All subjects · Chemistry · WCH15 (Unit 5)

Planning multi-step syntheses

Exam questions ask you to plan routes of up to four steps between compounds, using the reactions from the whole course, and to choose safe procedures.

Working out a route

1. Compare the starting material and target: count carbons and identify functional groups.

2. If the chain grows, you need KCN, HCN, a Grignard reagent or a Friedel-Crafts reaction.

3. Work backwards from the target if forwards is not obvious.

4. For each step give the reagent, conditions and intermediate.

Predict properties of unfamiliar compounds from their functional groups: a molecule with -COOH and -OH reacts with NaHCO3 and with acidified dichromate.

Key conversions

Hazards and control

Use hazard data to choose control measures: a fume cupboard for toxic or volatile substances (HCN, Br2, acyl chlorides releasing HCl), no naked flames and a water bath or heating mantle for flammable solvents such as ether, gloves and eye protection for corrosive acids and alkalis. Use the smallest practical quantities.

Worked example

Plan a two-step synthesis of butanoic acid from 1-bromopropane.

  1. Chain grows from 3 to 4 carbons: use cyanide.
  2. Step 1: KCN in ethanol, heat under reflux: CH3CH2CH2Br -> CH3CH2CH2CN
  3. Step 2: dilute HCl, heat under reflux: CH3CH2CH2CN -> CH3CH2CH2COOH

Answer: 1-bromopropane -> butanenitrile -> butanoic acid

Key idea: Count carbons and compare functional groups; add carbon with CN-, HCN, Grignard or Friedel-Crafts. Give reagent, conditions and intermediate for each step. Choose control measures from hazard data.

Check yourself

  1. Which two steps convert benzene into phenylamine?
    • NH3 with AlCl3, then water
    • Br2 with FeBr3, then NH3 at room temperature
    • Conc H2SO4, then NaNO2 and HCl
    • Conc HNO3 and conc H2SO4 at 50 °C, then Sn and conc HCl followed by NaOH
    Show answer

    Conc HNO3 and conc H2SO4 at 50 °C, then Sn and conc HCl followed by NaOH. Nitration, then reduction of the nitro group.

  2. Which route converts ethanal into 2-hydroxypropanoic acid, CH3CH(OH)COOH?
    • LiAlH4, then PCl5
    • HCN with KCN, then heat under reflux with dilute HCl
    • Acidified dichromate, then HCN
    • KCN in ethanol, then LiAlH4
    Show answer

    HCN with KCN, then heat under reflux with dilute HCl. Nucleophilic addition gives the hydroxynitrile; acid hydrolysis turns -CN into -COOH.

  3. Starting only from propan-1-ol, how could you make propyl propanoate?
    • Dehydrate propan-1-ol, then add steam
    • React propan-1-ol with PCl5 twice
    • Reduce propan-1-ol with LiAlH4, then add HCN
    • Oxidise some propan-1-ol to propanoic acid by refluxing with acidified dichromate, then warm the acid with more propan-1-ol and conc H2SO4
    Show answer

    Oxidise some propan-1-ol to propanoic acid by refluxing with acidified dichromate, then warm the acid with more propan-1-ol and conc H2SO4. Make the acid from part of the alcohol, then esterify.

  4. Which single step converts benzene into phenylethanone, C6H5COCH3?
    • Ethanoyl chloride with AlCl3, dry, warm
    • Ethanal with HCN
    • Ethanoic acid with conc H2SO4
    • Chloroethane with AlCl3
    Show answer

    Ethanoyl chloride with AlCl3, dry, warm. Friedel-Crafts acylation.

  5. Which route makes propylamine, CH3CH2CH2NH2, from bromoethane without forming secondary amines?
    • Excess NH3 in ethanol, sealed tube
    • KOH(aq), then NH3
    • KCN in ethanol under reflux, then LiAlH4 in dry ether
    • HBr, then Sn and HCl
    Show answer

    KCN in ethanol under reflux, then LiAlH4 in dry ether. The nitrile adds a carbon; reduction gives only the primary amine.

  6. A synthesis uses ethoxyethane (ether) as a solvent. Which control measure is most important?
    • Keep naked flames away and heat with a water bath
    • Use a fume cupboard because ether is corrosive
    • Add water to the ether before heating
    • Heat strongly with a Bunsen burner to drive off the ether quickly
    Show answer

    Keep naked flames away and heat with a water bath. Ether is extremely flammable with a very low boiling point.

  7. A compound contains a C=C bond and a -CHO group. What would you observe with bromine water and with Tollens' reagent?
    • No change with either reagent
    • Bromine water decolourised; no change with Tollens'
    • Bromine water decolourised; silver mirror formed
    • Orange precipitate with bromine water; red-brown precipitate with Tollens'
    Show answer

    Bromine water decolourised; silver mirror formed. C=C adds bromine; the aldehyde reduces Tollens' reagent to silver.

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Last updated 7 November 2026. Spotted a mistake? Tell us.