterification, and reduction.
· –X in halogenoalkanes: polar C–X bond allows nucleophilic substitution.
· –CN: useful in synthesis because it increases carbon chain length by one carbon.
· In molecules with multiple groups, ask: which functional group reacts under these conditions?

Planning multi-step synthesis routes

· Work backwards from the target molecule: identify the final functional group and ask what precursor could form it.
· Then write the route forwards with reagents, conditions, and intermediates.
· Choose reactions that change one key functional group at a time.
· Use nitriles when the carbon chain must be extended by one carbon atom.
· Use oxidation/reduction to move between alcohols, aldehydes, ketones, carboxylic acids, and amines.
· Use substitution/elimination/addition to move between alkenes, halogenoalkanes, and alcohols.
· In an exam, every arrow should have a clear reaction type, reagent, and condition.

Retrosynthesis means planning backwards from the target molecule to simpler starting materials. The actual synthesis is then written in the forward direction with reagents and conditions added to each step. Source

Common synthesis logic patterns

· Alkene → alcohol: addition of steam; useful when introducing –OH.
· Alcohol → alkene: dehydration; useful when removing –OH to form C=C.
· Alcohol → aldehyde/carboxylic acid: oxidation of a primary alcohol; conditions decide the product.
· Secondary alcohol → ketone: oxidation.
· Halogenoalkane → alcohol/amine/nitrile/alkene: choose nucleophile or elimination conditions.
· Aldehyde/ketone → alcohol: reduction.
· Aldehyde/ketone → hydroxynitrile: nucleophilic addition of HCN/KCN, increasing complexity.
· Nitrile → carboxylic acid: hydrolysis.
· Carboxylic acid + alcohol → ester: esterification.

Analysing a given synthetic route

· For each step, identify the functional group before and after the reaction.
· State the reaction type: addition, substitution, elimination, hydrolysis, condensation, oxidation, or reduction.
· State the reagents and conditions needed for the step.
· Check whether the reaction forms by-products, e.g. H₂O, HCl, salts, or mixtures of products.
· Check if the route is chemically sensible: each intermediate must be able to form the next compound using a known syllabus reaction.

This diagram demonstrates how an alkene can be converted into an amine using a sequence of functional group changes. It is useful practice for identifying intermediates and assigning reagents to each arrow. Source

Exam strategy for synthesis questions

· Circle the starting functional group and the target functional group.
· Write a rough conversion chain before adding reagents, e.g. alkene → alcohol → halogenoalkane → amine.
· Add reagents and conditions only after the route is logically complete.
· If the carbon chain gets longer, look for KCN/nitrile formation as a key step.
· If the molecule is oxidised or reduced, check the correct level of oxidation required.
· Always name or describe possible by-products when asked to analyse a route.