Organic synthesis: the reaction map for A-level Chemistry

An organic synthesis map shows how the main families of organic compounds can be turned into one another, and what reagents and conditions each step needs. A-level Chemistry uses it to plan synthetic routes: making a target molecule from a starting material in a few steps. Learn the arrows, the reagents and the conditions, and you can work out most routes.

The organic synthesis map (aliphatic compounds)

More conversions to know

How to plan an organic synthesis

1. Name the functional groups in the starting material and the target.
2. Compare the carbon chains. If the target has one more carbon, you need a step that adds one, such as KCN (making a nitrile) or KCN with a carbonyl (making a hydroxynitrile).
3. Find the route on the map from the starting group to the target group. Work backwards from the target if it is easier.
4. Write each step with the intermediate compound, the reagents and the conditions.
5. Check for problems: mixtures of products, unwanted oxidation, or a step that also attacks another group in the molecule.

AQA A-level Chemistry (3.3.14) expects you to devise syntheses with up to four steps using the reactions in the specification, and to explain why chemists prefer routes with fewer steps, a high atom economy, no solvent and non-hazardous starting materials.

Worked examples of synthetic routes

Example 1: ethene to ethanoic acid (2 steps)

1. Ethene + steam, with a phosphoric acid catalyst → ethanol.
2. Ethanol + excess acidified potassium dichromate(VI), heated under reflux → ethanoic acid.

Example 2: propene to propanone (2 steps)

Propanone is a ketone, so it must come from a secondary alcohol, propan-2-ol.

1. Propene + steam, with an acid catalyst → propan-2-ol. This is the major product, because it forms through the more stable secondary carbocation.
2. Propan-2-ol + acidified potassium dichromate(VI), reflux → propanone.

The hydration also makes some propan-1-ol, which would oxidise to propanoic acid, so the product needs purifying. Spotting by-products like this is part of evaluating a route.

Example 3: adding a carbon atom (2 steps)

Make 3-methylbutan-1-amine, (CH₃)₂CHCH₂CH₂NH₂, from 1-bromo-2-methylpropane, (CH₃)₂CHCH₂Br. The target has one more carbon, so use cyanide:

1. 1-bromo-2-methylpropane + KCN in ethanol and water, warm → 3-methylbutanenitrile, (CH₃)₂CHCH₂CN. The CN carbon adds one carbon to the chain.
2. Reduce the nitrile with LiAlH₄ (or H₂ with a nickel catalyst) → 3-methylbutan-1-amine.

Reacting the bromoalkane with ammonia instead would give an amine with the same number of carbons, (CH₃)₂CHCH₂NH₂, so it would not reach this target.

Check your understanding.

Common mistakes

• Mixing up aqueous and ethanolic KOH. Aqueous gives an alcohol (substitution); in ethanol, hot, it gives an alkene (elimination).
• Leaving out conditions. "Potassium dichromate" alone isn't enough: say acidified, and reflux or distil.
• Forgetting chain length. KCN adds a carbon; ammonia doesn't.
• Oxidising a tertiary alcohol. Tertiary alcohols are not easily oxidised.
• Using UV and a halogen on a big molecule and expecting one product. Free radical substitution gives a mixture.
• Writing reagents without names or formulae. Give both where you can, such as "acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺".

Practice questions

1. Give the reagent and conditions to convert bromoethane into ethanol.
2. Give a two-step route from ethene to bromoethane and then to ethylamine.
3. How would you convert propan-1-ol into propanal rather than propanoic acid?
4. Suggest a two-step route from ethanal to 2-hydroxypropanoic acid. (Hint: hydroxynitriles can be hydrolysed to carboxylic acids.)

Answers: 1) Aqueous NaOH, warm. 2) Ethene + HBr → bromoethane; then excess ammonia in ethanol, heated in a sealed tube → ethylamine. 3) Use acidified potassium dichromate(VI) and distil the propanal off as it forms. 4) Ethanal + KCN, then dilute acid → 2-hydroxypropanenitrile; then hydrolyse the nitrile group to a carboxylic acid group.

Practise with a short timed paper

Synthetic routes draw on the whole of A-level organic chemistry. The daily A-level Chemistry: Year 2 and A-level Chemistry: Year 1 papers are short timed papers of 8 questions, with a new one every day. The mechanisms behind the arrows are in the guides to free radical substitution, electrophilic addition, nucleophilic substitution and nucleophilic addition.

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Chemistry · Organic chemistryElimination reactions: mechanism, conditions, examplesWhat an elimination reaction is, the mechanism for halogenoalkanes with hot ethanolic KOH and for dehydrating alcohols, conditions, products and exam tips.