🔹 1. Identifying Functional Groups, Predicting Properties and Reactions
Organic synthesis begins with understanding functional groups, their chemical behavior, and how they transform in reactions. Common functional groups include:
| Functional Group | General Formula | Detection / Reactions |
|---|---|---|
| Alkene | C=C | Decolourises Br₂(aq), electrophilic addition |
| Alcohol | –OH | Oxidised to aldehydes, ketones, or carboxylic acids (K₂Cr₂O₇ / H⁺) |
| Halogenoalkane | –X (Cl, Br, I) | Undergoes nucleophilic substitution with NaOH, NH₃, CN⁻ |
| Aldehyde | –CHO | Oxidised to –COOH, reduces Tollens’ or Fehling’s |
| Ketone | RCOR’ | Does not reduce Tollens’ or Fehling’s |
| Carboxylic acid | –COOH | Reacts with carbonates, bases; forms esters with alcohols |
| Ester | –COOR | Hydrolysis to acid and alcohol |
| Amine | –NH₂ | Basic, forms salts with acids |
✅ Functional Group Identification (Based on reactions from the syllabus):
- Alkene: Add Br₂(aq) → orange to colourless (test for C=C)
- Alcohol: Oxidation with acidified K₂Cr₂O₇:
- Primary alcohol → aldehyde → carboxylic acid
- Secondary alcohol → ketone
- Aldehyde:
- Tollens’ reagent → silver mirror
- Fehling’s solution → red precipitate
- Carboxylic acid:
- Reacts with Na₂CO₃ → effervescence (CO₂)
- Forms esters with alcohols in presence of H₂SO₄ (catalyst)
🔹 2. Devising Multi-step Synthetic Routes
To plan a multi-step synthesis, students should:
- Identify the target molecule and its functional group.
- Work backwards to identify possible starting materials (retrosynthesis).
- Select appropriate reagents and conditions for each transformation.
- Ensure that functional groups are compatible with chosen reagents.
🛠️ Common synthetic pathways:
| Transformation | Reagents & Conditions | Type of Reaction |
|---|---|---|
| Alkene → Alcohol | H₂O(g), H₃PO₄ catalyst, 300°C | Electrophilic addition |
| Alkene → Halogenoalkane | HBr (g) | Electrophilic addition |
| Halogenoalkane → Alcohol | NaOH (aq), reflux | Nucleophilic substitution |
| Halogenoalkane → Amine | NH₃ in ethanol, heat in sealed tube | Nucleophilic substitution |
| Alcohol → Aldehyde | K₂Cr₂O₇ / H⁺, distill (primary alcohol) | Oxidation |
| Alcohol → Ketone | K₂Cr₂O₇ / H⁺, reflux (secondary alcohol) | Oxidation |
| Alcohol → Carboxylic Acid | Excess K₂Cr₂O₇ / H⁺, reflux (primary alcohol) | Oxidation |
| Carboxylic Acid + Alcohol → Ester | Conc. H₂SO₄, heat | Esterification |
| Ester → Alcohol + Acid | HCl(aq) or NaOH(aq), heat under reflux | Hydrolysis |
🧩 Example of Multi-step Synthesis:
Target: Ethyl propanoate (an ester)
Route:
- Starting material: Propan-1-ol
➝ Oxidised with K₂Cr₂O₇ / H⁺, reflux
➝ Product: Propanoic acid - React with ethanol + conc. H₂SO₄
➝ Esterification → Ethyl propanoate
🔹 3. Analyzing Synthetic Routes
Students must be able to:
- Identify reaction type for each step (e.g., oxidation, nucleophilic substitution, addition, esterification).
- Name the reagents and conditions used.
- Recognize by-products formed (e.g., H₂O in esterification, HX in substitution).
📘 Example Analysis:
Synthesis: CH₃CH₂Br → CH₃CH₂OH → CH₃CHO → CH₃COOH
| Step | Reaction Type | Reagents & Conditions | By-products |
|---|---|---|---|
| 1. CH₃CH₂Br → CH₃CH₂OH | Nucleophilic substitution | NaOH(aq), reflux | NaBr |
| 2. CH₃CH₂OH → CH₃CHO | Oxidation | K₂Cr₂O₇ / H⁺, distill | H₂O |
| 3. CH₃CHO → CH₃COOH | Oxidation | K₂Cr₂O₇ / H⁺, reflux | H₂O |
⚠️ Tips for Success
- Always consider reaction compatibility (some reagents might affect multiple groups).
- Include reflux/distillation when necessary to control reaction products.
- Watch for isomer formation or side reactions (especially in substitution).
- Draw structural formulas clearly at each step in exam answers.
>>> Go to the main course
Have Questions? Speak to an Expert: 01817 122800
