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Organic synthesis questions are among the most challenging in the Edexcel Chemistry papers — and among the most rewarding. They test your ability to link reactions together into multi-step routes, applying reagents, conditions, and mechanisms to convert one molecule into another. This lesson covers what examiners expect, every key functional group interconversion, retrosynthetic thinking, and worked multi-step examples.
When a question asks you to outline a synthesis route, you must provide:
Simply naming the reaction type is not enough. You must specify both reagents and conditions. "Oxidation" alone scores zero — "K₂Cr₂O₇/H₂SO₄, reflux" scores full marks.
The heart of organic synthesis is knowing how to convert one functional group into another. Here are the essential transformations:
The diagram below shows how the major functional groups are interconnected. Alcohols sit at the centre because they can be converted to or from almost every other group.
graph TD
ALK[Alkene] -->|"H₂O/H₃PO₄, 300°C"| ALC[Alcohol]
ALK -->|"HBr, RT"| HAL[Halogenoalkane]
HAL -->|"NaOH(aq), reflux"| ALC
HAL -->|"NaOH in ethanol, reflux"| ALK
HAL -->|"KCN in ethanol, reflux"| NIT[Nitrile]
HAL -->|"excess NH₃, ethanol, heat"| AMI[Amine]
NIT -->|"dilute HCl, reflux"| CA[Carboxylic Acid]
NIT -->|"LiAlH₄ in dry ether"| AMI
ALC -->|"K₂Cr₂O₇/H₂SO₄, distil"| ALD[Aldehyde]
ALC -->|"K₂Cr₂O₇/H₂SO₄, reflux"| CA
ALC -->|"H₂SO₄, heat"| ALK
ALC -->|"NaBr/H₂SO₄ or SOCl₂"| HAL
ALD -->|"K₂Cr₂O₇/H₂SO₄, reflux"| CA
ALD -->|"NaBH₄"| ALC
CA -->|"Alcohol + H₂SO₄, reflux"| EST[Ester]
CA -->|"SOCl₂"| ACL[Acyl Chloride]
ACL -->|"Alcohol, RT"| EST
ACL -->|"NH₃, RT"| AMD[Amide]
For complex multi-step problems, work backwards from the target molecule:
This approach prevents you from going down dead-end routes and helps you find the shortest pathway.
Target: CH₃CH₂COOH (propanoic acid, 3 carbons) Starting material: CH₃CH₂OH (ethanol, 2 carbons)
Working backwards:
Forward route:
This is a classic 3-step route involving carbon chain extension via the nitrile route.
The nitrile route is the key to extending a carbon chain:
This is one of the most commonly tested synthesis pathways. If the target molecule has more carbons than the starting material, think nitrile.
Alcohols are central to organic synthesis because they can be converted to or from almost every other functional group. Many synthesis routes pass through an alcohol intermediate. When stuck, ask yourself: "Can I get to an alcohol from here?"
A critical skill is controlling oxidation of primary alcohols:
| Product desired | Reagent | Conditions | Why |
|---|---|---|---|
| Aldehyde | K₂Cr₂O₇/H₂SO₄ | Distil immediately | Removes aldehyde before further oxidation |
| Carboxylic acid | K₂Cr₂O₇/H₂SO₄ | Reflux | Keeps aldehyde in contact with oxidising agent |
If asked to explain why different conditions give different products, discuss the fact that distillation removes the aldehyde from the reaction mixture as it forms, preventing further oxidation. Reflux keeps the aldehyde in contact with the oxidising agent, allowing oxidation to continue to the carboxylic acid.
Question: Outline a synthesis route to convert ethene into ethyl ethanoate.
Step 1: Identify start and target
Step 2: Retrosynthesis
Step 3: Forward route
Note: Step 3 requires ethanol as a reactant, which was produced in step 1. A good answer would note that some ethanol from step 1 is set aside for step 3 while the rest is oxidised in step 2.
| Mistake | Why it costs marks | Correct approach |
|---|---|---|
| "Oxidise the alcohol" | No reagent or conditions specified | "K₂Cr₂O₇/H₂SO₄, reflux" |
| "React with NaOH" | Solvent not specified | "NaOH(aq), reflux" |
| "Heat" | No specific method | "Reflux" or "Distil" |
| Missing carbon chain extension | Target has more carbons than starting material | Use the nitrile route (KCN) |
| Wrong Markovnikov product | Ignoring regioselectivity | H adds to C with more H atoms |
Organic synthesis is the part of Paper 2 most candidates either love or fear, and there is no middle ground. The skill rewards a structured "starting material → target → reagents and conditions → mechanism → product" workflow that is almost identical across the spec. A candidate fluent on the workflow can secure 12–18 marks on Paper 2 and another 6–10 on Paper 3 with high reliability. This deeper strategy section codifies the workflow and the mark-scheme expectations.
Edexcel 9CH0 examines organic synthesis in three recurring formats:
These appear principally on Paper 2, where Topics 6 (Organic I), 17 (Organic II), and 18 (Organic III) cluster. Synthesis questions also appear on Paper 3 in synoptic form, often combined with spectroscopy or yield/atom economy calculations. Paper 1 occasionally tests reagents-and-conditions for organic in inorganic-context redox questions (e.g. oxidation by acidified dichromate(VI)).
The reagent toolkit candidates must master is small but precise: roughly 25 distinct reagent-condition pairs cover 95% of the spec. Memorising these is non-negotiable. Beyond memorisation, the strategic skill is retrosynthetic thinking — working backwards from target to starting material by identifying the disconnections that map onto known reactions.
Synthesis pacing is usually generous: the marks are awarded for each correct step, and the steps are independent. A 6-mark synthesis route is typically 6 marks for 6 steps (reagent + condition + product per step ≈ 2 marks/step). Pace at 1.17 min/mark.
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