CAPE CHEMISTRY
UNIT 2
AMINES
MODULE 1
INTRODUCING AMINES
This page explains what amines are, and what the difference is
between primary, secondary and tertiary amines. It looks in
some detail at their simple physical properties such as solubility
and boiling points. Details of the chemical reactions of amines
are described on separate pages.
Note: This page only deals with amines where the functional
group is not attached directly to a benzene ring. Aromatic
amines such as phenylamine (aniline) are sufficiently different
that they are covered in a separate section. Follow this link if
you are mainly interested in phenylamine.
What are amines?
The easiest way to think of amines is as near relatives of
ammonia, NH3.
In amines, the hydrogen atoms in the ammonia have been
replaced one at a time by hydrocarbon groups. On this page, we
are only looking at cases where the hydrocarbon groups are
simple alkyl groups.
The different kinds of amines
Amines fall into different classes depending on how many of the
hydrogen atoms are replaced.
Primary amines
In primary amines, only one of the hydrogen atoms in the
ammonia molecule has been replaced. That means that the
formula of the primary amine will be RNH2 where "R" is an alkyl
group.
Examples include:
Naming amines can be quite confusing because there are so
many variations on the names. For example, the simplest amine,
CH3NH2, can be called methylamine, methanamine or
aminomethane.
The commonest name at this level is methylamine and, similarly,
the second compound drawn above is usually called ethylamine.
Where there might be confusion about where the -NH2 group is
attached to a chain, the simplest way of naming the compound
is to use the "amino" form.
For example:
Secondary amines
In a secondary amine, two of the hydrogens in an ammonia
molecule have been replaced by hydrocarbon groups. At this
level, you are only likely to come across simple ones where both
of the hydrocarbon groups are alkyl groups and both are the
same.
For example:
There are other variants on the names, but this is the
commonest and simplest way of naming these small secondary
amines.
Tertiary amines
In a tertiary amine, all of the hydrogens in an ammonia molecule
have been replaced by hydrocarbon groups. Again, you are only
likely to come across simple ones where all three of the
hydrocarbon groups are alkyl groups and all three are the same.
The naming is similar to secondary amines. For example:
Physical properties of amines
Boiling points
The table shows the boiling points of some simple amines.
type
formula
boiling point (°C)
primary
CH3NH2
-6.3
primary
CH3CH2NH2
16.6
primary
CH3CH2CH2NH2
48.6
secondary
(CH3)2NH
7.4
tertiary
(CH3)3N
3.5
We will need to look at this with some care to sort out the
patterns and reasons. Concentrate first on the primary amines.
Primary amines
It is useful to compare the boiling point of methylamine, CH3NH2,
with that of ethane, CH3CH3.
Both molecules contain the same number of electrons and have,
as near as makes no difference, the same shape. However, the
boiling point of methylamine is -6.3°C, whereas ethane's boiling
point is much lower at -88.6°C.
The reason for the higher boiling points of the primary amines is
that they can form hydrogen bonds with each other as well as
van der Waals dispersion forces and dipole-dipole interactions.
Note: If you aren't happy about intermolecular forces (including
van der Waals dispersion forces and hydrogen bonds) then you
really ought to follow this link before you go on. The next bit
won't make much sense to you if you aren't familiar with the
various sorts of intermolecular forces.
Use the BACK button on your browser to return to this page.
Hydrogen bonds can form between the lone pair on the very
electronegative nitrogen atom and the slightly positive hydrogen
atom in another molecule.
The hydrogen bonding isn't as efficient as it is in, say, water,
because there is a shortage of lone pairs. Some slightly positive
hydrogen atoms won't be able to find a lone pair to hydrogen
bond with. There are twice as many suitable hydrogens are
there are lone pairs.
The boiling points of the primary amines increase as you
increase chain length because of the greater amount of van der
Waals dispersion forces between the bigger molecules.
Secondary amines
For a fair comparison you would have to compare the boiling
point of dimethylamine with that of ethylamine. They are isomers
of each other - each contains exactly the same number of the
same atoms.
The boiling point of the secondary amine is a little lower than the
corresponding primary amine with the same number of carbon
atoms.
Secondary amines still form hydrogen bonds, but having the
nitrogen atom in the middle of the chain rather than at the end
makes the permanent dipole on the molecule slightly less.
The lower boiling point is due to the lower dipole-dipole
attractions in the dimethylamine compared with ethylamine.
Tertiary amines
This time to make a fair comparison you would have to compare
trimethylamine with its isomer 1-aminopropane.
If you look back at the table further up the page, you will see that
the trimethylamine has a much lower boiling point (3.5°C) than
1-aminopropane (48.6°C).
