Why do amines act as
nucleophiles?
A nucleophile is
something which is attracted to, and then attacks, a positive or slightly
positive part of another molecule or ion.
All amines contain an
active lone pair of electrons on the very electronegative nitrogen atom. It
is these electrons which are attracted to positive parts of other molecules
or ions.
The reactions of
primary amines with halogenoalkanes
You get a complicated
series of reactions on heating to give a mixture of products - probably one
of the most confusing sets of reactions you will meet at this level. The
products of the reactions include secondary and tertiary amines and their
salts, and quaternary ammonium salts.
Making secondary
amines and their salts
In the first stage of
the reaction, you get the salt of a secondary amine formed. For example if
you started with ethylamine and bromoethane, you would get diethylammonium
bromide
In the presence of
excess ethylamine in the mixture, there is the possibility of a reversible
reaction. The ethylamine removes a hydrogen from the diethylammonium ion to
give free diethylamine - a secondary amine.
Making tertiary amines
and their salts
But it doesn't stop
here! The diethylamine also reacts with bromoethane - in the same two stages
as before. This is where the reaction would start if you reacted a secondary
amine with a halogenoalkane.
In the first stage,
you get triethylammonium bromide.
There is again the
possibility of a reversible reaction between this salt and excess ethylamine
in the mixture.
The ethylamine removes
a hydrogen ion from the triethylammonium ion to leave a tertiary amine -
triethylamine.
Making a quaternary
ammonium salt
The final stage! The
triethylamine reacts with bromoethane to give tetraethylammonium bromide - a
quaternary ammonium salt (one in which all four hydrogens have been replaced
by alkyl groups).
This time there isn't
any hydrogen left on the nitrogen to be removed. The reaction stops here.
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Saturday, 29 April 2017
The nucleophilic properties of amines
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