CIE AS/A Level Chemistry 36.1 Organic synthesis Study Notes- 2025-2027 Syllabus
CIE AS/A Level Chemistry 36.1 Organic synthesis Study Notes – New Syllabus
CIE AS/A Level Chemistry 36.1 Organic synthesis Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Chemistry latest syllabus with Candidates should be able to:
for an organic molecule containing several functional groups:
(a) identify organic functional groups using the reactions in the syllabus
(b) predict properties and reactionsdevise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus
analyse a given synthetic route in terms of type of reaction and reagents used for each step of it, and
possible by-products
Organic Molecules with Several Functional Groups
Many organic molecules contain more than one functional group. You must be able to:
- (a) identify the functional groups present using reactions from the syllabus
- (b) predict the properties and reactions of the molecule
(a) Identifying Functional Groups
To identify functional groups, you should:
- inspect the structure for characteristic groups
- use specific chemical tests from the syllabus
- match positive results to functional groups
Common Functional Groups and Tests
| Functional group | Test / reaction | Positive observation |
|---|---|---|
| Alkene (C=C) | Bromine water | Orange → colourless |
| Alcohol (–OH) | Oxidation with acidified \( \mathrm{K_2Cr_2O_7} \) | Orange → green (1° / 2°) |
| Aldehyde (–CHO) | Tollens’ reagent | Silver mirror |
| Carboxylic acid (–COOH) | \( \mathrm{Na_2CO_3} \) | Effervescence (CO₂) |
| Phenol | Bromine water | White precipitate |
| Amine | Reaction with acids | Salt formation |
When more than one group is present, several tests may be positive.
(b) Predicting Properties and Reactions
Once the functional groups are identified, you must predict:
- acid–base behaviour
- polarity and solubility
- likely chemical reactions
Predicting Properties
- –OH, –NH₂, –COOH → hydrogen bonding, higher boiling point
- –COOH → acidic behaviour
- –NH₂ → basic behaviour
- non-polar regions → low water solubility
Predicting Reactions
Each functional group reacts independently, using reactions from the syllabus:
- –OH → oxidation or esterification
- –COOH → salt formation, esterification, amide formation
- –NH₂ → salt formation, acylation
- C=C → addition reactions
In multifunctional molecules, reactions may be:
- selective (one group reacts preferentially)
- competing (more than one possible reaction)
Example
A molecule contains a –COOH group and an –OH group. Identify one test for each functional group and one property of the molecule.
▶️ Answer / Explanation
The –COOH group gives effervescence with \( \mathrm{Na_2CO_3} \).
The –OH group can be oxidised using acidified \( \mathrm{K_2Cr_2O_7} \).
The molecule is capable of hydrogen bonding and has a relatively high boiling point.
Example
A compound contains an aromatic ring, a phenol group and an amine group. Predict two reactions and one property of the compound.
▶️ Answer / Explanation
The phenol group reacts with bromine water to form a white precipitate.
The amine reacts with acids to form an ammonium salt.
The molecule is polar and capable of hydrogen bonding, giving a relatively high boiling point.
Devising Multi-Step Organic Synthetic Routes
In synthesis questions, you are required to devise multi-step routes to prepare a target organic molecule using only reactions in the syllabus. You must show suitable starting materials, reagents, conditions, and logical sequencing of steps.
General Strategy for Multi-Step Synthesis
- Identify the functional group(s) in the target molecule
- Work backwards to simpler compounds (retrosynthesis)
- Choose reactions that introduce or modify functional groups
- Ensure each step uses approved syllabus reagents
- Check the order of steps avoids unwanted side reactions
Common Functional-Group Interconversions
- alkene → alcohol (acid-catalysed hydration)
- alcohol → halogenoalkane (PCl₅ / SOCl₂)
- halogenoalkane → alcohol (aqueous OH⁻)
- halogenoalkane → nitrile → carboxylic acid / amine
- alcohol → aldehyde / ketone → carboxylic acid
- carboxylic acid → ester / amide
- nitro compound → amine (reduction)
Key Rules
- each arrow must have reagents and conditions
- avoid unnecessary steps
- routes must be chemically sensible
- functional groups must survive earlier steps
Example
Devise a synthetic route to prepare ethyl ethanoate from ethene.
