What is organic synthesis in the context of AS-Level Organic Chemistry?

Organic synthesis is the process of constructing organic compounds through chemical reactions. In the context of AS-Level Organic Chemistry, it involves understanding the mechanisms and pathways used to create complex molecules from simpler ones, focusing on reaction types such as substitution, addition, and elimination.

Why is organic synthesis important in the Cambridge International A Levels Chemistry curriculum?

Organic synthesis is crucial in the Cambridge International A Levels Chemistry curriculum because it helps students understand how complex organic molecules are formed. This knowledge is foundational for further studies in chemistry and related fields, as it applies to pharmaceuticals, materials science, and biochemistry.

What are some common techniques used in organic synthesis at the AS-Level?

At the AS-Level, common techniques in organic synthesis include refluxing, distillation, recrystallization, and chromatography. These methods are used to purify and isolate organic compounds, allowing students to explore the practical aspects of chemical reactions and product formation.

How do substitution reactions play a role in organic synthesis?

Substitution reactions are a key component of organic synthesis, where one atom or group of atoms in a molecule is replaced by another. This type of reaction is fundamental in creating a wide variety of organic compounds and is extensively covered in AS-Level Organic Chemistry to illustrate the transformation of functional groups.

What safety precautions should be taken during organic synthesis experiments in a laboratory setting?

During organic synthesis experiments, it's essential to wear appropriate personal protective equipment, such as lab coats, gloves, and safety goggles. Students should also be familiar with the properties of the chemicals they are using, ensure proper ventilation, and be aware of emergency procedures to handle spills or accidents safely.

Why is "Giving vague reagents (e.g. 'oxidise') without conditions." a common mistake in Organic synthesis?

Why it happens: Knowing the transformation but not the specifics. How to avoid it: State the reagent and conditions (e.g. K₂Cr₂O₇/H₂SO₄, reflux/distil). Source: 9701 Examiner Reports 2022-2024

Why is "Changing the carbon number without a KCN/HCN step." a common mistake in Organic synthesis?

Why it happens: Forgetting only cyanide adds a carbon. How to avoid it: Plan the carbon count first; use KCN or HCN to add a carbon.

Why is "Refluxing when an aldehyde intermediate is needed." a common mistake in Organic synthesis?

Why it happens: Not matching conditions to the target. How to avoid it: Distil to STOP at the aldehyde; reflux to go all the way to the acid.

Why is "Including a step that doesn't work (e.g. NaBH₄ on a carboxylic acid)." a common mistake in Organic synthesis?

Why it happens: Forcing a route. How to avoid it: Check every step is a real reaction with valid reagents/conditions.

Organic synthesis(AS-Level Organic Chemistry)Cambridge International A Level Chemistry 9701

Organic synthesis

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Organic Synthesis — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How to identify functional groups, devise multi-step synthetic routes between organic compounds, and analyse a given route — pulling together every reaction in the AS organic course.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

21.1.1 — For an organic molecule with several functional groups: (a) identify the functional groups using the reactions in the syllabus; (b) predict its properties and reactions.
21.1.2 — Devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus.
21.1.3 — Analyse a given synthetic route in terms of the type of reaction and the reagents used for each step, and possible by-products.

The AS functional-group reaction map

Learn the standard one-step conversions; routes are just chains of these.

Every synthesis route is built from the standard one-step conversions of the AS course. Learn this map:

From	To	Reagents / conditions
Alkene	Alkane	H₂, Ni/Pt, heat
Alkene	Halogenoalkane	HX(g) (room temp)
Alkene	Alcohol	steam, H₃PO₄ catalyst
Alkene	Diol	cold dilute acidified KMnO₄
Halogenoalkane	Alcohol	NaOH(aq), reflux
Halogenoalkane	Nitrile (+1 C)	KCN in ethanol, reflux
Halogenoalkane	Amine	NH₃ in ethanol, heat under pressure
Halogenoalkane	Alkene	NaOH in ethanol, reflux (elimination)
Alcohol	Halogenoalkane	HX / PCl₅ / SOCl₂ / PCl₃
Alcohol	Alkene	Al₂O₃ or conc H₂SO₄ (dehydration)
1° Alcohol	Aldehyde	K₂Cr₂O₇/H₂SO₄, distil
1° Alcohol	Carboxylic acid	K₂Cr₂O₇/H₂SO₄, reflux
2° Alcohol	Ketone	K₂Cr₂O₇/H₂SO₄
Aldehyde/ketone	Alcohol	NaBH₄ (or LiAlH₄)
Aldehyde/ketone	Hydroxynitrile (+1 C)	HCN, KCN catalyst
Nitrile	Carboxylic acid	dilute acid, reflux
Carboxylic acid	Ester	alcohol, conc H₂SO₄
Carboxylic acid	1° Alcohol	LiAlH₄

Alcohols and halogenoalkanes are the central 'hubs'.
Add a carbon via KCN (nitrile) or HCN (hydroxynitrile).
Oxidation ladder: 1° alcohol → aldehyde → acid; 2° → ketone.

