3.11 Amines
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Amines
Compounds based on ammonia with at least one hydrogen atom replaced by an alkyl or aryl group
How are amines classified?
By the number of R groups bonded to nitrogen
General formula for aliphatic amines
CₙH₂ₙ₊₃N
How are primary aliphatic amines produced?
• Nucleophilic substitution of halogenoalkanes with ammonia • Reduction of nitriles
Reagents and conditions for the reduction of nitriles to primary amines
• Hydrogen • Nickel (catalyst) • High temperature and pressure
Equation for the reduction of nitriles to primary amines
R-C≡N + 2H₂ –> RCH₂NH₂
Reagents and conditions for the preparation of primary aliphatic amines from halogenoalkanes
• Excess ammonia (prevents further substitution) • Ethanol as a solvent
Why are ammonia and amines weak bases?
The lone pair on nitrogen can accept a proton from an acid
Product of the reaction between amines and acids
Alkylammonium salts
Basicity
The ability of a substance to act as a base
Amines in order of increasing basicity
• Aromatic • Ammonia • Primary aliphatic amines
Why are primary amines stronger bases than ammonia?
• Alkyl group releases electron density towards N (positive inductive effect) • Increases electron density around N • Lone pair is more available to accept H⁺
Why are aromatic amines weaker bases than ammonia?
• Lone pair on N is delocalised into the pi-ring • Decreases electron density on N • Lone pair is less available to accept H⁺
Product when further nucleophilic substitution occurs with halogenoalkanes and ammonia?
Quaternary ammonium salts (or secondary/tertiary amines if reaction is stopped before this point)
Nucleophilic substitution of halogenoalkanes with excess ammonia/amine
Only one alkyl group is added to the amine/ammonia
Nucleophilic substitution of halogenoalkanes with excess halogenoalkane
More than one alkyl group is added (forms quaternary ammonium salts)
Use of quaternary ammonium salts
Cationic surfactants (used in detergents and fabric softeners)
Surfactants
Compounds with regions that are partly soluble and partly insoluble in water
Why do quaternary ammonium salts act as surfactants?
• The ionic region is soluble in water • The long non-polar hydrocarbon chains are insoluble