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Amines (6.2.1)
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Amines are organic derivatives of ammonia.
They have groups, (alkyl or aryl), substituted in place of ammonia hydrogens.
Amines, like ammonia, act as bases by donating the lone pair on the nitrogen atom in a dative covalent bond to protons.
Amines and ammonia react with inorganic acids, such as , in an acid–base reaction.
The nitrogen atom acts as a base, using its lone pair to form a dative covalent bond to the proton.
The product is a neutral ammonium or alkylammonium salt following reaction with ammonia or an amine.
Primary amines react with hydrochloric acid to form alkylammonium chloride.
The primary amine ethylamine () reacts with hydrochloric acid to form the salt ethylammonium chloride.
Secondary amines react with hydrochloric acid to form dialkylammonium chloride.
The secondary amine diethylamine will similarly react with forming the salt diethylammonium sulfate.
Note that charge balancing is required to form a charge-neutral salt.
Primary amines can be formed from haloalkanes refluxed with excess ethanolic ammonia. Ethanolic solvent is used to prevent hydrolysis of the haloalkane.
For a good yield of primary amines, must be in excess.
Secondary amines and tertiary amines can also be formed.
The amine products act as nucleophiles, and compete with the ammonia in subsequent substitutions. Further substitutions are less likely to occur if the ammonia is in excess, so there are many fewer R groups available for subsitiution and a higher concentration of the nucleophile.
Secondary amines can be formed as a major product in the nucleophilic substitution reaction between haloalkanes and excess ethanolic primary amine.
Ethanolic solvent is used to prevent hydrolysis of the haloalkane.
The carbon atom, of the polarised carbon-halogen bond, is readily attacked by nucleophiles, such as ammonia and amines.
To ensure the secondary amine is the major product the primary amine must be in excess.
The nucleophilic substitution reaction mechanism for the formation of primary amines from haloalkanes is shown below:
The carbon atom, of the polarised carbon-halogen bond of a haloalkane, is readily attacked by the nucleophilic nitrogen’s lone pair.
In the reaction, the initial nucleophilic attack is followed by deprotonation by a second ammonia molecule. In this case the ammonia acts as a base.
The halide ion is lost as an ammonium salt, and the substituted amine product is formed.
The mechanism for formation of a secondary or tertiary amine is primarily the same, but features an amine nucleophile.
Tertiary amines can be formed as a major product in the nucleophilic substitution reaction between haloalkanes and an excess secondary amine.
where is a halogen.
The reaction occurs in ethanolic conditions.
Ammonia and amines act as nucleophiles, substituting the halogen atom in haloalkanes.
If the haloalkane is in excess, a mixture of primary, secondary, and tertiary amine products will be formed.
Aromatic primary amines have the amino group directly bonded to a carbon atom in the benzene ring.
They are prepared by a reduction of the corresponding aromatic nitro compound.
The reaction is carried out by refluxing with metallic tin, , and concentrated hydrochloric acid.
The initial product formed is a salt, due to the strongly acidic conditions, so treatment of the product with is required to liberate the aromatic amine.
It is conventional to represent the reducing reagents as and are needed to reduce each aromatic group to .
Amines can be produced by the reduction of nitriles, , by hydrogenation.
is used as a reducing agent, under acidic conditions to reduce the nitrile to an amine.
can be used to represent a reducing agent. Four equivalents of are required to reduce a nitrile group.
Nitriles can also be reduced to primary amines through catalytic hydrogenation using a catalyst with hydrogen.