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Two-Stage Synthetic Routes

A concise revision guide to planning short organic syntheses with the AS reactions: the reaction map linking alkenes, halogenoalkanes, alcohols, carbonyl compounds and carboxylic acids, how to work backwards from a target in two stages, writing reagents and conditions for every step, and analysing a given route for its reaction types and by-products.

AS Level
Topic 21: Organic synthesis
9701 Papers 1 and 2
1

Before You Start

Every reaction on this page was met when its functional group was taught. Here they are joined into one map so that a target two steps away can be reached.

2

The AS Reaction Map

Five families sit on the map: alkenes, halogenoalkanes, alcohols, carbonyl compounds and carboxylic acids. Each arrow is one AS reaction with its reagents and conditions.

Reaction map of the AS organic conversions drawn with condensed formulae as nodes and the reagents and conditions written on the arrows: alkene to

The map with condensed structures as nodes and the reagents and conditions on every arrow: learn it as a picture.

  • Alkene → halogenoalkane: HBr(g) at room temperature, electrophilic addition; the reverse is elimination with KOH in ethanol, heated under reflux.
  • Alkene → alcohol: steam with a phosphoric acid catalyst, 300 °C and 60 atm (hydration).
  • Halogenoalkane → alcohol: NaOH(aq), heat under reflux (nucleophilic substitution, hydrolysis).
  • Alcohol → halogenoalkane: NaBr with concentrated H₂SO₄ (HBr made in the flask), or PCl₅, PCl₃ with heat or SOCl₂.
  • Alcohol → alkene: concentrated H₂SO₄ or H₃PO₄, heat (dehydration, an elimination); or the vapour passed over heated Al₂O₃.
  • Primary alcohol → aldehyde → carboxylic acid: acidified K₂Cr₂O₇; distil for the aldehyde, reflux for the acid.
  • Secondary alcohol → ketone: acidified K₂Cr₂O₇, heat; a tertiary alcohol is not oxidised.
  • Carboxylic acid + alcohol ⇌ ester: concentrated H₂SO₄ catalyst, heat under reflux; dilute acid or NaOH(aq) hydrolyses the ester back.
3

Extra Arrows on This Syllabus

Five more AS reactions extend the map. Two of them change the number of carbon atoms, which no other AS reaction does.

  • Halogenoalkane → nitrile: KCN in ethanol, heat under reflux; the chain grows by one carbon: CH₃CH₂Br + KCN → CH₃CH₂CN + KBr.
  • Nitrile → carboxylic acid: dilute HCl(aq), heat under reflux: CH₃CH₂CN(l) + 2H₂O(l) + HCl(aq) → CH₃CH₂COOH(aq) + NH₄Cl(aq).
  • Halogenoalkane → primary amine: excess concentrated NH₃ in ethanol, heated under pressure in a sealed tube.
  • Aldehyde or ketone → hydroxynitrile: HCN with a KCN catalyst, warm (nucleophilic addition); one more carbon again.
  • Aldehyde or ketone → alcohol: NaBH₄ in water or LiAlH₄ in dry ether (reduction), the reverse of the oxidation arrow.
  • Details: Making Amines and Nitriles, Hydroxynitriles and the Hydrolysis of Nitriles, Reduction and Nucleophilic Addition and Carbon–Carbon Bond Formation.
4

Reagents and Conditions: The Table to Learn

Reagent and conditions are separate marks. The table holds every shared conversion in the form the mark scheme expects.