In a tertiary amine there aren't any hydrogen atoms attached
directly to the nitrogen. That means that hydrogen bonding
between tertiary amine molecules is impossible. That's why the
boiling point is much lower.
Solubility in water
The small amines of all types are very soluble in water. In fact,
the ones that would normally be found as gases at room
temperature are normally sold as solutions in water - in much
the same way that ammonia is usually supplied as ammonia
solution.
All of the amines can form hydrogen bonds with water - even the
tertiary ones.
Although the tertiary amines don't have a hydrogen atom
attached to the nitrogen and so can't form hydrogen bonds with
themselves, they can form hydrogen bonds with water
molecules just using the lone pair on the nitrogen.
Solubility falls off as the hydrocarbon chains get longer noticeably so after about 6 carbons. The hydrocarbon chains
have to force their way between water molecules, breaking
hydrogen bonds between water molecules.
However, they don't replace them by anything as strong, and so
the process of forming a solution becomes less and less
energetically feasible as chain length grows.
Smell
The very small amines like methylamine and ethylamine smell
very similar to ammonia - although if you compared them side by
side, the amine smells are slightly more complex.
As the amines get bigger, they tend to smell more "fishy", or
they smell of decay.
If you are familiar with the smell of hawthorn blossom (and
similarly smelling things like cotoneaster blossom), this is the
smell of trimethylamine - a sweet and rather sickly smell like the
early stages of decaying flesh.
AMINES AS BASES
This page looks at the reactions of amines as bases. Their basic
properties include the reactions with dilute acids, water and
copper(II) ions.
It only deals with amines where the functional group is not
attached directly to a benzene ring. Aromatic amines such as
phenylamine (aniline) are much weaker bases than the amines
discussed on this page and are dealt with separately on a page
specifically about phenylamine. If you are interested in
phenylamine, read this page first and then follow the link at the
bottom.
The basic properties of amines
We are going to have to use two different definitions of the term
"base" in this page.
A base is


a substance which combines with hydrogen ions. This is
the Bronsted-Lowry theory.
an electron pair donor. This is the Lewis theory.
Note: If you aren't familiar with either of these terms, you
should follow this link to a page about theories of acids and
bases.
Use the BACK button on your browser to return to this page
when you are confident about these terms.
The easiest way of looking at the basic properties of amines is
to think of an amine as a modified ammonia molecule. In an
amine, one or more of the hydrogen atoms in ammonia has
been replaced by a hydrocarbon group.
Replacing the hydrogens still leaves the lone pair on the
nitrogen unchanged - and it is the lone pair on the nitrogen that
gives ammonia its basic properties. Amines will therefore
behave much the same as ammonia in all cases where the lone
pair is involved.
The reactions of amines with acids
These are most easily considered using the Bronsted-Lowry
theory of acids and bases - the base is a hydrogen ion acceptor.
We'll do a straight comparison between amines and the familiar
ammonia reactions.
A reminder about the ammonia reactions
Ammonia reacts with acids to produce ammonium ions. The
ammonia molecule picks up a hydrogen ion from the acid and
attaches it to the lone pair on the nitrogen.
If the reaction is in solution in water (using a dilute acid), the
ammonia takes a hydrogen ion (a proton) from a hydroxonium
ion. (Remember that hydrogen ions present in solutions of acids
in water are carried on water molecules as hydroxonium ions,
H3O+.)
If the acid was hydrochloric acid, for example, you would end up
with a solution containing ammonium chloride - the chloride
ions, of course, coming from the hydrochloric acid.
You could also write this last equation as:
. . . but if you do it this way, you must include the state symbols.
If you write H+ on its own, it implies an unattached hydrogen ion
- a proton. Such things don't exist on their own in solution in
water.
If the reaction is happening in the gas state, the ammonia
accepts a proton directly from the hydrogen chloride:
This time you produce clouds of white solid ammonium chloride.
The corresponding reactions with amines
The nitrogen lone pair behaves exactly the same. The fact that
one (or more) of the hydrogens in the ammonia has been
replaced by a hydrocarbon group makes no difference.
For example, with ethylamine:
If the reaction is done in solution, the amine takes a hydrogen
ion from a hydroxonium ion and forms an ethylammonium ion.
Or:
The solution would contain ethylammonium chloride or sulphate
or whatever.
Alternatively, the amine will react with hydrogen chloride in the
gas state to produce the same sort of white smoke as ammonia
did - but this time of ethylammonium chloride.
These examples have involved a primary amine. It would make
no real difference if you used a secondary or tertiary one. The
equations would just look more complicated.
The product ions from diethylamine and triethylamine would be
diethylammonium ions and triethylammonium ions respectively.