Multi-Step Organic Synthesis Revision Notes | A Level AQA Chemistry

1. Why Multi-Step Synthesis Matters

Pharmaceutical companies rarely find a drug molecule sitting in a bottle on a shelf. They build it — one reaction at a time — from simple, available starting materials. That process is organic synthesis, and it is one of the most intellectually demanding skills on the AQA A Level specification.

The AQA specification requires you to devise a synthesis of up to four steps for an organic compound, drawing on every reaction you have studied. This is not a memory exercise — it is a reasoning exercise. You need to look at a target molecule, identify its functional group, and ask: what reaction produces this group, and what did I need before that?

Think of it like navigating a map. You know your destination (the target molecule) and you know the roads available (the reactions in the specification). Your job is to plan the most efficient route. The skill that unlocks this is called retrosynthetic analysis — working backwards from the target to find a viable starting point.

2. The Reaction Toolkit: What You Can Do at Each Step

Before planning any route, you need a clear mental map of which functional groups can be converted into which others. The table below summarises the key interconversions available at A Level. Treat this as your synthesis toolkit — every step in a route must correspond to one of these transformations.

You do not need to memorise this table as a list — you need to internalise the logic of it. Notice that halogenoalkanes sit at the centre of the network: they can be made from alkenes or alcohols, and they can be converted into alcohols, amines, nitriles, or alkenes. They are the most versatile intermediate in A Level synthesis.

3. Retrosynthetic Thinking: Working Backwards from the Target

The most reliable way to plan a synthesis is to start at the end. Look at the target molecule, identify its functional group, and ask: which reaction in the specification produces this functional group? That tells you what the molecule in the previous step must have looked like. Repeat this process until you reach a starting material you are given or can recognise.

This backwards reasoning is called retrosynthetic analysis. In exam questions, you are usually given a starting material and a target, so you can work from both ends and meet in the middle. The double-headed arrow ($\Rightarrow$) is the conventional notation for a retrosynthetic step, meaning "this molecule can be made from..."

Here is the key question to ask at each retrosynthetic step: What functional group does the target contain, and which reaction type produces that group? Once you can answer that, you know the functional group of the precursor — and you can repeat the process.

A common trap is to plan a route that is chemically impossible — for example, trying to oxidise a tertiary alcohol to a carboxylic acid (tertiary alcohols cannot be oxidised under standard conditions). Always check that the reaction you are proposing is one that actually appears in the specification before committing to it.

4. Worked Example: A Three-Step Synthesis

Theory is clearest when you see it applied to a real problem. The worked example below takes you through a three-step synthesis, showing the retrosynthetic reasoning first and then the forward route with full reagents and conditions.

Worked example: Two-Step Synthesis: Bromoethane to Propanoic Acid via Nitrile

Starting from bromoethane (CH3CH2Br), devise a synthesis to produce propanoic acid (CH3CH2COOH). State reagents and conditions for each step.

1. Target: $CH_3CH_2COOH$ (carboxylic acid, 3 carbons) Starting material: $CH_3CH_2Br$ (halogenoalkane, 2 carbons) Key observation: the target has one more carbon than the starting material. — Whenever the target has more carbons than the starting material, you need a carbon chain extension step. At A Level, the reaction that adds one carbon is the reaction of a halogenoalkane with KCN to form a nitrile. This is the critical insight that unlocks the route.
2. CH3CH2Br + KCN -> CH3CH2CN + KBr Reagents: KCN in ethanol Conditions: heat under reflux Mechanism: nucleophilic substitution (SN2) — The cyanide ion ($CN^-$) acts as a nucleophile, attacking the electrophilic carbon bonded to bromine. This extends the carbon chain from 2 to 3 carbons, giving propionitrile ($CH_3CH_2CN$). State symbols: all reactants and products are in solution or liquid phase.
3. CH3CH2CN + 2H2O + H+ -> CH3CH2COOH + NH4+ Reagents: dilute H2SO4(aq) or dilute HCl(aq) Conditions: heat under reflux — Acid hydrolysis of the nitrile converts the $-CN$ group to $-COOH$. The nitrogen leaves as an ammonium ion. This gives propanoic acid with the correct 3-carbon chain. Alternatively, alkaline hydrolysis (NaOH(aq), heat under reflux) followed by acidification also works.
4. Step 0: $CH_3CH_2Br$ — 2 carbons, halogenoalkane Step 1: $CH_3CH_2CN$ — 3 carbons, nitrile Step 2: $CH_3CH_2COOH$ — 3 carbons, carboxylic acid — Always verify the carbon count at each stage. The chain extended by one carbon in Step 1 (KCN addition) and was preserved in Step 2 (hydrolysis). The target has been reached in two forward steps from bromoethane.