ConversionReagentConditionsReaction type
alkene → halogenoalkaneHBr(g)room temperatureelectrophilic addition
alkene → alcoholH₂O(g), H₃PO₄ catalyst300 °C, 60 atmelectrophilic addition (hydration)
halogenoalkane → alcoholNaOH(aq)heat under refluxnucleophilic substitution (hydrolysis)
alcohol → halogenoalkaneNaBr and concentrated H₂SO₄; or PCl₅; or SOCl₂warm; room temperature for PCl₅substitution
alcohol → alkeneconcentrated H₂SO₄ or H₃PO₄; or Al₂O₃heat; vapour over the hot catalystelimination (dehydration)
primary alcohol → aldehydeK₂Cr₂O₇ / H₂SO₄warm and distil at onceoxidation
primary alcohol → acid; secondary alcohol → ketoneexcess K₂Cr₂O₇ / H₂SO₄heat under refluxoxidation
acid + alcohol → esterconcentrated H₂SO₄ catalystheat under refluxesterification (condensation)
Table poster of the AS organic conversions as text with the reagents, conditions and reaction type for each: alkene to alkyl halide, alcohol or dibromoalkane, alkyl halide to alcohol or alkene, alcohol to alkyl halide, alkene, aldehyde, ketone or carboxylic acid, and carboxylic acid with alcohol to ester, with the by-products to mention.

The conversions, reagents, conditions and reaction types as a poster table to learn from.

Exam focus: “Acidified potassium dichromate(VI), heat under reflux” earns the reagent and the condition marks; “dichromate” alone earns neither.

Check your understanding

Check: The Map, Reagents and Conditions

Match each AS conversion to its reagent, conditions and reaction type, and pick the one arrow that joins two named compounds.

5

Planning Backwards from the Target

Start at the product, not at the starting material. Ask which single reaction makes the target group, then whether the compound that reaction needs is the one given.

  1. Name the target group and count its carbons (a change in carbon number points to a nitrile step).
  2. Choose the last step: the one AS reaction that makes that group. An acid comes from a primary alcohol or aldehyde; an alcohol from an alkene or a halogenoalkane.
  3. Write the intermediate that the last step needs, with its name and condensed formula.
  4. Choose the first step that turns the given starting material into that intermediate.
  5. Add reagents and conditions to both arrows, and the reaction type if asked.
  6. Check: carbon skeleton unchanged, group in the right position, each step a real AS reaction.
Checklist poster for planning a two-stage synthesis backwards as text: name the target and its functional group, ask which single reaction makes that group, find the intermediate, repeat to the starting material, then write reagents and conditions for every stage, check the carbon count and name the by-products.

The backwards-planning checklist as a flow: target group, last step, intermediate, first step, reagents, check.

Common mistake: Planning forwards and drifting: ethene → ethanol → bromoethane when the target was ethanoic acid. Backwards planning cannot drift, because every step is chosen to reach the target.

6

Worked Routes (1) and (2)

Route (1): ethene to ethanoic acid

Step 1.
Target: a carboxylic acid with two carbons
The last step is oxidation of a primary alcohol under reflux
So the intermediate is ethanol

Step 2. Ethene gives ethanol by hydration: C₂H₄(g) + H₂O(g) → CH₃CH₂OH(g), steam, H₃PO₄ catalyst, 300 °C, 60 atm.

Answer.
Ethanol is oxidised: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O, excess acidified K₂Cr₂O₇, heat under reflux
Two stages, two carbons throughout

Route (2): propene to propanone

Step 1.
Target: a ketone, made by oxidising a secondary alcohol
The intermediate is propan-2-ol, which comes from 2-bromopropane by hydrolysis
Step 2.
Propene + HBr(g) at room temperature gives mainly 2-bromopropane: the secondary carbocation is the more stable intermediate
So bromine joins the middle carbon
Step 3.
CH₃CHBrCH₃ + NaOH(aq) → CH₃CH(OH)CH₃ + NaBr(aq), heat under reflux
Then CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O with acidified K₂Cr₂O₇
Answer.
Three steps rather than two, but every arrow is an AS reaction
Hydration of propene would reach propan-2-ol in one step instead
Three worked two-stage routes with condensed structures and the reagents on each arrow: ethene through ethanol to ethanoic acid, propene through

Routes (1), (2) and (4) drawn with condensed structures and the reagents and conditions written over every arrow.