Devising a multi-step route

Compare start and target, fix the carbon count first, then change functional groups step by step.

A reliable strategy:

Identify the functional groups in the starting material and the target.
Compare the carbon count. If the target has one more carbon, plan a KCN or HCN step early. If fewer/same, no chain change is needed.
Bridge the functional groups using the reaction map, one step at a time — alcohols and halogenoalkanes are the most versatile intermediates.
Write the reagents and conditions for each step.

Worked — bromoethane → propanoic acid (needs +1 carbon):

$\text{CH}_3\text{CH}_2\text{Br} \xrightarrow{\text{KCN, ethanol}} \text{CH}_3\text{CH}_2\text{CN}$ (adds a carbon).
$\text{CH}_3\text{CH}_2\text{CN} \xrightarrow{\text{dilute HCl, reflux}} \text{CH}_3\text{CH}_2\text{COOH}$ .

Worked — ethene → ethyl ethanoate:

ethene + steam (H₃PO₄) → ethanol.
some ethanol → ethanoic acid (K₂Cr₂O₇, reflux).
ethanoic acid + ethanol (conc H₂SO₄) → ethyl ethanoate.

Identify FGs in start and target.
Fix the carbon count first (KCN/HCN if +1 C).
Bridge FGs via alcohols/halogenoalkanes, step by step.
Quote reagents and conditions for each step.

Identifying functional groups and analysing routes

For each step state the reaction type, reagents and by-products; predict reactions of multi-group molecules.

Identifying functional groups in a complex molecule lets you predict its reactions: a molecule with both an –OH and a –COOH, for example, can be oxidised (the alcohol), neutralised (the acid), esterified, and so on — apply the chemistry of each group.

Analysing a given route — for each step state:

the type of reaction (e.g. nucleophilic substitution, oxidation, condensation),
the reagents and conditions, and
any by-products (e.g. HBr, H₂O, NH₄⁺ salts).

Worked analysis. Step "CH₃CH₂Br → CH₃CH₂OH using NaOH(aq), reflux":

Reaction type: nucleophilic substitution (hydrolysis).
Reagent/conditions: aqueous NaOH, reflux.
By-product: NaBr (Br⁻).

Tip — test/identify clues: if a route uses a reagent that is also a test (e.g. 2,4-DNPH, Tollens', bromine water, K₂Cr₂O₇ colour change), mention what it confirms.

Multi-group molecule: apply the chemistry of each functional group.
Per step: reaction type + reagents/conditions + by-products.
Link reagents to the tests they represent where relevant.

How it’s examined

Multi-step synthesis is a high-mark Paper 2 question that integrates the whole organic course; it rewards correct reagents/conditions and recognising when a carbon must be added. Examiner reports flag vague conditions (e.g. 'oxidise' without the reagent/distil-vs-reflux) and routes that change the carbon number without a KCN/HCN step.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 21.1; 9701 Examiner Reports 2022-2024; 9701/22 & 9701/42 May/Jun 2024 question papers and mark schemes. Last reviewed 2026-05-20.

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Step-by-step worked examples — Organic synthesis

Step-by-step solutions to past-paper-style questions on organic synthesis, written exactly the way a tutor would explain them at the board.

