Mini-Lesson

Organic Synthesis

This mini-lesson covers AQA 3.3.14 Organic synthesis: putting all the reactions together into multi-step routes. You will choose reagents and conditions for each functional group interconversion, learn the two ways to lengthen a carbon chain, and calculate overall yields — the reason chemists keep routes short.

Work through each screen, answer the questions as you go (some are wordy, most are calculations) and collect ⭐ stars. Everything here is A-level standard — the maths is done properly, not skipped. Press Start when you're ready.

Synthesis · the toolkit

The functional group interconversions you must know

Synthesis is not a new topic — it is every reaction you already know, chained together. Learn this map:

Alkene → alcohol: steam, H₃PO₄ catalyst, high temperature and pressure (hydration).
Alkene → halogenoalkane: HBr at room temperature (electrophilic addition).
Alcohol → alkene: concentrated H₂SO₄ (or Al₂O₃), heat — dehydration/elimination.
Alcohol → halogenoalkane: e.g. NaBr with concentrated H₂SO₄.
Halogenoalkane → alcohol: warm aqueous NaOH (nucleophilic substitution).
Halogenoalkane → alkene: hot ethanolic KOH (elimination). Same reagent, different solvent, different product.
Halogenoalkane → nitrile: KCN in ethanol/water, reflux — this adds a carbon.
Halogenoalkane → amine: excess ethanolic NH₃ in a sealed tube.
1° alcohol → aldehyde: K₂Cr₂O₇/H₂SO₄, distil. → carboxylic acid: the same reagent, but reflux.
2° alcohol → ketone: K₂Cr₂O₇/H₂SO₄, reflux.
Carbonyl → alcohol: NaBH₄ (reduction).
Carbonyl → hydroxynitrile: KCN then dilute acid — this also adds a carbon.
Nitrile → 1° amine: LiAlH₄, or H₂/Ni. Nitrile → carboxylic acid: dilute HCl, reflux (hydrolysis).
Carboxylic acid → ester: alcohol + concentrated H₂SO₄ catalyst.

Sort it

Sort each reaction by its type

Tap a reaction, then tap the type of reaction it is.

🟩 Nucleophilic substitution

🟪 Elimination

🟦 Oxidation or reduction

Quick check

?You need to convert 1-bromopropane into propan-1-ol. Which reagent and conditions?

Synthesis · growing the chain

Two ways to add a carbon atom

Most reactions keep the carbon skeleton the same. Only two reactions on this specification lengthen the chain — and both go through a nitrile.

Route 1 — from a halogenoalkane, with KCN:

CH₃CH₂Br + KCN → CH₃CH₂CN + KBrnucleophilic substitution. C₂ → C₃

Route 2 — from a carbonyl, with KCN then dilute acid:

CH₃CHO + HCN → CH₃CH(OH)CNnucleophilic addition, giving a hydroxynitrile. C₂ → C₃ (and a racemic mixture, because the C=O is planar)

Then the nitrile can go two ways:

Reduction (LiAlH₄, or H₂ with a Ni catalyst) → a primary amine: CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂ (C 3, H 5 + 4 = 9 = 9, N 1 ✓)
Acid hydrolysis (dilute HCl, reflux) → a carboxylic acid: CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl

Friedel–Crafts acylation is the aromatic equivalent: it attaches a whole acyl group (and therefore extra carbons) to a benzene ring, making a C–C bond that cannot be made any other way on this course.

Quick check

?You need to convert ethanol (C₂) into propanoic acid (C₃). Which route works?

Calculate

Your turn

1A two-step synthesis has yields of 80% and 50%. Calculate the overall percentage yield.

%

Hint: Multiply the fractional yields: 0.80 × 0.50, then × 100.

Calculate

Your turn

2A three-step synthesis has yields of 90%, 80% and 75%. Calculate the overall percentage yield.

%

Hint: 0.90 × 0.80 × 0.75, then × 100.

Calculate

Your turn

3A chemist starts with 0.500 mol of a reactant and runs a synthesis whose overall yield is 40%. The product has M_r = 74.0 and forms in a 1 : 1 ratio. Calculate the mass of product obtained.

g

Hint: n(product) = 0.500 × 0.40 = 0.200 mol. m = 0.200 × 74.0.

Match it

Match the conversion to its reagent

Tap a conversion on the left, then the reagent and conditions on the right.

Conversion

Reagent and conditions

Synthesis · judging a route

Choosing between two routes

Two routes can give the same product. A chemist chooses between them on five grounds — and AQA expects you to argue with them, not just list them.

Number of steps. Yields multiply, so every extra step costs you badly. Three steps at 80% each gives only 51% overall.
Percentage yield of each step.
Atom economy. An addition reaction is 100%; a substitution or elimination always wastes atoms as by-product.
Cost, availability and hazard of the reagents. KCN and HCN are lethally toxic; LiAlH₄ reacts violently with water; concentrated H₂SO₄ is corrosive.
Waste. What must be disposed of, and at what cost and environmental damage?

Worked example — why short routes win

Route A: 2 steps, 80% and 80% → 0.80 × 0.80 = 64% overall

Route B: 4 steps, 90% each → 0.90⁴ = 0.656 → 65.6% overall

Even though every step of Route B is better, it barely beats the two-step route — and it needs twice the reagents, time and purification. Steps are expensive.

Calculate

Your turn

4Calculate the atom economy for the hydration of ethene: C₂H₄ + H₂O → C₂H₅OH.

%

Hint: There is only one product — so what fraction of the total product mass is the desired product?

Quick check

?Why does a synthesis with more steps usually give a much lower overall yield?

Quick check

?Which of these is the strongest argument that an addition reaction is "greener" than a substitution?

Synthesis · exam traps

Reading a route question properly

Count the carbons first. If the product has more carbons than the starting material, you must use a chain-lengthening step — KCN with a halogenoalkane, or HCN with a carbonyl. There is no other way on this specification.
State the SOLVENT, not just the reagent. "NaOH" is not enough: aqueous NaOH substitutes; ethanolic KOH eliminates.
Say "reflux" or "distil" when oxidising a primary alcohol. The apparatus is the answer.
Yields multiply. Never add or average them.

The two nitrile exits. Once you have made a nitrile you can go two ways: reduce it (LiAlH₄ or H₂/Ni) to a primary amine, or hydrolyse it (dilute HCl, reflux) to a carboxylic acid. Knowing both exits doubles the number of targets you can reach.

Quick check

?Which reagent and conditions convert a nitrile into a carboxylic acid?

Calculate

Your turn

5A four-step synthesis has a yield of 70% at every step. Calculate the overall percentage yield.

%

Hint: 0.70 × 0.70 × 0.70 × 0.70 = 0.70⁴, then × 100.

Recap

The big ideas to know

Route design: identify the functional group you have and the one you want, then find the shortest chain of known reactions between them

Lengthening the chain: KCN with a halogenoalkane, or HCN with a carbonyl — both add one carbon as a nitrile

Reducing the nitrile: LiAlH₄ or H₂/Ni gives a primary amine; acid hydrolysis gives a carboxylic acid

Substitution vs elimination: aqueous NaOH substitutes; ethanolic KOH eliminates

Overall yield: multiply the fractional yields: 0.80 × 0.50 = 0.40 → 40%. Fewer steps means a much better yield

Green chemistry: judge a route on yield, atom economy, number of steps, cost and hazard of the reagents, and the waste produced

That is AQA 3.3.14. Press Finish to see your score.

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