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7.8Advanced Organic Chemistry

Organic Synthesis — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 12

📚 AQA A Level Chemistry Hub / Topical Revision
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Lesson summary

Organic Synthesis: Functional Group Interconversions and Routes — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Plan multi-step synthetic routes using reagents and conditions for functional group interconversion.A wide range of organic products can be made from just a few startin…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Percentage yield
  • Recrystallisation

Part 1 of 6Principles of synthesis

A wide range of organic products can be made from just a few starting compounds, given the right reagents and conditions.

Principles of synthesis
Understanding how different organic functional groups interconvert is key to designing synthetic routes.
A good route uses few steps, high-yielding reactions, safe reagents, and gives a product that can be purified.
Work backwards from the target: identify its functional group, decide which reaction makes it, and find the precursor.

Part 2 of 6Aliphatic interconversions

1
Alkane → halogenoalkane: Cl2 or Br2, UV light (free radical substitution).
2
Alkene → halogenoalkane: HBr (electrophilic addition). Alkene → alcohol: steam, H3PO4, high pressure. Alkene → dihalide: Br2.
3
Halogenoalkane → alcohol: NaOH(aq), heat.
  • Halogenoalkane → nitrileKCN in ethanol, heat.
  • Halogenoalkane → amineexcess NH3 in ethanol.
  • Halogenoalkane → alkeneNaOH in ethanol, heat (elimination).
4
Nitrile → carboxylic acid: acid or alkali hydrolysis. Nitrile → amine: LiAlH4 or H2/Ni.
5
Primary alcohol → aldehyde: K2Cr2O7/H+, distil; → carboxylic acid: same, reflux. Secondary alcohol → ketone. Aldehyde/ketone → alcohol: NaBH4.
6
Alcohol → alkene: conc. H2SO4 or H3PO4, heat (dehydration).
7
Carboxylic acid → ester (alcohol, H+), → acyl chloride (SOCl2), acyl chloride → ester, amide, N-substituted amide.
8
Carbonyl → hydroxynitrile: HCN/KCN.

Part 3 of 6Aromatic interconversions

Benzene → nitrobenzene (conc. HNO3/conc. H2SO4, 50 °C) → phenylamine (Sn/HCl, then NaOH). Benzene → phenyl ketone (acyl chloride, AlCl3). Benzene → alkylbenzene (halogenoalkane, AlCl3). Phenylamine and acyl chlorides form amides.

Part 4 of 6Practical considerations

Use reflux when heating for long periods or with volatile liquids; use distillation to remove a lower-boiling product as it forms (e.g. aldehydes); purify solids by recrystallisation and check purity by melting point, and liquids by drying, then redistillation. Calculate percentage yield from moles of product compared with the theoretical yield. Atom economy is also considered.

Part 5 of 6Worked strategy for synthesis questions

Write the structures of the starting material and the target; count carbons; identify whether the chain must be lengthened (only via CN-, Grignard reagents are outside this specification, or Friedel–Crafts) or whether only the functional group changes.

1
Then write each step with the reagent, conditions and the name of the intermediate.
2
Label each intermediate clearly.
3
Check that every step is chemically sensible: for example, a primary alcohol cannot be converted to a ketone,
  • and oxidation of a secondary alcohol never gives an acid.
4
When two routes exist, prefer the one with fewer steps and higher yield, and mention hazards (HCN, LiAlH4) if asked to evaluate it.

Part 6 of 6Worked examples

  1. Suggest a two-step synthesis of propanoic acid from bromoethane. [3]3 marks

  2. Suggest a synthesis of phenylamine from benzene. [3]3 marks

  3. Propan-2-ol to propene: reagent and conditions. [2]2 marks

  4. Outline a route from ethene to ethylamine. [3]3 marks

Key terms

TermMeaning
Functional group interconversionChanging one functional group into another.
RefluxHeating with condenser so vapours return to the flask.
Percentage yieldActual yield / theoretical yield × 100.
RecrystallisationPurification of a solid by dissolving and crystallising.
Atom economyMass of desired product / total mass of reactants × 100.

Avoid these mistakes

1
Suggesting an impossible direct step such as alkane to alcohol without a halogenoalkane intermediate.
2
Omitting reaction conditions (solvent, heat, catalyst).
3
Confusing NaOH(aq) (substitution) with NaOH in ethanol (elimination).
4
Using distillation where reflux is needed and vice versa.

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