How do you plan a synthesis?

Work backwards from the target to the starting material, naming the reagent, conditions and product for each step.

How do you convert a haloalkane into a nitrile, and what is the effect on the carbon chain?

Heat under reflux with KCN in aqueous ethanol; the chain gains one carbon atom.

How is a nitrile reduced to a primary amine?

Use H₂ with a nickel catalyst, or LiAlH₄ in dry ether.

What reagent and conditions convert a haloalkane into an alkene?

Ethanolic NaOH or KOH, heated (elimination).

How is a secondary alcohol converted into a ketone?

Heat with acidified potassium dichromate(VI).

How is benzene converted into nitrobenzene?

Concentrated nitric acid and concentrated sulfuric acid at about 50 °C.

How is nitrobenzene converted into phenylamine?

Heat under reflux with tin and concentrated hydrochloric acid, then add sodium hydroxide.

Write the formula for percentage atom economy.

M(r) of desired product ÷ sum of M(r) of all reactants × 100.

How do you find the overall yield of a multi-step synthesis?

Multiply the fractional yields of each step together.

Why do chemists try to avoid using solvents?

Less waste, less energy for separation, and no hazardous flammable or toxic solvent vapours.

Why do chemists prefer processes with fewer steps?

Less energy, time and cost, a higher overall yield and less waste.

Why do chemists prefer high atom economy?

More reactant atoms end up in the product, so there is less waste and raw materials are used efficiently.

Organic synthesis routesAQA A-Level Chemistry: Flashcards

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What is a multi-step synthesis?

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What is a multi-step synthesis?: A sequence of reactions converting a starting material into a target compound, each step changing a functional group.
How do you plan a synthesis?: Work backwards from the target to the starting material, naming the reagent, conditions and product for each step.
How do you convert a haloalkane into a nitrile, and what is the effect on the carbon chain?: Heat under reflux with KCN in aqueous ethanol; the chain gains one carbon atom.
How is a nitrile reduced to a primary amine?: Use H₂ with a nickel catalyst, or LiAlH₄ in dry ether.
What reagent and conditions convert a haloalkane into an alcohol?: NaOH(aq), heated under reflux.
What reagent and conditions convert a haloalkane into an alkene?: Ethanolic NaOH or KOH, heated (elimination).
How is a secondary alcohol converted into a ketone?: Heat with acidified potassium dichromate(VI).
How is benzene converted into nitrobenzene?: Concentrated nitric acid and concentrated sulfuric acid at about 50 °C.
How is nitrobenzene converted into phenylamine?: Heat under reflux with tin and concentrated hydrochloric acid, then add sodium hydroxide.
Write the formula for percentage atom economy.: M(r) of desired product ÷ sum of M(r) of all reactants × 100.
How do you find the overall yield of a multi-step synthesis?: Multiply the fractional yields of each step together.
Why do chemists try to avoid using solvents?: Less waste, less energy for separation, and no hazardous flammable or toxic solvent vapours.
Why do chemists prefer processes with fewer steps?: Less energy, time and cost, a higher overall yield and less waste.
Why do chemists prefer high atom economy?: More reactant atoms end up in the product, so there is less waste and raw materials are used efficiently.

Exam questions on Organic synthesis routes

A pharmaceutical company is redesigning the production of a drug intermediate. The existing process has five steps, is carried out in a chlorinated organic solvent and starts from a toxic compound. The company wants a greener process.
Explain why chemists aim to design processes that use no solvent and that start from non-hazardous materials.2 marks
Ethanol can be made by two routes. Route A: hydration of ethene with steam, C₂H₄ + H₂O → C₂H₅OH. Route B: hydrolysis of bromoethane with aqueous sodium hydroxide, C₂H₅Br + NaOH → C₂H₅OH + NaBr. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Na = 23.0, Br = 79.9.
Use the atom economies to justify why a chemist would prefer Route A to Route B.2 marks
A research chemist plans to make organic compounds from simple starting materials. Only reactions from the A Level specification are to be used, and each synthesis may have up to four steps.
Devise a two-step synthesis of propan-1-amine, CH₃CH₂CH₂NH₂, from bromoethane. For each step, give the reagents and conditions and the name or formula of the organic product.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).