Organic Synthesis (Edexcel IAL Chemistry Unit 5)

Edexcel International A-Level Chemistry· 20.1–20.5 (WCH15)· 35 min read

1. Structure Determination of Organic Compounds★★★☆☆⏱ 8 min

📘 Definition

Structure determination

Process of deducing empirical, molecular and structural formulae of unknown organic compounds using quantitative and spectroscopic data.

Start by calculating empirical formula from percentage composition or combustion analysis data, using mole ratios of elements. Use the molecular ion peak from mass spectrometry to find relative molecular mass, then scale the empirical formula to get the molecular formula. Combine IR, ¹H NMR and ¹³C NMR data (using provided data booklet tables) to identify functional groups and their positions to get the full structural formula.

📐 Worked Example

An unknown compound has composition 62.1% C, 10.3% H, 27.6% O. Its molecular ion peak is at m/z = 58. It has a ¹H NMR peak at δ=2.1 (singlet, integration 6). Deduce its structure.

1
Calculate mole ratios of elements:
$C: \frac{62.1}{12.0} = 5.175, H: \frac{10.3}{1.0} = 10.3, O: \frac{27.6}{16.0} = 1.725$
2
Divide by the smallest value to get empirical formula:
$C: \frac{5.175}{1.725} = 3, H: \frac{10.3}{1.725} \approx 6, O: \frac{1.725}{1.725} = 1 \rightarrow Empirical = C_3H_6O, M_r = 58 which matches the molecular ion, so molecular formula = C_3H_6O$
3
The ¹H NMR peak is a singlet with integration 6, meaning two equivalent CH₃ groups with no adjacent H atoms. The only matching structure is propanone (CH₃COCH₃).

Exam tip:

Always cross-check your final structure against all given data points to avoid missing functional groups or isomer constraints.

2. Grignard Reagents for Chain Lengthening★★★★☆⏱ 7 min

📘 Definition

Grignard reagent

RMgX

Organometallic reagent formed by reacting a halogenoalkane/halogenoarene with magnesium turnings in dry ether.

Grignard reagents are nucleophilic, and are used to add one or more carbon atoms to an organic chain, making them powerful for synthesis. They react with four main carbonyl-containing species per the specification, followed by acid hydrolysis: 1. Carbon dioxide (CO₂) → carboxylic acid (adds 1 carbon atom) 2. Methanal → primary alcohol 3. Aldehyde → secondary alcohol 4. Ketone → tertiary alcohol. All reactions must be carried out in completely dry ether, as Grignard reagents react violently with water.

📐 Worked Example

Outline the reaction of ethylmagnesium bromide (CH₃CH₂MgBr) with propanone, including conditions and product structure.

1
Reaction is carried out in dry ether to avoid side reactions with water.
2
The nucleophilic ethyl group attacks the electrophilic carbonyl carbon in propanone, forming an intermediate alkoxide ion.
3
Acid hydrolysis (e.g. dilute HCl) protonates the alkoxide to form the final product: 2-methylbutan-2-ol, a tertiary alcohol.

3. Planning Multi-step Organic Synthesis Routes★★★★☆⏱ 8 min

All synthesis questions are capped at 4 steps, and only use reactions from the Edexcel IAL specification. When planning a route: 1. Compare the starting material and target product to identify changes in functional groups and carbon chain length. 2. Select reagents and conditions for each step, ensuring no side reactions will interfere with desired products. 3. Prioritise routes with higher yields and fewer steps where possible. You may also be asked to predict properties of unfamiliar intermediate compounds based on their functional groups.

📐 Worked Example

Plan a 2-step synthesis of butanoic acid starting from 1-bromopropane, including reagents and conditions for each step.

1
Step 1: Convert 1-bromopropane to propylmagnesium bromide (Grignard reagent) by reacting with magnesium turnings in dry ether.
2
Step 2: Bubble CO₂ through the Grignard solution, then add dilute HCl for hydrolysis. The product is butanoic acid, which has one extra carbon atom compared to the starting material.

Exam tip:

Always label reagents and conditions clearly for each step – partial marks are awarded for correct reagents even if the full route is wrong.

4. Organic Synthesis Practical Purification Techniques★★★☆☆⏱ 7 min

After synthesising an organic product, you need to purify it before analysis. The technique you choose depends on whether the product is a solid or liquid:
• Solids: Recrystallisation, melting point determination
• Liquids: Distillation, steam distillation, solvent extraction
General purification steps include washing with water or sodium carbonate solution to remove acidic impurities, drying with an anhydrous salt (e.g. MgSO₄), and filtering off drying agents.

📐 Worked Example

You have synthesised a liquid organic product that is insoluble in water, and contaminated with unreacted carboxylic acid starting material. Outline the purification steps.

