A-LEVEL AQA ChEMISTRY NOTES
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- Aliphatic amines are those without aromatic rings present. Primary aliphatic amines can be prepared by reacting ammonia with a halogenoalkane. X = an electronegative species e.g. Cl, Br
- Primary aliphatic amines can also be made by the reduction of nitriles using a nickel catalyst and hydrogen gas:
R-C≡N + 2H2 → R-CH2NH2
A reducing agent such as LiAlH4, can be used instead of hydrogen gas:
R-C≡N + 4[H] → R-CH2NH2
- Aromatic amines are made by reducing nitroarenes such as nitrobenzene, refluxed at 100°C using a reducing agent of tin and concentrated HCl. The tin and concentrated HCl react to form hydrogen
- Aromatic amines are used in the production of dyes as they’re brightly coloured
Basic & Nucleophilic Properties of Amines
- Amines are weak bases
- In primary amines the lone pair of electrons on the N atom is readily donated due to the extra electron density from the alkyl groups. Aryl groups are electron-withdrawing so the lone pair of electrons on the nitrogen atom in an aryl amine is attracted into the benzene ring, so the electron pair is less easily donated
- The lone pair of electrons on the nitrogen atom in amines allows them to act as nucleophiles.
- Haloalkanes undergo nucleophilic substitution with an ammonia nucleophile to form amines. This reaction can also happen with primary amines acting as the nucleophile, using the same mechanism as ammonia, to form secondary amines
- Reaction of haloalkanes with secondary amine forms tertiary amines. Reaction of haloalkanes with tertiary alkanes forms quaternary ammonium salts.
- Quaternary ammonium salts containing a large alkyl group are often used as cationic surfactants. This is because they have one long non-polar hydrocarbon chain and a charged end.
- Ammonia and primary amines can also act as nucleophiles in addition-elimination reactions with acyl chlorides and acid anhydrides
- When ammonia reacts, the product formed is a primary amide.
- Primary amines react by the same mechanism, but the product formed is an N-substituted (secondary) amide.