▶️ Answer / Explanation
Step 1: Convert ethene to ethanol
\( \mathrm{CH_2{=}CH_2 \rightarrow CH_3CH_2OH} \)
Reagents: steam, phosphoric acid catalyst
Step 2: Oxidise ethanol to ethanoic acid
\( \mathrm{CH_3CH_2OH \rightarrow CH_3COOH} \)
Reagents: acidified \( \mathrm{K_2Cr_2O_7} \), reflux
Step 3: Esterification
\( \mathrm{CH_3COOH + CH_3CH_2OH \rightarrow CH_3COOCH_2CH_3 + H_2O} \)
Reagents: concentrated \( \mathrm{H_2SO_4} \), heat
Example
Devise a synthetic route to prepare phenylamine from benzene.
▶️ Answer / Explanation
Step 1: Nitration of benzene
\( \mathrm{C_6H_6 \rightarrow C_6H_5NO_2} \)
Reagents: concentrated \( \mathrm{HNO_3} \) / concentrated \( \mathrm{H_2SO_4} \), 50–60°C
Step 2: Reduction of nitrobenzene
\( \mathrm{C_6H_5NO_2 \rightarrow C_6H_5NH_2} \)
Reagents: hot Sn / concentrated HCl, followed by \( \mathrm{NaOH(aq)} \)
Analysing a Multi-Step Synthetic Route
In synthesis questions, you may be given a completed synthetic route. You must be able to analyse each step in terms of:
- the type of reaction
- the reagents and conditions
- any possible by-products
Step-by-Step Analysis Method (Exam Technique)
- Identify what functional group changes between steps
- Name the reaction type causing this change
- State the reagents and conditions
- State any by-products formed
Common Reaction Types and By-Products
| Reaction type | Functional group change | Typical by-product |
|---|---|---|
| Oxidation | Alcohol → aldehyde / acid | \( \mathrm{H_2O} \) |
| Nucleophilic substitution | Halogenoalkane → alcohol / amine | Halide ion (e.g. \( \mathrm{Cl^-} \)) |
| Elimination | Halogenoalkane → alkene | \( \mathrm{KBr} \), \( \mathrm{H_2O} \) |
| Condensation | Acid + alcohol → ester | \( \mathrm{H_2O} \) |
| Reduction | Nitro → amine | Metal salts |
Example of Route Analysis
Consider the following synthetic route:
alkene → alcohol → carboxylic acid → ester
Step 1: Alkene → Alcohol
Reaction type: addition (hydration)
Reagents: steam, phosphoric acid catalyst
By-products: none (addition reaction)
Step 2: Alcohol → Carboxylic Acid
Reaction type: oxidation
Reagents: acidified \( \mathrm{K_2Cr_2O_7} \), reflux
By-products: \( \mathrm{H_2O} \), reduced chromium ions
Step 3: Carboxylic Acid → Ester
Reaction type: condensation (esterification)
Reagents: alcohol, concentrated \( \mathrm{H_2SO_4} \), heat
By-product: \( \mathrm{H_2O} \)
Common By-Products Expect
- \( \mathrm{H_2O} \) from condensation reactions
- hydrogen halides (e.g. \( \mathrm{HCl} \)) from acyl chloride reactions
- halide ions from substitution reactions
- metal salts from reduction reactions
Example
An alcohol is converted into a halogenoalkane using \( \mathrm{PCl_5} \). Identify the reaction type and a by-product.
▶️ Answer / Explanation
This is a substitution reaction.
By-products include \( \mathrm{POCl_3} \) and \( \mathrm{HCl} \).
Example
A synthetic route contains a step where a halogenoalkane reacts with \( \mathrm{NH_3} \) in ethanol under pressure. Analyse this step.
▶️ Answer / Explanation
This is a nucleophilic substitution reaction.
Reagents are ethanolic ammonia and heat under pressure.
By-products include ammonium halide salts.