7

Worked Routes (3), (4) and (5)

Route (3): 1-bromobutane to butanal

Step 1.
An aldehyde comes from a primary alcohol by gentle oxidation with distillation
The intermediate is butan-1-ol
Answer.
CH₃CH₂CH₂CH₂Br + NaOH(aq) → CH₃CH₂CH₂CH₂OH + NaBr(aq), heat under reflux
Then warm with acidified K₂Cr₂O₇ and distil the butanal (boiling point 75 °C) as it forms

Route (4): propan-1-ol to 1,2-dibromopropane

Step 1.
Two bromines on adjacent carbons come from adding Br₂ to an alkene
The intermediate is propene
Answer.
Dehydrate: CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O, concentrated H₂SO₄, heat
Then CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br at room temperature, orange to colourless

Route (5): ethanol to ethyl ethanoate

Step 1.
An ester needs an acid and an alcohol
Both can come from ethanol: oxidise one portion to ethanoic acid under reflux and keep the rest
Answer.
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O, concentrated H₂SO₄ catalyst, heat under reflux
Two stages from a single starting material

Exam focus: Every intermediate is named and drawn, and every arrow carries reagents and conditions. A route with the right compounds but bare arrows scores half marks at best.

Check your understanding

Quick Check: Moving a Group Along the Chain

Plan backwards from a target that has its functional group in a new position.

8

Worked Route (6): Adding a Carbon Atom

Only the nitrile step changes the carbon count. A target with one carbon more than the starting material almost always runs through a nitrile.

Step 1.
Target: propanoic acid, three carbons, from bromoethane, two carbons
The acid comes from hydrolysing propanenitrile

Step 2. CH₃CH₂Br + KCN → CH₃CH₂CN + KBr, KCN in ethanol, heat under reflux (nucleophilic substitution).

Answer.
CH₃CH₂CN(l) + 2H₂O(l) + HCl(aq) → CH₃CH₂COOH(aq) + NH₄Cl(aq), dilute hydrochloric acid, heat under reflux
Or dilute NaOH(aq) then acidify
The chain-lengthening route from bromoethane through propanenitrile to propanoic acid with condensed structures and reagents on the arrows: potassium

Route (6) drawn: bromoethane to propanenitrile to propanoic acid, reagents over the arrows and the carbon count under each structure.

Check your understanding

Check: Planning Two-Stage Routes

Work backwards from a target to the intermediate, name the reagents and conditions for both steps, and spot the route that changes the carbon skeleton by mistake.

9

Analysing a Given Route

A question may give the route and ask for the reaction type, the reagents and the likely by-products at each step. By-products come from competing reactions and from equilibria.

Step in route (2)Reaction typeReagents and conditionsPossible by-product
propene → 2-bromopropaneelectrophilic additionHBr(g), room temperature1-bromopropane from the less stable primary carbocation
2-bromopropane → propan-2-olnucleophilic substitution (hydrolysis)NaOH(aq), heat under refluxpropene by elimination if the hydroxide is in ethanol or the mixture is hot and concentrated
propan-2-ol → propanoneoxidationacidified K₂Cr₂O₇, heatnone from the ketone; a primary alcohol would over-oxidise to the acid
  • Over-oxidation: refluxing a primary alcohol gives the acid when the aldehyde was wanted; distil instead.
  • Esterification equilibrium: the ester is mixed with water, unreacted acid and alcohol; distil, then wash with NaHCO₃(aq).
  • Substitution against elimination: KOH in water substitutes (alcohol), KOH in ethanol eliminates (alkene); a mixed solvent gives both products.
  • Addition to an unsymmetrical alkene: both bromoalkanes form; the one from the more stable carbocation is the major product.
  • Incomplete hydrolysis: a nitrile refluxed too briefly leaves unreacted nitrile or the amide in the product.
  • Dehydration of a longer alcohol can give more than one alkene: positional isomers and E/Z isomers.

