Amines

Amines are found in a wide variety of living organism including plants and animals. Many amines have physiological effects on the body e.g. dopamine, adrenaline and amphetamine are three amines that you will probably have heard of each of which has a large physiological effect on the human body.

Amines can be thought as substituted ammonia molecules, where one or more of the hydrogen atoms on the ammonia molecule has been replaced by an alkyl group. Amines are classified as either primary, secondary or tertiary depending on the number of hydrogen atoms in an ammonia molecule that have been replaced by an alkyl group e.g.

In a primary amine one of the hydrogen atoms in an ammonia molecule has been replaced by an alkyl group; a methyl group (-CH₃) in the examples above. In a secondary amine two of the hydrogen atoms in an ammonia molecule have been replaced by alkyl groups whereas in a tertiary amine all three hydrogen atoms in an ammonia molecule have been replaced by an alkyl group.

In all the amines shown above the nitrogen atom makes 3 covalent bonds and has one lone pair of electrons. However it is also possible for the nitrogen atom to form four bonds. In this case it will end up with a forming an ionic compound and the nitrogen atom will end up with a positive charge, simply because it can use its lone pair of electrons to form a dative covalent bond, for example amines can act as Lewis bases and accept a hydrogen ion from an acid.

The ionic compounds formed fall into two distinct groups:

• amines salts.
and
• quaternary ammonium salts.

If the nitrogen atom is bonded to one or more hydrogen atoms then the ionic compound formed is an amine salt. However if the nitrogen atom is bonded to alkyl or aryl groups, that is there are no hydrogen atoms attached to the nitrogen then the ionic salt is called a quaternary ammonium salt, an example of an amine salt and a quaternary ammonium salt are shown below:

Nomenclature- naming amines

Naming amines can be a bit of a nightmare at times, there are several naming systems in use but luckily most exam boards seem to prefer the traditional naming system I have covered below. Simple amine molecules are named by simply named by adding the suffix -amine to the name of the alkyl or aryl group, for example the molecules below can be thought of as simply an ammonia molecule where one or more of the hydrogen atoms has been replaced by an alkyl group.

If more than one alkyl group is attached to the nitrogen atom then the largest alkyl group is chosen as the parent name and the other alkyl group is assigned as a N-Alkyl group, for example:

You may come across another method used to name amines whereby instead of using the suffix -amine to name the compounds instead the prefix -amino is used, the image below gives several examples of this naming system. However most exam boards seem to prefer the traditional naming system above so I would use this unless your teacher tells you otherwise.

Amine and quaternary ammonium salts

The image below shows number of amine salts and a quaternary ammonium salt. The naming of these salts is straightforward. The 1⁰, 2⁰ and 3⁰ amines form 1⁰, 2⁰ and 3⁰ ammonium salts while tertiary amines (3⁰) form quaternary ammonium salts.

Physical properties of amines

Amines molecules contain a N-H bond and so will undergo hydrogen bonding. The hydrogen bonding in amines is weaker than in water or alcohols since the N-H bond is not as polar as the O-H bond. This means that for example the boiling points of amines are higher than similar non- hydrogen bonded molecules such as alkanes but lower than similar molecular weight alcohols.

Since tertiary amines do not have any hydrogen atoms attached directly to the nitrogen atom they cannot undergo any hydrogen bonding and as a result tertiary amines have lower boiling points than comparable primary and secondary amines.

Base properties of amines and ammonia

Amines are perhaps best known for their rather foul smelling odours, most have a fishy odour but many especially diamines have a rather putrid smell of decaying matter. The chemistry of ammonia and amines is largely dominated by one aspect of their structure- the lone pair of electrons on the nitrogen atom. The presence of this lone pair of electrons makes amines excellent Brønsted-Lowry bases (H⁺ or proton acceptors). Ammonia and amines can form dative covalent with hydrogen ions (H⁺) to form ammonium salts, as shown above:

The presence of an alkyl substituent on an ammonia molecules increases the base strength due to the positive inductive effect of the alkyl group which will push electron density onto the nitrogen atom. Therefore in general secondary amines will be stronger bases than primary amines and tertiary amines will be stronger bases than secondary amines.

However aromatic amines, such as phenylamine or aniline are poor bases. In an aromatic amine the nitrogen atom is bonded directly to an aromatic ring, this means that the lone pair of electrons on the nitrogen atom can be delocalised through the aromatic ring and so are not readily available to form dative covalent bonds in the same way aliphatic amines can. This is outlined in the diagram below:

Key Points

Practice questions

Check your understanding - Questions on amines

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