1Single-step interconversions (3 marks)
Getting started• reagents
Question
Give the reagents and conditions for (a) ethanol → ethene, (b) ethene → ethanol, (c) ethanol → ethanoic acid.
Step-by-step solution
1. Step 1
  (a) Dehydration: hot conc H₂SO₄ (or Al₂O₃).
2. Step 2
  (b) Hydration: steam, H₃PO₄ catalyst.
3. Step 3
  (c) Oxidation: K₂Cr₂O₇/H₂SO₄, reflux.
Answer
(a) hot conc H₂SO₄; (b) steam + H₃PO₄; (c) K₂Cr₂O₇/H₂SO₄, reflux.
Examiner tip
Always give the reagent AND the conditions — 'oxidise' alone scores nothing.
2Analysing a step (3 marks)
Getting started• analysis
Question
For the step CH₃CH₂Br + NaOH(ethanol, reflux) → CH₂=CH₂, state the reaction type, and a by-product.
Step-by-step solution
1. Step 1
  Ethanolic NaOH + heat → elimination.
2. Step 2
  By-products: NaBr and H₂O.
Answer
Elimination; by-products NaBr and H₂O.
3Two-step route with chain extension (4 marks)
Building confidence• synthesis
Question
Devise a route to convert 1-bromopropane into butanoic acid, giving reagents for each step.
Step-by-step solution
1. Step 1
  Add a carbon: KCN in ethanol, reflux → butanenitrile.
  $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CN}$
2. Step 2
  Hydrolyse: dilute HCl, reflux → butanoic acid.
  $\text{CH}_3\text{CH}_2\text{CH}_2\text{CN} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$
Answer
1-bromopropane → butanenitrile (KCN/ethanol) → butanoic acid (dilute HCl, reflux).
Examiner tip
The +1 carbon needs the KCN step; quote reflux conditions.
4Multi-step route from an alkene (4 marks)
Building confidence• synthesis
Question
Outline how to convert ethene into ethyl ethanoate.
Step-by-step solution
1. Step 1
  Ethene → ethanol: steam, H₃PO₄ catalyst.
2. Step 2
  Some ethanol → ethanoic acid: K₂Cr₂O₇/H₂SO₄, reflux.
3. Step 3
  Esterify: ethanoic acid + ethanol, conc H₂SO₄ → ethyl ethanoate.
Answer
Ethene → ethanol (steam/H₃PO₄) → ethanoic acid (K₂Cr₂O₇, reflux) → ethyl ethanoate (conc H₂SO₄).
5Alcohol to amine (3 marks)
Stretch• synthesis
Question
Devise a two-step route from propan-1-ol to propylamine (same number of carbons).
Step-by-step solution
1. Step 1
  Step 1: propan-1-ol → 1-bromopropane (HBr, or PBr₃).
  $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$
2. Step 2
  Step 2: excess NH₃ in ethanol, heat under pressure → propylamine.
  $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$
Answer
Propan-1-ol → 1-bromopropane (HBr/PBr₃) → propylamine (excess NH₃/ethanol, pressure).
Examiner tip
No carbon is added here, so use NH₃ substitution (not KCN, which would add a carbon).
6Three-step route with chain extension (4 marks)
Stretch• synthesis, chain extension
Question
Devise a route from ethanol to 2-hydroxypropanoic acid (lactic acid).
Step-by-step solution
1. Step 1
  Ethanol → ethanal: K₂Cr₂O₇/H₂SO₄, distil off the aldehyde.
2. Step 2
  Ethanal → 2-hydroxypropanenitrile: HCN with KCN catalyst (adds a carbon).
  $\text{CH}_3\text{CHO} \rightarrow \text{CH}_3\text{CH(OH)CN}$
3. Step 3
  Hydrolyse: dilute acid, reflux → 2-hydroxypropanoic acid.
  $\text{CH}_3\text{CH(OH)CN} \rightarrow \text{CH}_3\text{CH(OH)COOH}$
Answer
Ethanol → ethanal (oxidise, distil) → 2-hydroxypropanenitrile (HCN/KCN) → lactic acid (hydrolysis).
Examiner tip
Plan the carbon count first: the +1 carbon comes from HCN; oxidise to aldehyde (distil) before adding HCN.

Key Definitions and Keywords — Organic synthesis

Definitions to memorise and the exact keywords mark schemes credit for organic synthesis answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Synthetic route
Examiner keyword
A planned sequence of reactions converting a starting material into a target molecule.
Functional group interconversion
Examiner keyword
Changing one functional group into another with the right reagent (e.g. alcohol → halogenoalkane → amine).
Chain extension
Examiner keyword
Adding a carbon to the chain using KCN (→ nitrile) or HCN (→ hydroxynitrile).
Working backwards (retrosynthesis)
Examiner keyword
Planning a route by starting from the target and deciding what could make it, step by step.

Common Mistakes and Misconceptions — Organic synthesis

The traps other students keep falling into on organic synthesis questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Giving vague reagents (e.g. 'oxidise') without conditions.
9701 Examiner Reports 2022-2024
Why it happens
Knowing the transformation but not the specifics.
How to avoid it
State the reagent and conditions (e.g. K₂Cr₂O₇/H₂SO₄, reflux/distil).
✕Changing the carbon number without a KCN/HCN step.
Why it happens
Forgetting only cyanide adds a carbon.
How to avoid it
Plan the carbon count first; use KCN or HCN to add a carbon.
✕Refluxing when an aldehyde intermediate is needed.
Why it happens
Not matching conditions to the target.
How to avoid it
Distil to STOP at the aldehyde; reflux to go all the way to the acid.
✕Including a step that doesn't work (e.g. NaBH₄ on a carboxylic acid).
Why it happens
Forcing a route.
How to avoid it
Check every step is a real reaction with valid reagents/conditions.

Practice questions

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Organic synthesis — frequently asked questions

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Organic synthesis

Short Study Notes in the form of Slides

Short Study Notes — Organic synthesis

Detailed Notes

What you’ll learn

How it’s examined

1Single-step interconversions (3 marks)

2Analysing a step (3 marks)

3Two-step route with chain extension (4 marks)

4Multi-step route from an alkene (4 marks)

5Alcohol to amine (3 marks)

6Three-step route with chain extension (4 marks)

Synthetic route

Functional group interconversion

Chain extension

Working backwards (retrosynthesis)

✕Giving vague reagents (e.g. 'oxidise') without conditions.

✕Changing the carbon number without a KCN/HCN step.

✕Refluxing when an aldehyde intermediate is needed.

✕Including a step that doesn't work (e.g. NaBH₄ on a carboxylic acid).

Practice questions

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Organic synthesis — frequently asked questions