Organic Synthesis: Planning Multi-Step Routes

This guide covers subtopic 21.1, Organic synthesis, from Topic 21 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content — and unlike every other organic topic, it introduces no new reactions of its own. It’s a skills topic: using everything from Topics 13–20 together to identify, predict, plan and analyse.

Before studying this

This resource is a capstone — it draws on every AS organic resource published so far: Organic Mechanisms: An Introduction, Hydrocarbons: Alkanes and Alkenes, Halogenoalkanes, Alcohols: Reactions and Oxidation, Carbonyl Compounds, Carboxylic Acids and Esters, Nitrogen Compounds and Addition Polymerisation. If any single reaction below is unfamiliar, the linked resource covering it is the place to go, not this one.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 21

21.1 Organic synthesis — for a molecule containing several functional groups: identifying the functional groups present using the syllabus’s reactions, and predicting properties and reactions; devising multi-step synthetic routes for preparing organic molecules using the syllabus’s reactions; analysing a given synthetic route in terms of reaction type and reagents for each step, and possible by-products.

Identifying functional groups from reactions

Rather than memorising a separate list of “identification tests,” recognise that every characteristic reaction you’ve already learned doubles as an identification test for the functional group it belongs to:

Observation	Functional group indicated
Decolourises aqueous bromine	C=C (alkene)
Effervesces with a carbonate	–COOH (carboxylic acid)
Orange precipitate with 2,4-DNPH	C=O (aldehyde or ketone)
Silver mirror with Tollens’ reagent	–CHO (aldehyde, specifically)
Yellow precipitate with alkaline I₂(aq)	CH₃CO– or CH₃CH(OH)–
Colour change (orange → green) with acidified K₂Cr₂O₇	–OH on a primary or secondary carbon (oxidisable alcohol)
White precipitate with AgNO₃(aq)/ethanol	halogenoalkane (C–X)

Given an unknown molecule’s reactions with a set of reagents, work through this table in reverse: each positive or negative result rules functional groups in or out.

Predicting properties and reactions

Once you know which functional groups a molecule contains, its behaviour follows directly from what you already know about each group individually — a molecule with both an –OH and a C=C, for example, will show both sets of reactions (oxidation/esterification from the –OH, and electrophilic addition from the C=C), each behaving independently of the other unless the syllabus specifically states otherwise.

Planning a multi-step synthesis

A reliable method: identify the functional group you start with and the one you need to end with, then find the shortest chain of reactions already in the syllabus that connects them — checking at each step that the reagents and conditions are exactly right, not just “roughly the right idea.”

Worked example. Starting from ethene, suggest a route (reagents and conditions for each step) to prepare ethyl ethanoate.

Step 1: react ethene with steam, using an H₃PO₄ catalyst (electrophilic addition — see Hydrocarbons):

CH₂=CH₂ + H₂O → CH₃CH₂OH

Step 2: oxidise the ethanol with acidified K₂Cr₂O₇, under reflux (see Alcohols):

CH₃CH₂OH → CH₃COOH

Step 3: react the ethanoic acid with a further portion of ethanol (kept from step 1, or made via a second batch of ethene), using concentrated H₂SO₄ as catalyst (see Carboxylic Acids and Esters):

CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O

Three steps, each using a named reagent and condition from a syllabus reaction already covered — a good check on any proposed route is whether every step can be pointed to a specific, real reaction like this, rather than a vague “it gets oxidised” or “it reacts.”

Analysing a given route

The reverse skill — given someone else’s route, identify what’s actually happening at each step, including any by-products.

Worked example. A route from ethane is given as: Step 1, Br₂ / UV light; Step 2, NaOH(aq), heat. Identify the type of reaction and any likely by-product at each step.

Step 1 is free-radical substitution (see Hydrocarbons: Alkanes and Alkenes): CH₃CH₃ + Br₂ → CH₃CH₂Br + HBr. Because this mechanism isn’t very selective, a likely by-product is further substitution — some dibromoethane forming alongside the desired monobromoethane, since a C–H bond in the monobromo-product can itself be substituted again.

Step 2 is nucleophilic substitution (see Halogenoalkanes): CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr. If ethanolic (rather than aqueous) NaOH had been used instead, the likely by-product would be different again — elimination to ethene rather than substitution to ethanol, which is exactly why the solvent (aqueous vs ethanolic) is always worth checking carefully in a given route.

Common mistakes

• Proposing a reaction that isn’t actually in the syllabus. A plausible-sounding step (“it gets converted directly”) isn’t good enough — every step needs a real, named reagent and condition from a reaction you’ve studied.
• Skipping a necessary intermediate step. Some conversions need more steps than they first appear to — a halogenoalkane cannot become a hydroxynitrile directly, for instance; it has to pass through an alcohol and then a carbonyl compound first (see Nitrogen Compounds for that exact example).
• Forgetting that reaction conditions determine the product, not just the reagent. The same halogenoalkane with the same base gives a different major product depending on whether the solvent is aqueous or ethanolic — always state conditions, not just reagents.
• Ignoring realistic by-products when analysing a route. Free-radical substitution in particular is rarely perfectly selective — over-substitution is a genuine, examinable by-product to mention.

Quick revision checklist

• Using characteristic reactions as identification tests, read in reverse
• Predicting a multi-functional-group molecule’s behaviour from its individual functional groups
• Planning a route: identify start and target groups, find the shortest verified reaction chain between them, state exact reagents/conditions at each step
• Analysing a given route: name the reaction type at each step, and consider realistic by-products
• Checking that solvent/conditions (not just reagent identity) determine which product forms

Related resources

• Addition Polymerisation — the last individual organic reaction this synthesis toolkit draws on
• Halogenoalkanes: Nucleophilic Substitution and Elimination — the substitution/elimination conditions used throughout
• Nitrogen Compounds: Amines and Nitriles — a worked multi-step route example
• A Level Organic Synthesis: Multi-Step Routes with Aromatic Chemistry — the A Level continuation, adding benzene chemistry to the toolkit
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.