Every AS reaction is a tool. Spot the functional groups in a molecule, predict how it will react, then chain reactions together into two- and three-step routes, with the right reagents and conditions at every step.
Organic synthesis means making a target molecule from a starting material by a sequence of known reactions. First identify the functional groups present, because each group has its own reactions and tests. Then work backwards from the target: compare the carbon skeleton and functional group of the target with the starting material. If the number of carbon atoms goes up by one, a cyanide step is needed (KCN on a halogenoalkane, or HCN with KCN on a carbonyl compound). Halogenoalkanes and alcohols are the key intermediates that connect most AS routes. For every step give the reagent, conditions and type of reaction (substitution, addition, elimination, oxidation, reduction, hydrolysis, condensation), and remember that the overall yield is the product of the step yields.
Build displayed, structural and skeletal formulae in the simulator and pick out each functional group before you predict any reaction.
| Group | Typical reactions you can predict | Quick test |
|---|---|---|
| C=C (alkene) | electrophilic addition of H2, HX, X2, steam; oxidation by KMnO4 | decolourises orange bromine water |
| C–X (halogenoalkane) | nucleophilic substitution by OH−, CN−, NH3; elimination by ethanolic OH− | warm with NaOH(aq), acidify with HNO3, add AgNO3(aq) |
| –OH (alcohol) | Na, halogenation, dehydration, esterification; oxidation if primary or secondary | PCl5 gives steamy HCl fumes |
| –CHO (aldehyde) | oxidation to RCOOH; reduction by NaBH4; nucleophilic addition of HCN | orange ppt with 2,4-DNPH; silver mirror with Tollens’ |
| C=O (ketone) | reduction by NaBH4; nucleophilic addition of HCN; not oxidised | orange ppt with 2,4-DNPH; no change with Tollens’ |
| –COOH (carboxylic acid) | salts with metals, bases and carbonates; esterification; reduction by LiAlH4 | fizzes with Na2CO3(aq), giving CO2 |
| –COO– (ester) | hydrolysis by dilute acid or by NaOH(aq), heat under reflux | fruity smell |
| –C≡N (nitrile) | hydrolysis to RCOOH by dilute acid, heat under reflux | – |
Click the arrows on the reaction map in the simulator: the halogenoalkane is the hub of most AS routes, and every arrow carries its reagents and conditions.
| Conversion | Reagent and conditions | Type of reaction |
|---|---|---|
| alkane → halogenoalkane | Cl2 or Br2, UV light | free-radical substitution |
| alkene → halogenoalkane | HBr(g), room temperature | electrophilic addition |
| alkene → alcohol | steam, H3PO4 catalyst, 300 °C, 6 MPa | electrophilic addition (hydration) |
| halogenoalkane → alcohol | NaOH(aq), heat under reflux | nucleophilic substitution (hydrolysis) |
| halogenoalkane → alkene | NaOH in ethanol, heat | elimination |
| halogenoalkane → nitrile | KCN in ethanol, heat under reflux | nucleophilic substitution (adds one C) |
| halogenoalkane → amine | excess NH3 in ethanol, heat in a sealed tube | nucleophilic substitution |
| alcohol → halogenoalkane | HBr (KBr + conc. H2SO4), PCl5, PCl3 + heat, or SOCl2 | substitution |
| alcohol → alkene | Al2O3 heated, or hot conc. H2SO4 / H3PO4 | elimination (dehydration) |
| 1° alcohol → aldehyde | acidified K2Cr2O7, warm, distil off product as it forms | oxidation |
| 1° alcohol → carboxylic acid | excess acidified K2Cr2O7, heat under reflux | oxidation |
| 2° alcohol → ketone | acidified K2Cr2O7, heat | oxidation |
| aldehyde or ketone → alcohol | NaBH4 (or LiAlH4) | reduction |
| aldehyde or ketone → hydroxynitrile | HCN with a KCN catalyst | nucleophilic addition (adds one C) |
| nitrile → carboxylic acid | dilute HCl(aq), heat under reflux (or NaOH(aq), then acidify) | hydrolysis |
| carboxylic acid + alcohol → ester | conc. H2SO4 catalyst, heat | condensation (esterification) |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which functional group is shown by a compound that decolourises bromine water?
What converts bromoethane into propanenitrile?
Which step increases the length of a carbon chain by one carbon atom?
Ethene reacts with steam and an H3PO4 catalyst. What type of reaction is this?
Use the work-it-backwards tab to ask what each target could have been made from, then follow the two worked routes.
| Route A: ethene → ethyl ethanoate (3 steps; the ethanol from step 1 is split into two portions) | ||
|---|---|---|
| Step | Reagent and conditions | Type of reaction |
| 1. CH2=CH2 + H2O → CH3CH2OH | steam, H3PO4, 300 °C, 6 MPa | electrophilic addition |
| 2. CH3CH2OH + 2[O] → CH3COOH + H2O | excess acidified K2Cr2O7, heat under reflux | oxidation |
| 3. CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O | conc. H2SO4 catalyst, heat | condensation (esterification) |
| Route B: bromoethane → propanoic acid (2 steps; carbon count 2 → 3) | ||
|---|---|---|
| Step | Reagent and conditions | Type of reaction |
| 1. CH3CH2Br + KCN → CH3CH2CN + KBr | KCN in ethanol, heat under reflux | nucleophilic substitution |
| 2. CH3CH2CN + 2H2O + HCl → CH3CH2COOH + NH4Cl | dilute HCl(aq), heat under reflux | hydrolysis |
Do not try to oxidise straight from an alkane or alkene to an acid at AS: go through the halogenoalkane or alcohol. Give conditions as well as reagents: NaOH(aq) gives an alcohol but NaOH in ethanol gives an alkene; distilling gives an aldehyde but reflux gives a carboxylic acid. Use KCN in ethanol on a halogenoalkane, but HCN with a KCN catalyst on a carbonyl. And the overall yield is the product of the step yields, not their average or the lowest one.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
A three-step synthesis has step yields of 80.0%, 75.0% and 60.0%. What is the overall yield?
Which two-step route converts ethanal into 2-hydroxypropanoic acid, CH3CH(OH)COOH?
Ethane is converted into ethanal: step 1 Cl2, UV light; step 2 NaOH(aq), reflux; step 3 acidified K2Cr2O7, distil. What are the types of reaction for steps 1, 2 and 3?
Which route converts propan-1-ol into butanoic acid?
This skill is now recorded on your profile. Keep the chain going.