1
1. Wash the crude product mixture with dilute sodium carbonate solution in a separating funnel – this neutralises the carboxylic acid impurity to form a soluble sodium salt, which will be in the aqueous layer.
2
1. Run off the lower aqueous layer and discard, then wash the organic layer with distilled water to remove any remaining carbonate salt.
3
1. Add anhydrous magnesium sulfate to the organic layer to dry it, then filter off the solid drying agent.
4
1. Distill the remaining organic liquid to collect the pure product at its known boiling point.

5. Core Practical 16: Preparation of Aspirin★★★☆☆A2 only⏱ 5 min

Aspirin (2-ethanoyloxybenzoic acid) is synthesised by reacting 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride, using concentrated phosphoric acid as a catalyst. The reaction is heated under reflux for 10 minutes, then the crude product is purified via recrystallisation from ethanol, and its purity is checked by measuring its melting point (pure aspirin melts at 136°C).

📐 Worked Example

State why sodium carbonate solution is used during the work-up of crude aspirin, and what observation you would expect during this step.

1
Sodium carbonate solution neutralises any unreacted 2-hydroxybenzoic acid and phosphoric acid catalyst, converting them to soluble sodium salts.
2
You will observe effervescence (bubbles of CO₂ gas) as the carbonate reacts with the acidic impurities.

6. Common Pitfalls

Wrong move:

Forgetting to include dry ether as a condition for Grignard reactions

Why:

Grignard reagents react violently with water, producing unwanted alkane side products and posing a safety risk

Correct move:

Always explicitly state dry ether as the solvent for Grignard formation and reactions, followed by acid hydrolysis

Wrong move:

Using out-of-spec named reactions (e.g. Friedel-Crafts acylation) for chain lengthening

Why:

The specification only allows Grignard reagents for chain lengthening in this topic, and extra reactions will not be awarded marks

Correct move:

Only use the 4 allowed Grignard reactions and prior specification organic reactions for synthesis routes

Wrong move:

Planning synthesis routes longer than 4 steps

Why:

All Edexcel IAL WCH15 synthesis questions are capped at 4 steps, and longer routes will be marked incorrect even if they work

Correct move:

Find the shortest possible route (max 4 steps) using allowed reactions

Wrong move:

Using washing with water alone to remove acidic organic impurities

Why:

Water only removes soluble ionic impurities, not unreacted organic acids which are mostly non-polar

Correct move:

Wash with dilute sodium carbonate solution to neutralise organic acids to soluble ionic salts, then wash with water to remove remaining carbonate

Wrong move:

Assigning a molecular formula directly from empirical formula without checking the molecular ion peak

Why:

The empirical formula is the simplest whole number ratio, which may be a fraction of the actual molecular mass

Correct move:

Use the molecular ion peak from MS to find the relative molecular mass, then scale the empirical formula to get the correct molecular formula

7. Quick Reference Cheatsheet

Task	Method / Reagent	Key Condition
Form Grignard reagent	Mg + halogenoalkane	Dry ether solvent
Make carboxylic acid from Grignard	CO₂ + dilute acid hydrolysis	Dry ether, then H⁺(aq)
Purify solid organic product	Recrystallisation	Use minimum volume of hot solvent
Purify liquid organic product	Distillation	Collect fraction at boiling point of product
Deduce molecular formula	Empirical formula + MS molecular ion	Match Mᵣ to scaled empirical mass
Prepare aspirin	Salicylic acid + ethanoic anhydride, conc H₃PO₄	Heat under reflux 10 mins

8. Frequently Asked

Why do Grignard reactions require dry ether?

Grignard reagents react violently with water to produce alkanes, so all glassware and solvents must be completely dry to avoid side reactions and safety hazards.

What is the maximum number of steps in a synthesis route question?

All Edexcel IAL WCH15 synthesis questions are capped at 4 steps, using only reactions from the specification (no extra named reactions are required or accepted).

Going deeper

syllabusEdexcel IAL Chemistry Specification (2018+)Refer to section 20 for full topic guidance
core_practicalCore Practical 16: Aspirin Preparation Guide

What's Next

Now that you have mastered organic synthesis for Edexcel IAL Chemistry Unit 5, you are ready to practice past paper questions to apply your knowledge to exam-style scenarios. Synthesis questions are often synoptic, so revisiting all prior organic reaction content from Units 1, 2 and 4 will help you quickly identify required steps for multi-step routes. You should also practice interpreting combined spectroscopy data, as structure determination questions frequently appear alongside synthesis questions in WCH15 papers. Don’t forget to review Core Practical 16 thoroughly, as practical-based questions on aspirin preparation and purification are common and carry high mark weight.