Remember: Reaction type names: electrophilic addition, nucleophilic substitution, elimination, oxidation, esterification, hydrolysis, nucleophilic addition, reduction. Use the full name every time.

Check your understanding

Check: Analysing a Route

Name the reaction type and reagents at each step of a given route and predict the by-product that each step could form.

10

Common Exam Points

  • Plan backwards: name the target group, choose the last step, write the intermediate, then choose the first step.
  • Every arrow carries reagents and conditions: HBr(g) room temperature; steam / H₃PO₄ 300 °C 60 atm; NaOH(aq) reflux; K₂Cr₂O₇ / H₂SO₄ distil or reflux.
  • Distil for an aldehyde, reflux for a carboxylic acid or a ketone; concentrated H₂SO₄ is the catalyst for esterification and the reagent for dehydration.
  • A halogenoalkane and an alcohol interconvert: NaOH(aq) one way, NaBr with concentrated H₂SO₄ (or PCl₅) the other.
  • HBr adds to an unsymmetrical alkene to give mainly the product from the more stable carbocation; the other isomer is a by-product.
  • KCN in ethanol adds a carbon atom through a nitrile; hydrolysis with dilute acid then gives the acid with one more carbon.
  • By-products: over-oxidation, the esterification equilibrium, the minor addition isomer, elimination alongside substitution.
  • Name the reaction type in full: electrophilic addition, nucleophilic substitution, elimination, oxidation, esterification, hydrolysis.

FAQs

Use these quick answers to check how to plan a two-stage route with the AS reactions of alkenes, halogenoalkanes, alcohols and carbonyl compounds, how to write reagents and conditions for every step, how a nitrile adds a carbon and what by-products to mention.

How do I start planning a two-stage synthesis?

Work backwards from the target. Ask which single reaction could make it, write that compound down, then ask which reaction turns the starting material into it. Check the carbon skeleton is unchanged, then add reagents and conditions to each arrow before writing anything else.

How do I make ethanoic acid from ethene in two stages?

Stage 1: hydrate ethene with steam over a phosphoric(V) acid catalyst at about 300 °C and 60 atm to give ethanol. Stage 2: heat the ethanol under reflux with excess acidified potassium dichromate(VI), which oxidises it through ethanal to ethanoic acid.

How do I get from propene to propanone?

Add hydrogen bromide to propene to give 2-bromopropane (the hydrogen joins the end carbon, so bromine lands on the middle one). Hydrolyse the halogenoalkane with aqueous sodium hydroxide under reflux to propan-2-ol, then oxidise this secondary alcohol with acidified potassium dichromate(VI) to propanone.

How does a nitrile let me add one carbon to the chain?

Heat the halogenoalkane under reflux with potassium cyanide in ethanol: bromoethane gives propanenitrile, CH₃CH₂CN, with one carbon more. Hydrolysis of the nitrile with dilute hydrochloric acid under reflux then gives propanoic acid, so two stages convert a two-carbon compound into a three-carbon acid.

What by-products should I mention when I analyse a given route?

Ethanolic potassium hydroxide gives elimination to an alkene alongside substitution; a primary alcohol heated under reflux with dichromate is over-oxidised to the acid; esterification is an equilibrium, so acid and alcohol remain; and hydrogen bromide addition to an unsymmetrical alkene gives two isomers.

Dr. Mohammed Al-Fatah

Written by:
Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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Related Topic 21 Organic Synthesis Pages

Use these pages to connect the ideas across Topic 21 Organic Synthesis and the rest of the course.

Identifying the Functional Groups in a Molecule Topic 21 Organic Synthesis Overview Topic 36 Organic Synthesis Oxidation and Tests for Carbonyl Compounds Topic 15 Halogen Compounds Topic 9 Chemical Periodicity Topic 12 Nitrogen and Sulfur Topic 19 Nitrogen Compounds Topic 27 Group 2 Topic 31 Halogen Compounds

Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.