Part 1 of 5Aliphatic Conversions
Alkane → halogenoalkane
- Free radical substitution with Cl₂ or Br₂ in UV light.
Halogenoalkane → alcohol
- Warm with aqueous NaOH, a nucleophilic substitution;
- the same halogenoalkane with ethanolic NaOH gives an alkene by elimination.
Halogenoalkane → nitrile or amine
- Heat with ethanolic KCN to form a nitrile, or with excess ethanolic NH₃ in a sealed tube to form an amine.
Alkene → alcohol or halogenoalkane
- Steam with a H₃PO₄ catalyst gives an alcohol; HBr gives a halogenoalkane by electrophilic addition.
Alcohol → aldehyde or acid
- Acidified K₂Cr₂O₇ with distillation gives an aldehyde;
- with reflux a primary alcohol gives the carboxylic acid.
Nitrile → amine or acid
- Reduce with LiAlH₄ to make a primary amine, or hydrolyse with dilute acid to make a carboxylic acid.
Part 2 of 5Carboxylic Acid Derivatives and Aromatics
Carboxylic Acid Derivatives and Aromatics
Acid → ester
- Heat the acid with an alcohol and a concentrated H₂SO₄ catalyst, and ester hydrolysis reverses this.
Acyl chloride
- Acyl chlorides react with alcohols to give esters, with water to give acids and with NH₃ or amines to give amides.
Benzene reactions
- Nitration,
- then reduction with Sn / HCl gives phenylamine;
- Friedel–Crafts with CH₃Cl or CH₃COCl and AlCl₃ gives alkyl or acyl benzenes.
- then reduction with Sn / HCl gives phenylamine;
Carbonyl compounds
- Aldehydes and ketones are reduced to alcohols by NaBH₄ in water, and add HCN (from KCN and acid) to give hydroxynitriles.
Part 3 of 5Designing a Route
1
Identify the functional groups in the starting material and the target, and note whether the carbon chain must get longer.
2
Work backwards from the target to find which compounds could be turned into it in one step.
3
Choose the reagents and conditions for each step, using the standard conversions.
4
Consider the green chemistry principles: fewer steps, high atom economy, no solvent if possible, and non-hazardous starting materials.
5
Check that each step gives the right product and that any product can be separated and purified.
Part 4 of 5Worked Example
Question Suggest a two-step synthesis of propylamine, CH₃CH₂CH₂NH₂, from bromoethane.
- The target has three carbons and the starting material has two, so a carbon must be added;
- a nitrile does this.
- Step 1: heat bromoethane under reflux with ethanolic KCN to give propanenitrile, CH₃CH₂CN, by nucleophilic substitution.
- Step 2: reduce the nitrile with LiAlH₄ in dry ether, or H₂ with a Ni catalyst, to give CH₃CH₂CH₂NH₂.
Answer CH₃CH₂Br → CH₃CH₂CN (KCN, ethanol, reflux) → CH₃CH₂CH₂NH₂ (LiAlH₄ in dry ether).
Part 5 of 5Exam Checklist & Tips
3.14.1 – Key Points for the Exam
- Reaction pathway A series of reactions converting one compound into another.
- Atom economy Mass of desired product as a percentage of the total mass of reactants.
- Reflux Heating with a condenser so that volatile liquids are not lost.
- Green chemistry Designing chemical processes to reduce waste and hazards.
Common Mistakes
- Giving a reagent but forgetting the conditions, such as reflux, ethanol or aqueous solvent.
- Writing reagents for a nucleophilic substitution without the solvent, such as ethanolic KCN or aqueous NaOH.
- Missing that nitrile formation extends the carbon chain by one.
- Choosing a long route when a shorter pathway with the same result exists.