Answer: Step 1: CH3CH2Br + KCN -> CH3CH2CN (KCN in ethanol, heat under reflux). Step 2: CH3CH2CN + 2H2O + H+ -> CH3CH2COOH + NH4+ (dilute acid, heat under reflux).

Notice how the retrosynthetic step — spotting the carbon count difference — immediately pointed to the KCN reaction. This is the kind of analytical reasoning that earns marks in AQA synthesis questions. The examiner is not just checking that you know the reactions; they are checking that you can select and sequence them logically.

5. A Four-Step Synthesis: Applying the Full Toolkit

Now that the two-step route is clear, it is time to tackle the full four-step challenge that AQA can set. The key difference is that longer routes require you to plan more carefully — a wrong choice at Step 1 can make Steps 2, 3, and 4 impossible.

The strategy remains the same: identify the functional group of the target, work backwards one step at a time, and check at each stage that the reaction you are proposing is in the specification. The example below converts ethene into an amine — a four-step route that uses four different reaction types.

Worked example: Four-Step Synthesis: Ethene to Ethylamine

Starting from ethene ($CH_2=CH_2$), devise a four-step synthesis to produce ethylamine ($CH_3CH_2NH_2$). State reagents and conditions for each step.

1. Target: $CH_3CH_2NH_2$ (primary amine) Retrosynthetic step 1: amine $\Rightarrow$ halogenoalkane (via $NH_3$ substitution) or nitrile (via reduction) Choose: halogenoalkane route — $CH_3CH_2Br \Rightarrow CH_3CH_2NH_2$ Retrosynthetic step 2: halogenoalkane $\Rightarrow$ alcohol — $CH_3CH_2OH \Rightarrow CH_3CH_2Br$ Retrosynthetic step 3: alcohol $\Rightarrow$ alkene — $CH_2=CH_2 \Rightarrow CH_3CH_2OH$ — Working backwards reveals a clean three-step route from ethanol to ethylamine. Since we start from ethene, we need one step to make ethanol first — giving four steps total. This is a classic AQA-style route.
2. Target: CH3CH2NH2 (primary amine) Ethene -> ethanol -> bromoethane -> ethylamine — Working backwards shows a three-step forward route from ethene to ethylamine.
3. $CH_3CH_2OH + HBr \rightarrow CH_3CH_2Br + H_2O$ Reagents: $NaBr$ and concentrated $H_2SO_4$ (generates HBr in situ), or $PBr_3$ Conditions: heat Mechanism: nucleophilic substitution — The hydroxyl group is converted to a bromine leaving group. Using $PBr_3$ or generating HBr from NaBr and concentrated $H_2SO_4$ are both acceptable. The $OH$ group is a poor leaving group, so it must be protonated first before substitution can occur.
4. CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br Reagents: excess ammonia in ethanol Conditions: heat under pressure in a sealed tube Mechanism: nucleophilic substitution — Excess ammonia is essential. If ammonia is not in excess, the primary amine product ($CH_3CH_2NH_2$) will itself act as a nucleophile and react with more bromoethane, producing secondary and tertiary amines and a quaternary ammonium salt — a mixture rather than a pure product.
5. Ethene ($CH_2=CH_2$) — alkene $\downarrow$ Step 1: hydration Ethanol ($CH_3CH_2OH$) — alcohol $\downarrow$ Step 2: substitution with HBr Bromoethane ($CH_3CH_2Br$) — halogenoalkane $\downarrow$ Step 3: substitution with excess $NH_3$ Ethylamine ($CH_3CH_2NH_2$) — primary amine — Three forward steps from ethanol, four from ethene. Each step uses a different reaction type: hydration, nucleophilic substitution (twice), giving a varied and logical route. The carbon skeleton is unchanged throughout — only the functional group changes at each step.

Answer: Step 1: CH2=CH2 + H2O -> CH3CH2OH (steam, H3PO4 catalyst, 300 C, 60-70 atm). Step 2: CH3CH2OH -> CH3CH2Br (NaBr + concentrated H2SO4, heat). Step 3: CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br (excess NH3 in ethanol, heat under pressure).

6. Choosing Between Alternative Routes

In many synthesis problems, more than one route is possible. AQA questions sometimes ask you to evaluate alternative routes or suggest why one route is preferred. This requires you to think beyond just whether a route works chemically — you need to consider yield, selectivity, and practicality.

