Flashcards · 9620 · A Level Topic 29

Amines (A Level) flashcards

12 cards on A Level Topic 29, subtopics 3.3.11.1 to 3.3.11.3. Pick a subtopic or study them all.

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Primary, secondary and tertiary amines: Classified by the number of carbon atoms bonded directly to the N: one, two or three.
Two ways to make a primary aliphatic amine: Halogenoalkane + excess NH₃ in ethanol, heated in a sealed vessel (gives a mixture). Reduce a nitrile, e.g. with H₂ and a nickel catalyst: RCN + 4[H] → RCH₂NH₂ (only the primary amine).
Making an aromatic amine from a nitro compound: Reduce with tin and concentrated HCl, heat under reflux, then add NaOH to free the amine. C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
Use of aromatic amines made by reducing nitro compounds: Manufacture of dyes.

Why are amines bases?: The lone pair on the nitrogen accepts a proton (H⁺). RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻
Order of base strength: ammonia, primary aliphatic amines, primary aromatic amines: Primary aromatic amine < ammonia < primary aliphatic amine.
Why is a primary aliphatic amine a stronger base than ammonia?: The alkyl group is electron-releasing, increasing the electron density on N, so the lone pair is more available to accept H⁺.
Why is phenylamine a weaker base than ammonia?: The nitrogen lone pair is delocalised into the benzene ring, lowering the electron density on N, so it is less available to accept H⁺.

Successive substitution of ammonia by halogenoalkanes: NH₃ → RNH₂ (primary) → R₂NH (secondary) → R₃N (tertiary) → R₄N⁺X⁻ (quaternary ammonium salt). Excess NH₃ favours the primary amine; excess halogenoalkane favours the quaternary salt.
Why can a quaternary ammonium ion not act as a nucleophile or a base?: Its nitrogen has used its lone pair in four bonds, so it has no lone pair to donate.
Cationic surfactants: Quaternary ammonium salts with a long hydrocarbon chain: a positive N⁺ head that interacts with water and a non-polar tail. Used in fabric softeners and hair conditioners.
Product of an acyl chloride with a primary amine: An N-substituted amide, by nucleophilic addition–elimination. E.g. heptanoyl chloride + excess methylamine → N-methylheptanamide + methylammonium chloride.