A Level Chemistry - How to Solve: Organic Synthesis Pathways (Reactions, Reagents, Conditions)

How to Solve: Organic Synthesis Pathways (Reactions, Reagents, Conditions)

Complete Guide For GCSE/A-Level Chemistry – Ace Your Exam with Confidence

? Introduction

"Mastering organic synthesis pathways doesn’t just get you marks—it unlocks 10-15% of your A-Level Chemistry exam (e.g., AQA Paper 2 or Edexcel Topic 6). Miss this, and you’re handing over easy points. But nail it, and you’ll confidently predict exactly which reagent turns an alkene into an alcohol, or how to convert benzene into a dye—just like real chemists do in labs and industry."

? WHAT YOU NEED TO KNOW FIRST

Before diving in, you must already understand:
1. Functional groups – Names, structures, and properties (e.g., alkenes, alcohols, carboxylic acids).
2. Reaction types – Addition, substitution, elimination, oxidation, reduction, hydrolysis.
3. Bonding & mechanisms – How electrons move in reactions (e.g., electrophilic addition, nucleophilic substitution).

If you’re shaky on these, pause and review them first—this guide won’t make sense without them!

? KEY TERMS & FORMULAS

1. Functional Groups (MEMORISE THESE)

2. Key Reagents & Conditions (MEMORISE THESE)

3. Formulas to Know

• General formula for alkanes: CₙH₂ₙ₊₂ (MEMORISE)
• General formula for alkenes: CₙH₂ₙ (MEMORISE)
• General formula for alcohols: CₙH₂ₙ₊₁OH (MEMORISE)
• Oxidation state of Cr in K₂Cr₂O₇: +6 → +3 (orange → green) (MEMORISE)

? STEP-BY-STEP METHOD

How to Plan an Organic Synthesis Pathway (6 Steps)

1. Identify the starting material and target molecule.
2. Write their structures and names.

3. Circle the functional groups in both.

4. Compare functional groups.

5. What changes between start and end?

6. Example: Alcohol → Aldehyde = oxidation.

7. List possible reactions that change the functional group.

8. Use the reagent table above.

9. Example: Primary alcohol → Aldehyde = K₂Cr₂O₇/H₂SO₄, distil.

10. Check for side reactions or conditions.

11. Does the reaction need specific conditions (e.g., reflux vs. distil)?

12. Example: Primary alcohol → Carboxylic acid = reflux (not distil).

13. Plan the sequence step-by-step.

14. If one step isn’t possible, break it into smaller steps.

15. Example: Alkene → Carboxylic acid = Alkene → Alcohol → Aldehyde → Carboxylic acid.

16. Write the full pathway with reagents and conditions.

17. Use arrows (→) and label each step.
18. Example: Ethene → Ethanol → Ethanal → Ethanoic acid (Steam/H₃PO₄) → (K₂Cr₂O₇/distil) → (K₂Cr₂O₇/reflux)

✏️ WORKED EXAMPLES

Example 1 – Basic: Ethene to Ethanoic Acid

Question: Plan a synthesis pathway from ethene to ethanoic acid. Include reagents and conditions.

Step-by-Step Solution:
1. Start: Ethene (C₂H₄) – alkene. Target: Ethanoic acid (CH₃COOH) – carboxylic acid.
2. Functional group change: Alkene → Carboxylic acid.
3. Possible reactions: - Alkene → Alcohol (hydration) - Alcohol → Aldehyde (oxidation) - Aldehyde → Carboxylic acid (oxidation)
4. Conditions check: - Ethene → Ethanol: Steam, H₃PO₄ catalyst, 300°C, 60 atm. - Ethanol → Ethanal: K₂Cr₂O₇/H₂SO₄, distil. - Ethanal → Ethanoic acid: K₂Cr₂O₇/H₂SO₄, reflux.
5. Pathway: Ethene → Ethanol → Ethanal → Ethanoic acid (Steam/H₃PO₄) → (K₂Cr₂O₇/distil) → (K₂Cr₂O₇/reflux)

What we did and why: - We broke the problem into smaller steps because you can’t go directly from an alkene to a carboxylic acid. - Each step uses specific reagents and conditions to control the reaction.

Example 2 – Medium: Propan-1-ol to Propanone

Question: Plan a synthesis pathway from propan-1-ol to propanone. Include reagents and conditions.

Step-by-Step Solution:
1. Start: Propan-1-ol (CH₃CH₂CH₂OH) – primary alcohol. Target: Propanone (CH₃COCH₃) – ketone.
2. Problem: Primary alcohols cannot directly form ketones (ketones come from secondary alcohols).
3. Solution: First, convert the primary alcohol to a secondary alcohol. - Propan-1-ol → Propene (elimination) - Propene → Propan-2-ol (hydration)
4. Conditions: - Propan-1-ol → Propene: Conc. H₂SO₄, 170°C, reflux. - Propene → Propan-2-ol: Steam, H₃PO₄, 300°C, 60 atm. - Propan-2-ol → Propanone: K₂Cr₂O₇/H₂SO₄, reflux.
5. Pathway: Propan-1-ol → Propene → Propan-2-ol → Propanone (Conc. H₂SO₄/reflux) → (Steam/H₃PO₄) → (K₂Cr₂O₇/reflux)

What we did and why: - We spotted the trick—primary alcohols can’t directly make ketones. - We used elimination then addition to rearrange the molecule into a secondary alcohol first.

Example 3 – Exam-Style: Benzene to Phenylamine (via Nitrobenzene)

Question: A student wants to make phenylamine from benzene. They start by reacting benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid. a) Name the product formed in this first step. b) State the conditions needed for this reaction. c) The student then reduces the product from (a) to form phenylamine. Give the reagent and conditions for this reduction. d) Write the full synthesis pathway from benzene to phenylamine.

Step-by-Step Solution: a) Product: Nitrobenzene (C₆H₅NO₂). b) Conditions: 50°C (heat gently). c) Reduction: - Reagent: Tin (Sn) + concentrated HCl. - Conditions: Reflux, then add NaOH to neutralise. d) Pathway: Benzene → Nitrobenzene → Phenylamine (Conc. HNO₃ + conc. H₂SO₄, 50°C) → (Sn + conc. HCl, reflux, then NaOH)

What we did and why: - We answered each part separately (common in exams). - We remembered the reduction conditions (Sn/HCl is classic for nitro → amine). - We included the NaOH step—examiners love this detail!

❌ COMMON MISTAKES

? EXAM TRAPS