For example, converting a primary alcohol to a primary amine can be done via a halogenoalkane (two steps) or via oxidation to an aldehyde followed by reductive amination. At A Level, the halogenoalkane route is the one on the specification, so that is the route you should use unless the question explicitly directs you otherwise.

• Prefer routes where each step has a high yield — more steps means more material lost at each stage.
• Avoid routes that produce unwanted side products (e.g. using insufficient $NH_3$ in amine synthesis gives a mixture of primary, secondary, and tertiary amines).
• Check that each intermediate is stable enough to isolate and carry forward to the next step.
• Where two routes are equally valid chemically, the shorter route is generally preferred.
• Always check that the reagents for one step will not destroy a functional group introduced in a previous step.

A particularly important selectivity issue arises with oxidation of alcohols. A primary alcohol can be oxidised to an aldehyde (by distilling the product immediately) or to a carboxylic acid (by heating under reflux). If your target is an aldehyde, you must specify distillation — otherwise the reaction continues to the acid. This is a classic AQA mark point that students frequently miss.

7. Handling Carbonyl Chemistry in Synthesis

Carbonyl compounds — aldehydes, ketones, and carboxylic acids — appear frequently as intermediates or targets in synthesis questions. The key distinction to keep in mind is that aldehydes and ketones are produced by oxidation of alcohols, while carboxylic acids can be produced either by further oxidation of aldehydes or by hydrolysis of nitriles.

Reduction of carbonyl compounds is equally important. Sodium tetrahydridoborate ($NaBH_4$) in water or methanol reduces aldehydes to primary alcohols and ketones to secondary alcohols. This is a nucleophilic addition mechanism — the hydride ion ($H^-$) from $NaBH_4$ acts as the nucleophile, attacking the electrophilic carbonyl carbon.

In a synthesis context, $NaBH_4$ reduction is useful when you need to convert a ketone intermediate into a secondary alcohol, or an aldehyde into a primary alcohol, without affecting other functional groups such as carbon-carbon double bonds or ester groups. This selectivity makes it more useful in multi-step synthesis than lithium aluminium hydride ($LiAlH_4$), which is a more powerful but less selective reducing agent — though $LiAlH_4$ is not required at AQA A Level.

The best synthesis answers are not just lists of reagents. They show that you are tracking both the functional group and the carbon skeleton at every stage, so you can tell when a chain extension is needed and when a route only changes functionality.

8. Exam Technique: Writing Synthesis Answers for Full Marks

Synthesis questions on AQA papers are typically worth 3–6 marks, with marks awarded for each correct step. The mark scheme usually awards one mark per step for the correct reagents and conditions, and sometimes an additional mark for the correct structural formula of the intermediate or product. Knowing the chemistry is necessary but not sufficient — you need to present it in the format the examiner expects.

The most reliable format is to number each step clearly, draw or write the structural formula of the product of that step, and then state the reagents and conditions underneath. If the question asks you to "describe" the synthesis, you must include what happens at each step in sequence. If it asks you to "suggest" a route, there may be more than one acceptable answer — but you still need to give full reagents and conditions for each step you propose.

Every synthesis question on an AQA paper can be approached with the same systematic framework. Applying this framework consistently — even under exam pressure — is what separates students who get full marks from those who lose marks on avoidable errors.

9. Bringing It Together: A Synthesis Planning Framework

• Step A — Identify the functional group of the target molecule and the functional group of the starting material.
• Step B — Count the carbons in the target and the starting material. If the target has more carbons, you need a KCN step somewhere in the route.
• Step C — Work backwards from the target: what reaction produces the target functional group? What must the precursor functional group have been?
• Step D — Repeat Step C until you reach the given starting material or a recognisable intermediate.
• Step E — Write the forward route, numbering each step and stating full reagents and conditions.
• Step F — Check: is every reaction in the specification? Is the carbon count correct at each stage? Have you specified 'excess' where required and 'reflux' vs 'distil' for oxidations?

This framework works for two-step, three-step, and four-step routes alike. The only thing that changes is the number of times you apply Step C. With practice, the retrosynthetic reasoning becomes fast and reliable — and that is exactly the skill AQA is assessing when it asks you to "devise a synthesis".

Multi-step synthesis draws together every area of organic chemistry you have studied: alkenes, halogenoalkanes, alcohols, carbonyls, carboxylic acids, amines, nitriles, and esters. Mastering it means you have genuinely understood the connections between all of these functional groups — which is precisely what the specification requires.