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Amines

Key Terminology
Term Definition
Amines Organic compounds containing an NH2 group, deriving from ammonia, where alkyl or
aryl groups substitute for hydrogens of NH3
Amines are organic compounds containing an NH2 group. In a primary amine, one of the hydrogens
from NH3 has been swapped for an alkyl group, in a secondary amine two hydrogens have been
swapped out, and so on.
• Primary amines have the structure RNH2 and are named alkylamine
• Secondary amines have the structure RR’NH, and tertiary amines have the structure RR’R’’N.
These may need to be named as N-substituted amines
• If the lone pair on the tertiary amine attracts another alkyl group, then a quaternary ammonium
salt has been produced, with a positively charged N atom
• Hydrogen bonding causes a higher boiling point of 1° and 2° amines, compared to 3° amines.
• Short-chain primary amines dissolve in water and ethanol
Synthesising Amines
Nucleophilic Substitution with Ammonia
Primary amines can be produced by nucleophilic substitution of halogenoalkanes with ammonia – the
halogenoalkane is heated in a seal tube with ammonia dissolved in aqueous ethanol. The primary amine
may also act as a nucleophile to produce a secondary amine by RNH2 + RX + NH3 → R2NH + NH4X.
Multiple substitutions occur, until a quaternary ammonium salt is produced. A mixture of amines can
be separated by fractional distillation. Production of a 1° amine is favoured by a large excess of NH3.
Nucleophilic Substitution with Cyanide
Nucleophilic substitution of halogenoalkanes with cyanide ions can also occur. Remember that this
lengthens the carbon chain. The C≡N bond can be reduced by a nickel/hydrogen catalyst. This produces
a purer product than ammonia as only a primary amine can be formed.
Aromatic Amines
Aromatic amines, such as phenylamine, must be produced from nitrobenzene (see aromatic section
above). Nitrobenzene is reduced to phenylamine, using tin and hydrochloric acid as a reducing agent.
The Sn and HCl react to form hydrogen, which replaces the O atoms from NO2 and with hydrogens.
Amines as Bases
Amines can accept a proton and so are Brønsted-Lowry bases. Amines react with bases to form salts
e.g. ethylamine reacts with HCl to form ethylammonium chloride.
Both ammonia and methylamine has a lone pair of electrons that attract a proton. However,
methylamine has an alkyl group that is electron releasing, releasing some negative charge towards the
N atom by the inductive effect. As a result, the electron density on the N atom is higher, and the lone
pair of electrons accept a proton more readily – hence alkylamines are stronger bases than ammonia.
Phenylamine is a weaker base than ammonia. The lone pair on the N atom overlaps and with the
delocalised electron system, becoming part of the delocalisation. This means that the lone pair is less
available to form a coordinate bond with a proton and so the arylamine is a weaker base than NH3.
Using Amines
Amines react with acid chlorides/anhydrides to produce amides. This reaction is useful in forming
polymers such as nylon. Aromatic amines are used in the production of dyes.
Quaternary ammonium salts with long chain alkyl groups are used as cationic surfactants. In solution,
the ions cluster with the charged ends in the water and the hydrocarbon tails on the surface. In fabric
softeners and hair conditioners, where the surfaces being treated are negatively charged, the cationic
surfactants prevent build-up of static, keeping surfaces smooth.

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