Organic Synthesis (A Level)

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Cambridge International AS & A Level Chemistry (9701)

Organic Synthesis (A Level)

The big idea: This builds on the AS chapter Organic Synthesis. Synthesis questions test whether you can use every reaction in the syllabus together: recognise each functional group in a large molecule, predict what it does, and chain reactions into routes that turn one compound into another.

Summary — What This Chapter Covers

Identify functional groups with syllabus test reactions and predict how a multi-functional molecule will react.
Devise multi-step routes using AS and A Level reactions, including steps that add a carbon atom.
Analyse a given route: the type of each reaction, the reagents and conditions, and possible by-products.

1. Organic Synthesis

36.1

Identifying functional groups

Functional group	Test	Positive result
C=C (alkene)	Br₂(aq)	Orange to colourless (no ppt)
Phenol or phenylamine	Br₂(aq)	Decolourised, white precipitate
Primary or secondary alcohol, aldehyde	Acidified K₂Cr₂O₇, warm	Orange to green
C=O (aldehyde or ketone)	2,4-DNPH	Orange precipitate
Aldehyde (not ketone)	Tollens' reagent / Fehling's solution, warm	Silver mirror / red-brown precipitate
CH₃CO– or CH₃CH(OH)–	Alkaline I₂, warm	Pale yellow precipitate of CHI₃
–COOH	Na₂CO₃(aq)	Effervescence, CO₂
–OH in any form (alcohol, phenol, acid)	Na(s)	Effervescence, H₂
Halogenoalkane	Warm with NaOH(aq), add HNO₃, then AgNO₃(aq)	White (Cl), cream (Br), yellow (I) precipitate
Acyl chloride	Water; AgNO₃(aq)	Steamy fumes of HCl; white precipitate at once
Primary amide	Warm with NaOH(aq)	NH₃ released, turns damp red litmus blue

Worked Example

Tyrosine is HO–C₆H₄–CH₂CH(NH₂)COOH (the OH is at the 4-position of the ring). Predict the reaction of 1 mol of tyrosine with: (a) NaOH(aq), (b) Na₂CO₃(aq), (c) Na, (d) excess Br₂(aq), (e) HCl(aq).

The reaction toolkit

Change	Reagents and conditions
Alkane → halogenoalkane	Cl₂ or Br₂, UV light
Alkene → halogenoalkane / alcohol / alkane	HX(g); steam with H₃PO₄ catalyst; H₂ with Ni/Pt, heat
Halogenoalkane → alcohol	NaOH(aq), heat under reflux
Halogenoalkane → nitrile (+1 C)	KCN in ethanol, heat under reflux
Halogenoalkane → amine	NH₃ in ethanol, heat under pressure
Halogenoalkane → alkene	NaOH in ethanol, heat
Primary alcohol → aldehyde / carboxylic acid	Acidified K₂Cr₂O₇: distil off the aldehyde / heat under reflux for the acid
Aldehyde or ketone → hydroxynitrile (+1 C)	HCN with KCN catalyst
Aldehyde/ketone → alcohol	NaBH₄ (or LiAlH₄)
Nitrile → carboxylic acid	Dilute HCl(aq), heat under reflux
Nitrile → amine	LiAlH₄, or H₂/Ni
Carboxylic acid → acyl chloride	SOCl₂, PCl₅, or PCl₃ + heat
Acyl chloride → ester / amide / acid	Alcohol or phenol / NH₃ or amine / water, room temperature
Benzene → nitrobenzene → phenylamine	Conc HNO₃ + conc H₂SO₄, 25–60 °C; then Sn + conc HCl, heat, then NaOH(aq)
Phenylamine → diazonium salt → phenol / azo dye	NaNO₂ + dilute HCl below 10 °C; warm with water / phenol in NaOH(aq)
Benzene → alkylbenzene / ketone (+C)	RCl or RCOCl with AlCl₃, heat
Alkylbenzene → benzoic acid	Hot alkaline KMnO₄, then dilute acid
Amide → amine	LiAlH₄

Devising routes

Analysing a given route

type of reaction

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Organic Synthesis (A Level)