Synthetic Routes

A synthetic route is a sequence of reactions that converts a starting material into a target. To plan one you need a mental map of how the functional groups you have met can be converted into each other, with the reagents and conditions for each step.Key conversions (Edexcel conditions):Alkene → halogenoalkane: hydrogen halide, e.g. HBr, at room temperature.
Halogenoalkane → alcohol: aqueous NaOH or KOH, heat under reflux (substitution).
Halogenoalkane → alkene: KOH in ethanol, heat under reflux (elimination).
Halogenoalkane → nitrile: KCN in ethanol, heat under reflux (adds one carbon).
Halogenoalkane → amine: excess ethanolic ammonia, heat in a sealed tube.
Primary alcohol → aldehyde: acidified potassium dichromate(VI), distil off as it forms.
Primary alcohol → carboxylic acid: excess acidified potassium dichromate(VI), heat under reflux.
Secondary alcohol → ketone: acidified potassium dichromate(VI), heat.
Aldehyde or ketone → alcohol: LiAlH₄ in dry ether.
Aldehyde or ketone → hydroxynitrile: HCN with KCN (adds one carbon).
Nitrile → carboxylic acid: dilute acid, heat under reflux.
Nitrile → amine: LiAlH₄ in dry ether.
Carboxylic acid + alcohol → ester: concentrated H₂SO₄ catalyst, heat.Three reactions lengthen a carbon chain: halogenoalkane with KCN, carbonyl with HCN, and Grignard reagents with CO₂ or carbonyl compounds.Worked example: convert bromoethane into propanoic acid. The target has one more carbon, so use cyanide. Step 1: heat bromoethane under reflux with KCN in ethanol to give propanenitrile, CH₃CH₂CN. Step 2: heat under reflux with dilute acid to hydrolyse it to propanoic acid, CH₃CH₂COOH.