# Organic Synthesis (Edexcel IAL Chemistry Unit 5)

> Edexcel International A-Level Chemistry · Edexcel IAL Chemistry
> Source: https://www.owlsprep.com/study/edexcel-ial-chemistry-u5-organic-synthesis/

This guide covers all Edexcel IAL Unit 5 (WCH15) organic synthesis content, including structure determination, Grignard reagents, multi-step route planning, practical purification, and Core Practical 16. It is aligned to specification points 20.1–20.5.

**Prerequisites:** [All organic reaction knowledge from Units 1, 2 and 4 of Edexcel IAL Chemistry](https://www.owlsprep.com/study/edexcel-ial-chemistry-organic-reactions-summary/); [Spectroscopy (IR, ¹H NMR, ¹³C NMR, Mass Spectrometry) fundamentals](https://www.owlsprep.com/study/edexcel-ial-chemistry-u4-spectroscopy/)

## Learning objectives

- Deduce organic formulae and structures using combustion analysis, mass spectrometry, IR and NMR data
- Explain Grignard reagent formation and reactions for chain lengthening
- Plan up to 4-step organic synthesis routes using specification-aligned organic reactions
- Describe practical purification procedures for organic products including Core Practical 16 (aspirin synthesis)
- Identify hazard controls for organic synthesis practical work

## Structure Determination of Organic Compounds

**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.

## Grignard Reagents for Chain Lengthening

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

*Notation:* RMgX

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.

> **warning**
>
> Never forget the dry ether condition for Grignard reactions – omitting this will cost you marks in the exam.

## Planning Multi-step Organic Synthesis Routes

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.

## Organic Synthesis Practical Purification Techniques

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: <br>• Solids: Recrystallisation, melting point determination <br>• Liquids: Distillation, steam distillation, solvent extraction <br> 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. 2. Run off the lower aqueous layer and discard, then wash the organic layer with distilled water to remove any remaining carbonate salt.
3. 3. Add anhydrous magnesium sulfate to the organic layer to dry it, then filter off the solid drying agent.
4. 4. Distill the remaining organic liquid to collect the pure product at its known boiling point.

## Core Practical 16: Preparation of Aspirin

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.

> **tip**
>
> Recrystallised aspirin should have a sharp melting point close to 136°C; a wide melting point range indicates impurities are present.

## Common pitfalls

- **Wrong:** Forgetting to include dry ether as a condition for Grignard reactions
  - Why it fails: Grignard reagents react violently with water, producing unwanted alkane side products and posing a safety risk
  - Correct: Always explicitly state dry ether as the solvent for Grignard formation and reactions, followed by acid hydrolysis
- **Wrong:** Using out-of-spec named reactions (e.g. Friedel-Crafts acylation) for chain lengthening
  - Why it fails: The specification only allows Grignard reagents for chain lengthening in this topic, and extra reactions will not be awarded marks
  - Correct: Only use the 4 allowed Grignard reactions and prior specification organic reactions for synthesis routes
- **Wrong:** Planning synthesis routes longer than 4 steps
  - Why it fails: All Edexcel IAL WCH15 synthesis questions are capped at 4 steps, and longer routes will be marked incorrect even if they work
  - Correct: Find the shortest possible route (max 4 steps) using allowed reactions
- **Wrong:** Using washing with water alone to remove acidic organic impurities
  - Why it fails: Water only removes soluble ionic impurities, not unreacted organic acids which are mostly non-polar
  - Correct: Wash with dilute sodium carbonate solution to neutralise organic acids to soluble ionic salts, then wash with water to remove remaining carbonate
- **Wrong:** Assigning a molecular formula directly from empirical formula without checking the molecular ion peak
  - Why it fails: The empirical formula is the simplest whole number ratio, which may be a fraction of the actual molecular mass
  - Correct: Use the molecular ion peak from MS to find the relative molecular mass, then scale the empirical formula to get the correct molecular formula

## 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 |

## 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.

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